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Materials Science and Engineering/Diagrams/Optimization of Devices

Responsivity
The ratio of generated photocurrent to incident light power, typically expressed in A/W when used in photoconductive mode. The responsivity may also be expressed as a quantum efficiency, or the ratio of the number of photogenerated carriers to incident photons and thus a unitless quantity.

Dark Current
The current through the photodiode in the absence of light, when it is operated in photoconductive mode. The dark current includes photocurrent generated by background radiation and the saturation current of the semiconductor junction. Dark current must be accounted for by calibration if a photodiode is used to make an accurate optical power measurement, and it is also a source of noise when a photodiode is used in an optical communication system.

Noise-Equivalent Power
(NEP) The minimum input optical power to generate photocurrent, equal to the rms noise current in a 1 hertz bandwidth. The related characteristic detectivity (D) is the inverse of NEP, 1/NEP; and the specific detectivity ($$D^\star$$) is the detectivity normalized to the area (A) of the photodetector, $$D^\star=D\sqrt{A}$$. The NEP is roughly the minimum detectable input power of a photodiode.

Comparison with Photomultipliers
Advantages compared to photomultipliers:

 * 1) Excellent linearity of output current as a function of incident light
 * 2) Spectral response from 190 nm to 1100 nm (silicon), longer wavelengths with other semiconductor materials
 * 3) Low noise
 * 4) Ruggedized to mechanical stress
 * 5) Low cost
 * 6) Compact and light weight
 * 7) Long lifetime
 * 8) High quantum efficiency, typically 80%
 * 9) No high voltage required

Disadvantages compared to photomultipliers:

 * 1) Small area
 * 2) No internal gain (except avalanche photodiodes, but their gain is typically 10²–10³ compared to up to 108 for the photomultiplier)
 * 3) Much lower overall sensitivity
 * 4) Photon counting only possible with specially designed, usually cooled photodiodes, with special electronic circuits
 * 5) Response time for many designs is slower

PN or PIN Diode

 * 1) Due to the intrinsic layer, a PIN photodiode must be reverse biased (Vr). The Vr increases the depletion region allowing a larger volume for electron-hole pair production, and reduces the capacitance thereby increasing the bandwidth.
 * 2) The Vr also introduces noise current, which reduces the S/N ratio. Therefore, a reverse bias is recommended for higher bandwidth applications and/or applications where a wide dynamic range is required.
 * 3) A PN photodiode is more suitable for lower light applications because it allows for unbiased operation.

Photodiode Array
Hundreds or thousands (up to 2048) photodiodes of typical sensitive area 0.025mmx1mm each arranged as a one-dimensional array, which can be used as a position sensor. One advantage of photodiode arrays (PDAs) is that they allow for high speed parallel read out since the driving electronics may not be built in like a traditional CMOS or CCD sensor.

Efficiency Terms
The proportion of electrons able to cross the base and reach the collector is a measure of the BJT efficiency. The heavy doping of the emitter region and light doping of the base region cause many more electrons to be injected from the emitter into the base than holes to be injected from the base into the emitter. The common-emitter current gain is represented by βF or hfe. It is approximately the ratio of the DC collector current to the DC base current in forward-active region, and is typically greater than 100. Another important parameter is the common-base current gain, αF. The common-base current gain is approximately the gain of current from emitter to collector in the forward-active region. This ratio usually has a value close to unity; between 0.98 and 0.998. Alpha and beta are more precisely related by the following identities (NPN transistor):

$$\alpha_T = \frac{I_\mathrm{C}}{I_\mathrm{E}}$$

$$\beta_F = \frac{I_\mathrm{C}}{I_\mathrm{B}}$$

$$\beta_F = \frac{\alpha_T}{1 - \alpha_T}\iff \alpha_T = \frac{\beta_F}{\beta_F+1}$$

Germanium
The germanium transistor was more common in the 1950s and 1960s, and while it exhibits a lower "cut off" voltage, making it more suitable for some applications, it also has a greater tendency to exhibit thermal runaway.

Heterojunction BJT
The heterojunction bipolar transistor (HBT) is an improvement of the BJT that can handle signals of very high frequencies up to several hundred GHz. It is common nowadays in ultrafast circuits, mostly RF systems.[5][6] Heterojunction transistors have different semiconductors for the elements of the transistor. Usually the emitter is composed of a larger bandgap material than the base. The figure shows that this difference in bandgap allows the barrier for holes to inject backward into the base, denoted in figure as Δφp, to be made large, while the barrier for electrons to inject into the base Δφn is made low. This barrier arrangement helps reduce minority carrier injection from the base when the emitter-base junction is under forward bias, and thus reduces base current and increases emitter injection efficiency.

The improved injection of carriers into the base allows the base to have a higher doping level, resulting in lower resistance to access the base electrode. In the more traditional BJT, also referred to as homojunction BJT, the efficiency of carrier injection from the emitter to the base is primarily determined by the doping ratio between the emitter and base, which means the base must be lightly doped to obtain high injection efficiency, making its resistance relatively high. In addition, higher doping in the base can improve figures of merit like the Early voltage by lessening base narrowing.

The grading of composition in the base, for example, by progressively increasing the amount of germanium in a SiGe transistor, causes a gradient in bandgap in the neutral base, denoted in the figure by ΔφG, providing a "built-in" field that assists electron transport across the base. That drift component of transport aids the normal diffusive transport, increasing the frequency response of the transistor by shortening the transit time across the base.

Two commonly used HBTs are silicon–germanium and aluminum gallium arsenide, though a wide variety of semiconductors may be used for the HBT structure. HBT structures are usually grown by epitaxy techniques like MOCVD and MBE.

Gate

 * 1) The threshold voltage (and consequently the drain to source on-current) is modified by the work function difference between the gate material and channel material. Because polysilicon is a semiconductor, its work function can be modulated by adjusting the type and level of doping. Furthermore, because polysilicon has the same bandgap as the underlying silicon channel, it is quite straightforward to tune the work function, so as to achieve low threshold voltages for both NMOS and PMOS devices. By contrast the work functions of metals are not easily modulated, so tuning the work function to obtain low threshold voltages becomes a significant challenge. Additionally, obtaining low threshold devices on both PMOS and NMOS devices would likely require the use of different metals for each device type, introducing additional complexity to the fabrication process.
 * 2) In the MOSFET IC fabrication process, it is preferable to deposit the gate material prior to certain high-temperature steps in order to make better performing transistors. Such high temperature steps would melt some metals, limiting the types of metals that could be used in a metal-gate based process.

Junction Design
The drain induced barrier lowering of the threshold voltage and channel length modulation effects upon I-V curves are reduced by using shallow junction extensions. In addition, halo doping can be used, that is, the addition of very thin heavily doped regions of the same doping type as the body tight against the junction walls to limit the extent of depletion regions.

The capacitive effects are limited by using raised source and drain geometries that make most of the contact area border thick dielectric instead of silicon.

These various features of junction design are shown (with artistic license) in the figure.

Junction leakage is discussed further in the section increased junction leakage.

FinFETS
The FinFET, see figure to right, is a double gate device, one of a number of geometries being introduced to mitigate the effects of short channels and reduce drain-induced barrier lowering.

NMOS Logic
n-channel MOSFETs are smaller than p-channel MOSFETs and producing only one type of MOSFET on a silicon substrate is cheaper and technically simpler. These were the driving principles in the design of NMOS logic which uses n-channel MOSFETs exclusively. However, unlike CMOS logic, NMOS logic consumes power even when no switching is taking place. With advances in technology, CMOS logic displaced NMOS logic in the 1980s to become the preferred process for digital chips.

Higher Subthreshold Conduction
Because of small MOSFET geometries, the voltage that can be applied to the gate must be reduced to maintain reliability. To maintain performance, the threshold voltage of the MOSFET has to be reduced as well. As threshold voltage is reduced, the transistor cannot be switched from complete turn-off to complete turn-on with the limited voltage swing available; the circuit design is a compromise between strong current in the "on" case and low current in the "off" case, and the application determines whether to favor one over the other. Subthreshold leakage (including subthreshold conduction, gate-oxide leakage and reverse-biased junction leakage), which was ignored in the past, now can consume upwards of half of the total power consumption of modern high-performance VLSI chips.

Increased Gate-Oxide Leakage
The gate oxide, which serves as insulator between the gate and channel, should be made as thin as possible to increase the channel conductivity and performance when the transistor is on and to reduce subthreshold leakage when the transistor is off. However, with current gate oxides with a thickness of around 1.2 nm (which in silicon is ~5 atoms thick) the quantum mechanical phenomenon of electron tunneling occurs between the gate and channel, leading to increased power consumption.

Insulators (referred to as high-k dielectrics) that have a larger dielectric constant than silicon dioxide, such as group IVb metal silicates e.g. hafnium and zirconium silicates and oxides are being used to reduce the gate leakage from the 45 nanometer technology node onwards. Increasing the dielectric constant of the gate dielectric allows a thicker layer while maintaining a high capacitance. (Capacitance is proportional to dielectric constant and inversely proportional to dielectric thickness.) All else equal, a higher dielectric thickness reduces the quantum tunneling current through the dielectric between the gate and the channel. On the other hand, the barrier height of the new gate insulator is an important consideration; the difference in conduction band energy between the semiconductor and the dielectric (and the corresponding difference in valence band energy) also affects leakage current level. For the traditional gate oxide, silicon dioxide, the former barrier is approximately 8 eV. For many alternative dielectrics the value is significantly lower, tending to increase the tunneling current, somewhat negating the advantage of higher dielectric constant.

Increased Junction Leakage
To make devices smaller, junction design has become more complex, leading to higher doping levels, shallower junctions, "halo" doping and so forth, all to decrease drain-induced barrier lowering. See also section on junction design. To keep these complex junctions in place, the annealing steps formerly used to remove damage and electrically active defects must be curtailed, increasing junction leakage. Heavier doping also is associated with thinner depletion layers and more recombination centers that result in increased leakage current, even without lattice damage.

Lower Transconductance
The transconductance of the MOSFET decides its gain and is proportional to hole or electron mobility (depending on device type), at least for low drain voltages. As MOSFET size is reduced, the fields in the channel increase and the dopant impurity levels increase. Both changes reduce the carrier mobility, and hence the transconductance. As channel lengths are reduced without proportional reduction in drain voltage, raising the electric field in the channel, the result is velocity saturation of the carriers, limiting the current and the transconductance.

Interconnect Capacitance
Traditionally switching time was roughly proportional to the gate capacitance of gates. However, with transistors becoming smaller and more transistors being placed on the chip, interconnect capacitance (the capacitance of the wires connecting different parts of the chip) is becoming a large percentage of capacitance. Signals have to travel through the interconnect, which leads to increased delay and lower performance.

Heat Production
The ever-increasing density of MOSFETs on an integrated circuit is creating problems of substantial localized heat generation that can impair circuit operation. Circuits operate slower at high temperatures, and have reduced reliability and shorter lifetimes. Heat sinks and other cooling methods are now required for many integrated circuits including microprocessors.

Process Variations
With MOSFETS becoming smaller, the number of atoms in the silicon that produce many of the transistor's properties is becoming fewer, with the result that control of dopant numbers and placement is more erratic. During chip manufacturing, random process variations affect all transistor dimensions: length, width, junction depths, oxide thickness etc., and become a greater percentage of overall transistor size as the transistor shrinks. The transistor characteristics become less certain, more statistical. The random nature of manufacture means we do not know which particular example MOSFETs actually will end up in a particular instance of the circuit. This uncertainty forces a less optimal design because the design must work for a great variety of possible component MOSFETs. See design for manufacturability, reliability engineering, six sigma and statistical process control.

First Generation Photovoltaics
The first generation photovoltaic cell consists of a large-area, single-crystal, single layer p-n junction diode, capable of generating usable electrical energy from light sources with the wavelengths of sunlight. These cells are typically made using a diffusion process with silicon wafers. First-generation photovoltaic cells (also known as silicon wafer-based solar cells) are the dominant technology in the commercial production of solar cells, accounting for more than 86% of the terrestrial solar cell market.

Second Generation Photovoltaics
The second generation of photovoltaic materials is based on the use of thin epitaxial deposits of semiconductors on lattice-matched wafers. There are two classes of epitaxial photovoltaics - space and terrestrial. Space cells typically have higher AM0 efficiencies (28-30%) in production, but have a higher cost per watt. Their "thin-film" cousins have been developed using lower-cost processes, but have lower AM0 efficiencies (7-9%) in production and are questionable for space applications. The advent of thin-film technology contributed to a prediction of greatly reduced costs for thin film solar cells that has yet to be achieved. There are currently (2007) a number of technologies/semiconductor materials under investigation or in mass production. Examples include amorphous silicon, polycrystalline silicon, micro-crystalline silicon, cadmium telluride, copper indium selenide/sulfide. An advantage of thin-film technology theoretically results in reduced mass so it allows fitting panels on light or flexible materials, even textiles. The advent of thin GaAs-based films for space applications (so-called "thin cells") with potential AM0 efficiencies of up to 37% are currently in the development stage for high specific power applications. Second generation solar cells now comprise a small segment of the terrestrial photovoltaic market, and approximately 90% of the space market.

Third Generation Photovoltaics
Third-generation photovoltaics are proposed to be very different from the previous semiconductor devices as they do not rely on a traditional p-n junction to separate photogenerated charge carriers. For space applications quantum well devices (quantum dots, quantum ropes, etc.) and devices incorporating carbon nanotubes are being studied - with a potential for up to 45% AM0 production efficiency. For terrestrial applications, these new devices include photoelectrochemical cells, polymer solar cells, nanocrystal solar cells, Dye-sensitized solar cells and are still in the research phase.

Fourth Generation Photovoltaics
A hypothetical 'fourth-generation' of solar cells may consist of composite photovoltaic technology, in which polymers with nano particles can be mixed together to make a single multispectrum layer. Then the thin multispectrum layers can be stacked to make multispectrum solar cells more efficient and cheaper based on polymer solar cell and multijunction technology used by NASA on Mars missions. The layer that converts different types of light is first, then another layer for the light that passes and last is an infra-red spectrum layer for the cell—thus converting some of the heat for an overall solar cell composite. Current research is being conducted under a DARPA grant to determine if this technology is viable. Companies working on fourth-generation photovoltaics include Xsunx, Konarka Technologies, Inc., Nanosolar, Dyesol and Nanosys. Research is also being done in this area by the USA's National Renewable Energy Laboratory.

Maximum Power Point
A solar cell may operate over a wide range of voltages (V) and currents (I). By increasing the resistive load on an irradiated cell continuously from zero (a short circuit) to a very high value (an open circuit) one can determine the maximum-power point, the point that maximizes V×I, that is, the load for which the cell can deliver maximum electrical power at that level of irradiation.

The maximum power point of a photovoltaic varies with incident illumination. For systems large enough to justify the extra expense, a maximum power point tracker tracks the instantaneous power by continually measuring the voltage and current (and hence, power transfer), and uses this information to dynamically adjust the load so the maximum power is always transferred, regardless of the variation in lighting.

Energy Conversion Factor
A solar cell's energy conversion efficiency (η, "eta"), is the percentage of power converted (from absorbed light to electrical energy) and collected, when a solar cell is connected to an electrical circuit. This term is calculated using the ratio of the maximum power point, Pm, divided by the input light irradiance (E, in W/m²) under standard test conditions (STC) and the surface area of the solar cell (Ac in m²).

$$\eta = \frac{P_{m}}{E \times A_c}$$

STC specifies a temperature of 25°C and an irradiance of 1000 W/m² with an air mass 1.5 (AM1.5) spectrum. These correspond to the irradiance and spectrum of sunlight incident on a clear day upon a sun-facing 37°-tilted surface with the sun at an angle of 41.81° above the horizon. This condition approximately represents solar noon near the spring and autumn equinoxes in the continental United States with surface of the cell aimed directly at the sun. Thus, under these conditions a solar cell of 12% efficiency with a 100 cm2 (0.01 m2) surface area can be expected to produce approximately 1.2 watts of power.

Fill Factor
Another defining term in the overall behavior of a solar cell is the fill factor (FF). This is the ratio of the maximum power point divided by the open circuit voltage (Voc) and the short circuit current (Isc):

$$FF = \frac{P_{m}}{V_{oc} \times I_{sc}} = \frac{\eta \times A_c \times E}{V_{oc} \times I_{sc}}$$

Quantum Efficiency
As described above, when a photon is absorbed by a solar cell it is converted to an electron-hole pair. This electron-hole pair may then travel to the surface of the solar cell and contribute to the current produced by the cell; such a carrier is said to be collected. Alternatively, the carrier may give up its energy and once again become bound to an atom within the solar cell without reaching the surface; this is called recombination, and carriers that recombine do not contribute to the production of electrical current.

Quantum efficiency refers to the percentage of photons that are converted to electric current (i.e., collected carriers). External quantum efficiency is the fraction of incident photons that are converted to electrical current, while internal quantum efficiency is the fraction of absorbed photons that are converted to electrical current. Mathematically, internal quantum efficiency is related to external quantum efficiency by the reflectance of the solar cell; given a perfect antireflection coating, they are the same.

Quantum efficiency should not be confused with energy conversion efficiency, as it does not convey information about the power collected from the solar cell. Furthermore, quantum efficiency is most usefully expressed as a spectral measurement (that is, as a function of photon wavelength or energy). Since some wavelengths are absorbed more effectively than others in most semiconductors, spectral measurements of quantum efficiency can yield information about which parts of a particular solar cell design are most in need of improvement.

Thin Films
The various thin-film technologies currently being developed reduce the amount (or mass) of light absorbing material required in creating a solar cell. This can lead to reduced processing costs from that of bulk materials (in the case of silicon thin films) but also tends to reduce energy conversion efficiency, although many multi-layer thin films have efficiencies above those of bulk silicon wafers.

CdTe
Cadmium telluride is an efficient light-absorbing material for thin-film solar cells. Compared to other thin-film materials, CdTe is easier to deposit and more suitable for large-scale production. Despite much discussion of the toxicity of CdTe-based solar cells, this is the only technology (apart from amorphous silicon) that can be delivered on a large scale, as shown by First Solar and Antec Solar. There is a 40 megawatt plant in Ohio (USA) and a 10 megawatt plant in Germany. First Solar is scaling up to a 100 MW plant in Germany and started building another 100 MW plant in Malaysia (2007).

The perception of the toxicity of CdTe is based on the toxicity of elemental cadmium, a heavy metal that is a cumulative poison. Scientific work, particularly by researchers of the National Renewable Energy Laboratories (NREL) in the USA, has shown that the release of cadmium to the atmosphere is lower with CdTe-based solar cells than with silicon photovoltaics and other thin-film solar cell technologies.

Copper-Indium Selenide
The materials based on CuInSe2 that are of interest for photovoltaic applications include several elements from groups I, III and VI in the periodic table. These semiconductors are especially attractive for thin film solar cell application because of their high optical absorption coefficients and versatile optical and electrical characteristics which can in principle be manipulated and tuned for a specific need in a given device. CIS is an abbreviation for general chalcopyrite films of copper indium selenide (CuInSe2).

When gallium is substituted for some of the indium in CIS, the material is sometimes called CIGS, or copper indium/gallium diselenide, a solid mixture of the semiconductors CuInSe2 and CuGaSe2, often abbreviated by the chemical formula CuInxGa(1-x)Se2. Unlike the conventional silicon based solar cell, which can be modelled as a simple p-n junction (see under semiconductor), these cells are best described by a more complex heterojunction model. The best efficiency of a thin-film solar cell as of December 2005 was 19.5% with CIGS absorber layer. Higher efficiencies (around 30%) can be obtained by using optics to concentrate the incident light. The use of gallium increases the optical bandgap of the CIGS layer as compared to pure CIS, thus increasing the open-circuit voltage. In another point of view, gallium is added to replace as much indium as possible due to gallium's relative availability to indium.

Gallium Arsenide (GaAs) Multijunction
High-efficiency cells have been developed for special applications such as satellites and space exploration. These multijunction cells consist of multiple thin films produced using molecular beam epitaxy. A triple-junction cell, for example, may consist of the semiconductors: GaAs, Ge, and GaInP2. Each type of semiconductor will have a characteristic band gap energy which, loosely speaking, causes it to absorb light most efficiently at a certain color, or more precisely, to absorb electromagnetic radiation over a portion of the spectrum./Pol The semiconductors are carefully chosen to absorb nearly all of the solar spectrum, thus generating electricity from as much of the solar energy as possible.

GaAs multijunction devices are the most efficient solar cells to date, reaching a record high of 40.7% efficiency under solar concentration and laboratory conditions.[19] These devices use 20 to 30 different semiconductors layered in series. At the National Renewable Energy Laboratory, a new cell of area 0.26685 cm² will generate a power of 2.6 W. They estimate that this technology could eventually produce electricity at a mere 8–10 cents/kWh. This is similar to the price of electricity today. Thus, this breakthrough could ultimately result in increased consumer use of solar cells.

Light-Absorbing Dyes
Typically a ruthenium metalorganic dye (Ru-centered) is used as a monolayer of light-absorbing material. The dye-sensitized solar cell depends on a mesoporous layer of nanoparticulate titanium dioxide to greatly amplify the surface area (200-300 m²/g TiO2, as compared to approximately 10 m²/g of flat single crystal). The photogenerated electrons from the light absorbing dye are passed on to the n-type TiO2, and the holes are passed to an electrolyte on the other side of the dye. The circuit is completed by a redox couple in the electrolyte, which can be liquid or solid. This type of cell allows a more flexible use of materials, and is typically manufactured by screen printing, with the potential for lower processing costs than those used for bulk solar cells. However, the dyes in these cells also suffer from degradation under heat and UV light, and the cell casing is difficult to seal due to the solvents used in assembly. In spite of the above, this is a popular emerging technology with some commercial impact forecast within this decade.

Organic/Polymer Solar Cells
Organic solar cells are built from thin films (typically 100 nm) of an active, semiconductor layer. In polymer cells, the active layer typically consists of a conjugated electron donor polymer such as poly(p-phenylene-vinylene) or poly(3-hexylthiopene) (P3HT) in conjunction with a fullerene electron acceptor (such as PCBM or C60). In small molecule cells, phthalocyanine compounds or other planar, conjugated compounds are used as donors with fullerenes as acceptors. Energy conversion efficiencies achieved to date using conductive polymers are much lower than standard silicon cells, reaching about 12% for the most efficient polymer cells and about 8% for the most efficient small molecule cells. However, organic solar cells are cost competitive with commercial silicon cells, as they feature inexpensive, abundant materials and relatively easy manufacturing, without need of cleanroom technology or high temperature processing. They are also well suited to applications where mechanical flexibility and disposability are important.

Silicon
Silicon thin-films are mainly deposited by chemical vapor deposition (typically plasma-enhanced (PE-CVD)) from silane gas and hydrogen gas. Depending on the deposition's parameters, this can yield:

 * 1) Amorphous silicon (a-Si or a-Si:H)
 * 2) Protocrystalline silicon or
 * 3) Nanocrystalline silicon (nc-Si or nc-Si:H).

These types of silicon present dangling and twisted bonds, which results in deep defects (energy levels in the bandgap) as well as deformation of the valence and conduction bands (band tails). The solar cells made from these materials tend to have lower energy conversion efficiency than bulk silicon, but are also less expensive to produce. The quantum efficiency of thin film solar cells is also lower due to reduced number of collected charge carriers per incident photon.

Amorphous silicon has a higher bandgap (1.7 eV) than crystalline silicon (c-Si) (1.1 eV), which means it absorbs the visible part of the solar spectrum more strongly than the infrared portion of the spectrum. As nc-Si has about the same bandgap as c-Si, the two material can be combined in thin layers, creating a layered cell called a tandem cell. The top cell in a-Si absorbs the visible light and leaves the infrared part of the spectrum for the bottom cell in nanocrystalline Si.

Nanocrystalline Solar Cells
These structures make use of some of the same thin-film light absorbing materials but are overlain as an extremely thin absorber on a supporting matrix of conductive polymer or mesoporous metal oxide having a very high surface area to increase internal reflections (and hence increase the probability of light absorption).

Concentrating Pholtovoltaics
Concentrating photovoltaic systems use a large area of lenses or mirrors to focus sunlight on a small area of photovoltaic cells. If these systems use single or dual-axis tracking to improve performance, they may be referred to as Heliostat Concentrator Photovoltaics (HCPV). The primary attraction of CPV systems is their reduced usage of semiconducting material which is expensive and currently in short supply. Additionally, increasing the concentration ratio improves the performance of general photovoltaic materials and also allows for the use of high-performance materials such as gallium arsenide.

Silicon Processing
One way of reducing the cost is to develop cheaper methods of obtaining silicon that is sufficiently pure. Silicon is a very common element, but is normally bound in silica, or silica sand. Processing silica (SiO2) to produce silicon is a very high energy process - at current efficiencies, it takes over two years for a conventional solar cell to generate as much energy as was used to make the silicon it contains. More energy efficient methods of synthesis are not only beneficial to the solar industry, but also to industries surrounding silicon technology as a whole.

Thin-Film Processing
Thin-film solar cells use less than 1% of the raw material (silicon or other light absorbers) compared to wafer based solar cells, leading to a significant price drop per kWh. There are many research groups around the world actively researching different thin-film approaches and/or materials, however it remains to be seen if these solutions can generate the same space-efficiency as traditional silicon processing.

One particularly promising technology is crystalline silicon thin films on glass substrates. This technology makes use of the advantages of crystalline silicon as a solar cell material, with the cost savings of using a thin-film approach.

Another interesting aspect of thin-film solar cells is the possibility to deposit the cells on all kind of materials, including flexible substrates (PET for example), which opens a new dimension for new applications.

Polymer Processing
The invention of conductive polymers (for which Alan Heeger, Alan G. MacDiarmid and Hideki Shirakawa were awarded a Nobel prize) may lead to the development of much cheaper cells that are based on inexpensive plastics. However, all organic solar cells made to date suffer from degradation upon exposure to UV light, and hence have lifetimes which are far too short to be viable. The conjugated double bond systems in the polymers, which carry the charge, are always susceptible to breaking up when radiated with shorter wavelengths. Additionally, most conductive polymers, being highly unsaturated and reactive, are highly sensitive to atmospheric moisture and oxidation, making commercial applications difficult.

Nanoparticle Processing
Experimental non-silicon solar panels can be made of quantum heterostructures, eg. carbon nanotubes or quantum dots, embedded in conductive polymers or mesoporous metal oxides. In addition, thin films of many of these materials on conventional silicon solar cells can increase the optical coupling efficiency into the silicon cell, thus boosting the overall efficiency. By varying the size of the quantum dots, the cells can be tuned to absorb different wavelengths. Although the research is still in its infancy, quantum dot-modified photovoltaics may be able to achieve up to 42 percent energy conversion efficiency due to multiple exciton generation(MEG).

Transparent Conductors
Many new solar cells use transparent thin films that are also conductors of electrical charge. The dominant conductive thin films used in research now are transparent conductive oxides (abbreviated "TCO"), and include fluorine-doped tin oxide (SnO2:F, or "FTO"), doped zinc oxide (e.g.: ZnO:Al), and indium tin oxide (abbreviated "ITO"). These conductive films are also used in the LCD industry for flat panel displays. The dual function of a TCO allows light to pass through a substrate window to the active light absorbing material beneath, and also serves as an ohmic contact to transport photogenerated charge carriers away from that light absorbing material. The present TCO materials are effective for research, but perhaps are not yet optimized for large-scale photovoltaic production. They require very special deposition conditions at high vacuum, they can sometimes suffer from poor mechanical strength, and most have poor transmittance in the infrared portion of the spectrum (e.g.: ITO thin films can also be used as infrared filters in airplane windows). These factors make large-scale manufacturing more costly.

A relatively new area has emerged using carbon nanotube networks as a transparent conductor for organic solar cells. Nanotube networks are flexible and can be deposited on surfaces a variety of ways. With some treatment, nanotube films can be highly transparent in the infrared, possibly enabling efficient low bandgap solar cells. Nanotube networks are p-type conductors, whereas traditional transparent conductors are exclusively n-type. The availability of a p-type transparent conductor could lead to new cell designs that simplify manufacturing and improve efficiency.

Silicon Wafer Based Solar Cell
Despite the numerous attempts at making better solar cells by using new and exotic materials, the reality is that the photovoltaics market is still dominated by silicon wafer-based solar cells (first-generation solar cells). This means that most solar cell manufacturers are equipped to produce these type of solar cells. Therefore, a large body of research is currently being done all over the world to create silicon wafer-based solar cells that can achieve higher conversion efficiency without an exorbitant increase in production cost. The aim of the research is to achieve the lowest $/watt solar cell design that is suitable for commercial production.

Sliver Cells
Professor Andrew Blakers and Dr Klaus Weber, working at Australian National University and Origin Energy have developed a technique for slicing a single silicon wafer, which allows a significantly larger collector surface area from each wafer, compared to usual solar cells. The technique involves taking a silicon wafer, typically 1 to 2 mm thick, and making a multitude of parallel, transverse slices across the wafer, creating a large number of slivers that have a thickness of 50 micrometres and a width equal to the thickness of the original wafer. These slices are rotated 90 degrees, so that the surfaces corresponding to the faces of the original wafer become the edges of the slivers. The result is to convert, for example, a 150 mm diameter, 2 mm-thick wafer having an exposed silicon surface area of about 175 cm² per side into about 1000 slivers having dimensions of 100 mm x 2 mm x 0.1 mm, yielding a total exposed silicon surface area of about 2000 cm² per side. As a result of this rotation, the electrical doping and contacts that were on the face of the wafer are located the edges of the sliver, rather than the front and rear as is the case with conventional wafer cells. This has the interesting effect of making the cell sensitive from both the front and rear of the cell (a property known as bifaciality)


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Radio Discussion/Living on Earth/2008.1.18/Segment 1

Biofuel the Best Bet?

Biofuels
Biofuels are controversial because they use the same resources as the human food supply.

Wikilinks

 * Biofuel
 * Virgin Atlantic Airways
 * Jatropha (detropha and getropha seem to be common synonyms)
 * Jatropha oil
 * Biofuels in India
 * (more articles on w:Category:Biofuels by country)
 * Flora of Indonesia
 * Palm oil
 * Forests Now Declaration
 * Canola
 * Soybean
 * Maize
 * Cellulosic ethanol
 * Woodchipper

Questions

 * Do you use biofuels?
 * Should biofuels be made from food crops?


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Materials Science and Engineering/Diagrams/Power Generating Devices

Fuel Cell
A fuel cell is an electrochemical energy conversion device. It produces electricity from various external quantities of fuel (on the anode side) and oxidant (on the cathode side). These react in the presence of an electrolyte. Generally, the reactants flow in and reaction products flow out while the electrolyte remains in the cell. Fuel cells can operate virtually continuously as long as the necessary flows are maintained.

Fuel cells are different from batteries in that they consume reactant, which must be replenished, while batteries store electrical energy chemically in a closed system. Additionally, while the electrodes within a battery react and change as a battery is charged or discharged, a fuel cell's electrodes are catalytic and relatively stable.

Many combinations of fuel and oxidant are possible. A hydrogen cell uses hydrogen as fuel and oxygen as oxidant. Other fuels include hydrocarbons and alcohols. Other oxidants include air, chlorine and chlorine dioxide.

Fuel Cell Design

In essence, a fuel cell works by catalysis, separating the component electrons and protons of the reactant fuel, and forcing the electrons to travel through a circuit, hence converting them to electrical power. The catalyst is typically comprised of a platinum group metal or alloy. Another catalytic process takes the electrons back in, combining them with the protons and the oxidant to form waste products (typically simple compounds like water and carbon dioxide).

Metal Hydride Fuel Cell
Metal hydride fuel cells are a subclass of alkaline fuel cells that are currently in the research and development phase. A notable feature is their ability to chemically bond and store hydrogen within the cell. This feature is shared with direct borohydride fuel cells, although the two differ in that MHFCs are refueled with pure hydrogen. Though the absorption characteristics of metal hydrides (around 2%) is far lower than sodium-borohydrides and other "light" metal hydrides (around 10,8%), prototypes have been claimed to demonstrate a number of interesting characteristics:

 * Ability to be recharged with electrical energy (similar to NiMH batteries);
 * Low operating temperatures (down to -20°C);
 * Fast kinetics;
 * Extended shelf life;
 * Fast "cold start" properties;
 * Ability to operate for limited periods of time with no external hydrogen source, enabling "hot swapping" of fuel canisters.

Electro-Galvanic Fuel Cell
A chemical reaction occurs in the fuel cell when the potassium hydroxide in the cell comes into contact with oxygen. This creates an electric current between the lead anode and the gold-plated cathode through a load resistance. The voltage produced is proportional to the concentration of oxygen present.

Formic Acid Fuel Cell
Direct-formic acid fuel cells or DFAFCs are a subcategory of proton-exchange fuel cells where, the fuel, formic acid, is not reformed, but fed directly to the fuel cell. Their applications include small, portable electronics such as phones and laptop computers.

Advantages
Similar to methanol, formic acid is a small organic molecule fed directly into the fuel cell, removing the need for complicated catalytic reforming. Storage of formic acid is much easier and safer than that of hydrogen because it does not need to be done at high pressures and (or) low temperatures, as formic acid is a liquid at ambient temperature.

There are two important advantages that formic acid possesses over methanol for use in the fuel cell. First, formic acid does not cross over the polymer membrane, so its efficiency can be higher than that of methanol. Second, formic acid does not cause blindness as does methanol, making it a somewhat safer fuel in case of leakage.

Zinc-Air Battery
Zinc-air batteries ( non-rechargeable), and zinc-air fuel cells, ( mechanically-rechargeable) are electro-chemical batteries powered by the oxidation of zinc with oxygen from the air. These batteries have high energy densities and are relatively inexpensive to produce. They are used in hearing aids and in experimental electric vehicles. They may be an important part of a future zinc economy.

Particles of zinc are mixed with an electrolyte (usually potassium hydroxide solution); water and oxygen from the air react at the cathode and form hydroxyls which migrate into the zinc paste and form zincate (Zn(OH)42-), at which point electrons are released and travel to the cathode. The zincate decays into zinc oxide and water is released back into the system. The water and hydroxyls from the anode are recycled at the cathode, so the water serves only as a catalyst. The reactions produce a maximum voltage level of 1.65 volts, but this is reduced to 1.4–1.35 V by reducing air flow into the cell; this is usually done for hearing aid batteries to reduce the rate of water drying out.

Microbial Fuel Cell
A microbial fuel cell (MFC) or biological fuel cell is a bio-electrochemical system that drives a current by mimicking bacterial interactions found in nature.

Generating Electricity
When micro-organisms consume a substrate such as sugar in aerobic conditions they produce carbon dioxide and water. However when oxygen is not present they produce carbon dioxide, protons and electrons as described below (Bennetto, 1990):

C12H22O11 + 13H2O ---> 12CO2 + 48H+ + 48e-   Eqt. 1

Microbial fuel cells use inorganic mediators to tap into the electron transport chain of cells and steal the electrons that are produced. The mediator crosses the outer cell lipid membranes and plasma wall; it then begins to liberate electrons from the electron transport chain that would normally be taken up by oxygen or other intermediates. The now-reduced mediator exits the cell laden with electrons that it shuttles to an electrode where it deposits them; this electrode becomes the electro-generic anode (negatively charged electrode). The release of the electrons means that the mediator returns to its original oxidised state ready to repeat the process. It is important to note that this can only happen under anaerobic conditions, if oxygen is present then it will collect all the electrons as it has a greater electronegativity than the mediator.

Reversible Fuel Cell
A reversible fuel cell (RFC) is a fuel cell that is designed to consume chemical A to produce electricity and chemical B and be reversed to consume electricity and chemical B to produce chemical A. A hydrogen fuel cell, for example, uses hydrogen (H2) and oxygen (O2) to produce electricity and water (H2O); a reversible hydrogen fuel cell could also use electricity and water to produce hydrogen and oxygen.

By definition, the process of any fuel cell could be reversed. However, a given device is usually optimized for operating in one mode and may not be built in such a way that it can be operated backwards. Fuel cells operated backwards generally do not make very efficient systems. Because of this, fuel cells operated in forward-reverse mode are not suited for energy storage systems in small and medium scale. Most fuel cells operated in the reverse mode are sold as learning kits or curiosities.

Direct Borohydride Fuel Cell
Direct borohydride fuel cells (DBFCs) are a subcategory of alkaline fuel cells that use a solution of sodium borohydride for fuel. The advantage of sodium borohydride over conventional hydrogen in an alkaline fuel cell is that the highly alkaline fuel and waste borax prevents poisoning of the fuel cell from carbon dioxide (CO2) in the air.

Sodium borohydride could potentially be used in more conventional hydrogen fuel cell systems as a means of storing hydrogen. The hydrogen can be regenerated for a fuel cell by catalytic decomposition of the borohydride:

 * $$NaBH_4 + 2H_2O \rightarrow NaBO_2 + 4H_2$$

Direct borohydride fuel cells decompose and oxidize the borohydride directly, side-stepping hydrogen production and even producing slightly higher energy yields:

 * Cathode: $$2O_2 + 4H_2O + 8e^- \rightarrow 8OH^- $$
 * $$E_0=.4 V\;$$
 * Anode: $$NaBH_4 + 8OH^- \rightarrow NaBO_2 + 6H_2O + 8e^-$$
 * $$E_0=1.24 V\;$$
 * Total $$E_0=1.64 V\;$$

DBFCs could be produced more cheaply than a traditional fuel cell because they do not need expensive platinum catalysts. In addition, they have a higher power density. Unfortunately, DBFCs do produce some hydrogen from a side step reaction of NaBH4 with water heated by the fuel cell. This hydrogen can either be piped out to the exhaust or piped to a conventional hydrogen fuel cell. Either fuel cell will produce water, and the water can be recycled to allow for higher concentrations of NaBH4.

After releasing its hydrogen and being oxidized, NaBO2 or borax is produced. Borax is a common detergent and soap additive and is relatively non-toxic. Borax can be hydrogenated back into borohydride fuel by several different techniques, some of which require nothing more than water and electricity or heat. These techniques are still in active development.

Sodium borohydride costs US$50 per kg, but with borax recycling and mass production projected prices for the fuel are as low as US$1/kg.

Alkaline Fuel Cell

The alkaline fuel cell (AFC), also known as the Bacon fuel cell after its British inventor, is one of the most developed fuel cell technologies and is the cell that flew Man to the Moon. NASA has used alkaline fuel cells since the mid-1960s, in Apollo-series missions and on the Space Shuttle. AFCs consume hydrogen and pure oxygen producing potable water, heat, and electricity. They are among the most efficient fuel cells, having the potential to reach 70%.

Direct Methanol Fuel Cell
Direct-methanol fuel cells or DMFCs are a subcategory of proton-exchange fuel cells where, the fuel, methanol (CH3OH), is not reformed, but fed directly to the fuel cell. Because methanol is fed directly into the fuel cell, complicated catalytic reforming is unneeded. Storage of methanol is much easier than that of hydrogen because it does not need to be done at high pressures or low temperatures, as methanol is a liquid from -97.0 °C to 64.7 °C (-142.6 °F to 148.5 °F). The energy density of methanol, the amount of energy contained in a given volume of methanol, is an order of magnitude greater than even highly compressed hydrogen.

However, the efficiency of direct-methanol fuel cells is low due to the high permeation of methanol through the membrane, which is known as methanol crossover, and the dynamic behaviour is sluggish. Other problems include the management of carbon dioxide created at the anode. Current DMFCs are limited in the power they can produce, but can still store a high energy content in a small space. This means they can produce a small amount of power over a long period of time. This makes them ill-suited for powering vehicles, but ideal for consumer goods such as mobile phones, digital cameras or laptops.

Reactions
The DMFC relies upon the oxidation of methanol on a catalyst layer to form carbon dioxide. Water is consumed at the anode and is produced at the cathode. Positive ions (H+) are transported across the proton exchange membrane (often Nafion) to the cathode where they react with oxygen to produce water. Electrons are transported via an external circuit from anode to cathode providing power to external devices.

The half-reactions are:

Anode: CH3OH + H2O → CO2 + 6H+ + 6e-

The methanol is adsorbed on a catalyst, usually made of platinum particles, and deprotonized until carbon dioxide is formed. Usually, the catalyst consists of another metallic component, usually ruthenium, which is used to catalyze methanol oxidation (see last paragraph for more details.

Cathode: (3/2)O2 + 6H+ + 6e- → 3H2O

Net reaction: CH3OH + (3/2)O2 → CO2 + 2H2O

Because water is consumed at the anode in the reaction, pure methanol cannot be used without provision of water via either passive transport such as back diffusion (osmosis), or active transport such as pumping. The need for water limits the energy density of the fuel.

Currently, platinum is used as a catalyst for both half-reactions. This contributes to the loss of cell voltage potential, as any methanol that is present in the cathode chamber will oxidize. If another catalyst could be found for the reduction of oxygen, the problem of methanol crossover would likely be significantly lessened. Furthermore, platinum is very expensive and contributes to the high cost per kilowatt of the fuel cell.

In one of the steps of the methanol oxidation reaction, a CO species is produced, which adsorbs strongly on the platinum catalyst, reducing the surface area for the catalyst reaction. The addition of another components, such as ruthenium or gold, to the catalyst, tends to ameliorate this problem because, according to the most well-established theory in the field, these catalysts oxidize water to yield OH radicals: H2O → OH• + H+ + e-. The OH species from the oxidized water molecule oxidizes CO to produce CO2 which can then be released as a gas: CO + OH• → CO2 + H+ + e-.

Reformed Methanol Fuel Cell
Reformed methanol fuel cell systems or RMFCs are a subcategory of proton-exchange fuel cells where, the fuel, methanol (CH3OH), is reformed, before being fed into the fuel cell. RMFC systems offer advantages over DMFC systems including higher efficiency, smaller cell stacks, no water management, better operation at low temperatures, and storage at sub-zero[vague] temperatures. The tradeoff is that RMFC systems operate at hotter temperatures and therefore need more advanced heat management and insulation.

Direct-Ethanol Fuel Cell
Direct-ethanol fuel cells or DEFCs are a subcategory of Proton-exchange fuel cells where, the fuel, ethanol, is not reformed, but fed directly to the fuel cell.

Advantages
DEFC uses Ethanol in the fuel cell instead of the more toxic methanol. Ethanol is an attractive alternative to methanol because it comes with a supply chain that's already in place. Ethanol also remains the easier fuel to work with for widespread use by consumers.

Proton Exchange Membrane
Proton exchange membrane fuel cells, also known as polymer electrolyte membrane fuel cells (PEMFC), are a type of fuel cell being developed for transport applications as well as for stationary and portable applications. Their distinguishing features include lower temperature/pressure ranges and a special polymer electrolyte membrane.

Molten Carbonate Fuel Cell
Molten-carbonate fuel cells (MCFCs) are high-temperature fuel cells, that operate at temperatures of 600°C and above.

Molten carbonate fuel cells (MCFCs) are currently being developed for natural gas and coal-based power plants for electrical utility, industrial, and military applications. MCFCs are high-temperature fuel cells that use an electrolyte composed of a molten carbonate salt mixture suspended in a porous, chemically inert ceramic matrix of beta-alumina solid electrolyte (BASE). Since they operate at extremely high temperatures of 650°C (roughly 1,200°F) and above, non-precious metals can be used as catalysts at the anode and cathode, reducing costs.

Protonic Ceramic Fuel Cell
The protonic ceramic fuel cell or PCFC is based on a ceramic electrolyte material that exhibits high protonic conductivity at elevated temperatures.


text: User:Grandmasterka/Survey responses

title: User:Grandmasterka/Survey responses


text:
User talk:Grandmasterka

Welcome
Hello Grandmasterka, and welcome to Wikiversity! If you need help, feel free to visit my talk page, or contact us and ask questions. After you leave a comment on a talk page, remember to sign and date; it helps everyone follow the threads of the discussion. The signature icon in the edit window makes it simple. To get started, you may
 * Take a guided tour and learn to edit;
 * Explore our learning projects;
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 * Chat with other Wikiversitans on #wikiversity-en.

And don't forget to explore Wikiversity with the links to your left. Be bold, and see you around Wikiversity! Erkan Yilmaz ( Wikiversity:Chat, wiki blog ) 19:00, 31 January 2008 (UTC)

title: User talk:Grandmasterka


text:
User:Grandmasterka

 * w:User:Grandmasterka
 * User:Grandmasterka/Survey responses

title: User:Grandmasterka


text:
Materials Science and Engineering/Diagrams/Transducers

A transducer is a device, usually electrical, electronic, electro-mechanical, electromagnetic, photonic, or photovoltaic that converts one type of energy to another for various purposes including measurement or information transfer (for example, pressure sensors). In a broader sense (for example in the Viable System Model) a transducer is sometimes defined as any device that converts a signal from one form to another.

Antenna

An antenna is a transducer designed to transmit or receive electromagnetic waves. In other words, antennas convert electromagnetic waves into electrical currents and vice versa. Antennas are used in systems such as radio and television broadcasting, point-to-point radio communication, wireless LAN, radar, and space exploration. Antennas usually work in air or outer space, but can also be operated under water or even through soil and rock at certain frequencies for short distances.

Cathode Ray Tube
The cathode ray tube (CRT) is an evacuated glass envelope containing an electron gun (a source of electrons) and a fluorescent screen, usually with internal or external means to accelerate and deflect the electrons. When electrons strike the fluorescent screen, light is emitted.

The electron beam is deflected and modulated in a way which causes it to display an image on the screen. The image may represent electrical waveforms (oscilloscope), pictures (television, computer monitor), echoes of aircr

Fluorescent Lamp

A fluorescent lamp or fluorescent tube is a gas-discharge lamp that uses electricity to excite mercury vapor in argon or neon gas, resulting in a plasma that produces short-wave ultraviolet light. This light then causes a phosphor to fluoresce, producing visible light.

Unlike incandescent lamps, fluorescent lamps always require a ballast to regulate the flow of power through the lamp. In common tube fixtures (typically 4 ft (122 cm) or 8 ft (244 cm) in length), the ballast is enclosed in the fixture. Compact fluorescent light bulbs may have a conventional ballast located in the fixture or they may have ballasts integrated in the bulbs, allowing them to be used in lampholders normally used for incandescent lamps.

Light Bulb
A lamp, in technical usage, is a replaceable component such as an incandescent light bulb, which is designed to produce light from electricity. These components usually have a base of ceramic, metal, glass or plastic, which makes an electrical connection in the socket of a light fixture. This connection may be made with a screw-thread base, two metal pins, 2 metal caps or a bayonet cap.

Magnetic Cartridge

A magnetic cartridge is a transducer used for the playback of gramophone records on a turntable or phonograph. It converts mechanical vibrational energy from a stylus riding in a spiral record groove into an electrical signal that is subsequently amplified and then converted back to sound by a loudspeaker system.

Photodetector
Photosensors or photodetectors are sensors of light or other electromagnetic energy. There are several varieties:

Most optical detectors are quantum devices in which an individual photon produces a discrete effect.

 * Chemical detectors, such as photographic plates, in which a silver halide molecule is split into an atom of metallic silver and a halogen atom. The photographic developer causes adjacent molecules to split similarly.
 * Photoresistors or Light Dependent Resistors (LDR) which change resistance according to light intensity
 * Photovoltaic cells or solar cells which produce a voltage and supply an electric current when illuminated
 * Photodiodes which can operate in photovoltaic mode or photoconductive mode
 * Photomultiplier tubes containing a photocathode which emits electrons when illuminated, the electrons are then amplified by a chain of dynodes.
 * Phototubes containing a photocathode which emits electrons when illuminated and in general behaves as a photoresistor.
 * Phototransistors incorporating one of the above sensing methods
 * Optical detectors that are effectively thermometers, responding purely to the heating effect of the incoming radiation, such as pyroelectric detectors, Golay cells, thermocouples and thermistors, but the latter two are much less sensitive.
 * Cryogenic detectors are sufficiently sensitive to measure the energy of single x-ray, visible and near infra-red photons (Enss 2005).

Photoresistor

A photoresistor or LDR is an electronic component whose resistance decreases with increasing incident light intensity. It can also be referred to as a light-dependent resistor (LDR), photoconductor, or photocell.

A photoresistor is made of a high-resistance semiconductor. If light falling on the device is of high enough frequency, photons absorbed by the semiconductor give bound electrons enough energy to jump into the conduction band. The resulting free electron (and its hole partner) conduct electricity, thereby lowering resistance.

A photoelectric device can be either intrinsic or extrinsic. An intrinsic semiconductor has its own charge carriers and is not an efficient semiconductor, eg. silicon. In intrinsic devices, the only available electrons are in the valence band, and hence the photon must have enough energy to excite the electron across the entire bandgap. Extrinsic devices have impurities added, which have a ground state energy closer to the conduction band — since the electrons don't have as far to jump, lower energy photons (i.e. longer wavelengths and lower frequencies) are sufficient to trigger the device. If a sample of silicon has some of its atoms replaced by phosphorus atoms(impurities), there will be extra electrons available for conduction. This is an example of an extrinsic semiconductor.

Tape Head
A tape head is a type of transducer used in tape recorders to convert electrical signals to magnetic fluctuations and vice versa.

The electromagnetic arrangement of a tape head is generally similar for all types, though the physical design varies considerably depending on the application - for example video recorders use rotating heads which implement a helical scan, whereas most audio recorders have fixed heads. A head consists of a core of magnetic material arranged into a doughnut shape or toroid, into which a very narrow gap has been let (an air gap or other insulating material). This gap causes the magnetic flux to spill out of the material at that point, which is where the tape is made to run. The flux thus magnetises the tape at that point. A coil of wire wrapped around the core opposite the gap interfaces to the electrical side of the apparatus; thus either supplying a signal in the case of recording, or being fed to an amplifier in the case of playback. The basic head design is fully reversible - a variable magnetic field at the gap will induce an electric current in the coil, and an electric current in the coil will induce a magnetic field in the core and hence in the tape drawn across the gap.

Hall Effect Sensor

A Hall effect sensor is a transducer that varies its output voltage in response to changes in magnetic field. Hall sensors are used for proximity switching, positioning, speed detection, and current sensing applications.

In its simplest form, the sensor operates as an analogue transducer, directly returning a voltage. With a known magnetic field, its distance from the Hall plate can be determined. Using groups of sensors, the relative position of the magnet can be deduced.

Electricity carried through a conductor will produce a magnetic field that varies with current, and a Hall sensor can be used to measure the current without interrupting the circuit. Typically, the sensor is integrated with a wound core or permanent magnet that surrounds the conductor to be measured.

pH Probes
A pH meter is an electronic instrument used to measure the pH (acidity or basicity) of a liquid (though special probes are sometimes used to measure the pH of semi-solid substances). A typical pH meter consists of a special measuring probe (a glass electrode) connected to an electronic meter that measures and displays the pH reading.

Electro-Galvanic Fuel Cell

An electro-galvanic fuel cell is an electrical device used to measure the concentration of oxygen gas in scuba diving and medical equipment.

A chemical reaction occurs in the fuel cell when the potassium hydroxide in the cell comes into contact with oxygen. This creates an electric current between the lead anode and the gold-plated cathode through a load resistance. The voltage produced is proportional to the concentration of oxygen present.

Electroactive Polymers
Electroactive Polymers or EAPs are polymers whose shape is modified when a voltage is applied to them. They can be used as actuators or sensors. As actuators, they are characterized by the fact that they can undergo a large amount of deformation while sustaining large forces. Due to the similarities with biological tissues in terms of achievable stress and force, they are often called artificial muscles, and have the potential for application in the field of robotics, where large linear movement is often needed.

Galvanometer

A galvanometer is a type of ammeter; an instrument for detecting and measuring electric current. It is an analog electromechanical transducer that produces a rotary deflection, through a limited arc, in response to electric current flowing through its coil. The term has expanded to include uses of the same mechanism in recording, positioning, and servomechanism equipment.

MEMS
Microelectromechanical systems (MEMS) is the technology of the very small, and merges at the nano-scale into nanoelectromechanical systems (NEMS) and Nanotechnology. MEMS are also referred to as micromachines (in Japan), or Micro Systems Technology - MST (in Europe). MEMS are separate and distinct from the hypothetical vision of Molecular nanotechnology or Molecular Electronics. MEMS generally range in size from a micrometer (a millionth of a meter) to a millimeter (thousandth of a meter). At these size scales, the standard constructs of classical physics do not always hold true. Due to MEMS' large surface area to volume ratio, surface effects such as electrostatics and wetting dominate volume effects such as inertia or thermal mass. Finite element analysis is an important part of MEMS design. The sensor technology made significant progress due to MEMS. Complexity and performance of advanced MEMS based sensors are described by different MEMS sensor generations.

Rotary and Linear Motor

An electric motor uses electrical energy to produce mechanical energy. The reverse process, that of using mechanical energy to produce electrical energy, is accomplished by a generator or dynamo. Traction motors used on locomotives often perform both tasks if the locomotive is equipped with dynamic brakes. Electric motors are found in household appliances such as fans, refrigerators, washing machines, pool pumps, floor vacuums, and fan-forced ovens.

Vibration Powered Generator
Vibration powered generators are mechanisms for converting kinetic energy derived from ambient vibration to electrical energy. (See Transducer)

Potentiometer
A potentiometer is a variable tapped resistor that can be used as a voltage divider.

A form of potentiometer is used as an instrument to measure the potential (or voltage) in a circuit by tapping off a fraction of a known voltage from a resistive slide wire and comparing it with the unknown voltage by means of a galvanometer. The sliding tap of the potentiometer is adjusted and the galvanometer briefly connected to both the sliding tap and the unknown potential; the deflection of the galvanometer is observed and the sliding tap adjusted until the galvanometer no longer deflects. At that point the galvanometer is drawing no current from the unknown source, and the magnitude of voltage can be calculated from the position of the sliding contact. This null balance method is a fundamental technique of electrical metrology.

As an electrical component, potentiometer (or 'pot' for short) describes a three-terminal resistor with a sliding contact that forms an adjustable voltage divider. If all three terminals are used, it can act as a variable voltage divider. If only two terminals are used (one side and the wiper), it acts as a variable resistor or rheostat. Potentiometers are commonly used as controls for electrical devices such as a volume control of a radio. Potentiometers operated by a mechanism can be used as position transducers, for example, in a joystick.

Load Cell
A load cell is typically an electronic device (transducer) that is used to convert a force into an electrical signal. This conversion is indirect and happens in two stages. Through a mechanical arrangement, the force being sensed deforms a strain gauge. The strain gauge converts the deformation (strain) to electrical signals. Normally, a load cell consists of four strain gauges in a Wheatstone bridge configuration, but is also available with one or two strain gauges. The electrical signal output is normally in the order of a few millivolts and requires amplification by an instrumentation amplifier before it can be used. The output of the transducer is plugged into an algorithm to calculate the force applied to the transducer.

Accelerometer
An accelerometer is a device for measuring the total specific external force on the sensor. This is sometimes referred to as the acceleration but this is not strictly correct. A DC-coupled accelerometer sitting still on a table top has zero acceleration but will read a specific force equal in magnitude to the local acceleration due to earth's gravity at that location, which is nominally one g (earth's gravity varies slightly from location to location). This is because the table is imparting a reaction force on the accelerometer equal to the weight of the accelerometer. Since weight is mass multipled by the local gravitational accleration, the table must exert this much force on the accelerometer for it to stay still. When an acclerometer is falling a vacuum, there are no reaction forces on it. Thus when falling in a vacuum, the accelerometer will read zero. Accelerometers may be part of an Inertial Navigation System, used to detect and measure vibrations, or for measuring acceleration due to gravity (inclination). An accelerometer inherently measures its own motion (locomotion), in contrast to a device based on remote sensing.

Strain Gauge

A strain gauge (alternatively: strain gage) is a device used to measure deformation (strain) of an object. Invented by Edward E. Simmons and Arthur C. Ruge in 1938, the most common type of strain gauge consists of an insulating flexible backing which supports a metallic foil pattern. The gauge is attached to the object by a suitable adhesive, such as cyanoacrylate. As the object is deformed, the foil is deformed, causing its electrical resistance to change. This resistance change, usually measured using a Wheatstone bridge, is related to the strain by the quantity known as the gauge factor.

String Potentiometer
A string potentiometer is a transducer used to detect and measure linear position and velocity using a flexible cable and spring-loaded spool. Other common names include "string pot", "cable-extension transducer", "draw wire sensor", and "yo-yo sensor".

Geophone

The term geophone derives from the Greek word "geo" meaning "earth" and "phone" meaning "sound".

A geophone is a device which converts ground movement (displacement) into voltage, which may be recorded at a recording station. The deviation of this measured voltage from the base line is called the seismic response and is analysed for structure of the earth.

Gramophone Pick-Up
The phonograph, or gramophone, was the most common device for playing recorded sound from the 1870s through the 1980s.

Hydrophone

A hydrophone is a microphone designed to be used underwater for recording or listening to underwater sound. Most hydrophones are based on a piezoelectric transducer that generates electricity when subjected to a pressure change. Such piezoelectric materials, or transducers can convert a sound signal into an electrical signal since sound is a pressure wave in fluids. Some transducers can also serve as a projector (emitter), but not all have this capability, and may be destroyed if used in such a manner.

Loudspeaker

A loudspeaker, speaker, or speaker system is an electromechanical transducer that converts an electrical signal into sound. The term loudspeaker can refer to individual devices (otherwise known as drivers), or to complete systems consisting of an enclosure incorporating one or more drivers and additional electronics. Loudspeakers are the most variable elements in any audio system, and are responsible for marked audible differences between otherwise identical sound systems.

Microphone

A microphone, sometimes referred to as a mike or mic, is an acoustic to electric transducer or sensor that converts sound into an electrical signal.

Microphones are used in many applications such as telephones, tape recorders, hearing aids, motion picture production, live and recorded audio engineering, in radio and television broadcasting and in computers for recording voice, VoIP, and for non-acoustic purposes such as ultrasonic checking.

Piezoelectric Crystal

Piezoelectricity is the ability of some materials (notably crystals and certain ceramics) to generate an electric potential in response to applied mechanical stress. This may take the form of a separation of electric charge across the crystal lattice. If the material is not short-circuited, the applied charge induces a voltage across the material. The word is derived from the Greek piezein, which means to squeeze or press.

The piezoelectric effect is reversible in that materials exhibiting the direct piezoelectric effect (the production of electricity when stress is applied) also exhibit the converse piezoelectric effect (the production of stress and/or strain when an electric field is applied). For example, lead zirconate titanate crystals will exhibit a maximum shape change of about 0.1% of the original dimension.

Tactile Transducer
A tactile transducer or "bass shaker" is a device which is made on the principle that low bass frequencies can be felt as well as heard. A shaker transmits low-frequency vibrations into various surfaces so that they can be felt by people. This is called tactile sound. Tactile transducers may augment or in some cases substitute for a subwoofer.

A bass-shaker is meant to be firmly attached to some surface such as a seat, couch or floor. The shaker houses a small weight which is driven by a voice coil similar to those found in dynamic loudspeakers. The voice-coil is driven by a low-frequency audio signal from an amplifier; common shakers typically handle 25 to 50 watts of amplifier power. The voice coil exerts force on both the weight and the body of the shaker, with the latter forces being transmitted into the mounting surface.

Tactile transducers may be used in a home theater, a commercial movie theater, or for special effects in an arcade game, amusement park ride or other application.


text: User talk:Renegade legend

title: User talk:Renegade legend


text:
Category talk:Celtic languages

Does anybody know what language the iceni spoke and where I could find samples of it? Cheers
 * Hello, I am unfamiliar with this topic, but one way to find an answer could be to:
 * use internet search engine
 * find people interested in the topic who might know the answer (e.g. contributors to the Wikipedia article Iceni here or pages that refer to Iceni here - like this you can also find contributors here at Wikiversity on every page with the "history" tab or on the left bottom navigation with "What links here").
 * Hope, that might help, Erkan Yilmaz ( Wikiversity:Chat, wiki blog ) 22:53, 31 January 2008 (UTC)

title: Category talk:Celtic languages


text:
User talk:Aran~enwikiversity

Hi, ping. Below more info about Wikiversity, Erkan Yilmaz ( Wikiversity:Chat, wiki blog ) 22:54, 31 January 2008 (UTC)

Welcome
Hello Aran, and welcome to Wikiversity! If you need help, feel free to visit my talk page, or contact us and ask questions. After you leave a comment on a talk page, remember to sign and date; it helps everyone follow the threads of the discussion. The signature icon in the edit window makes it simple. To get started, you may
 * Take a guided tour and learn to edit;
 * Explore our learning projects;
 * Browse our portals, schools, and research activities;
 * Read and help develop our community policies；or
 * Chat with other Wikiversitans on #wikiversity-en.

And don't forget to explore Wikiversity with the links to your left. Be bold, and see you around Wikiversity! Erkan Yilmaz ( Wikiversity:Chat, wiki blog ) 22:54, 31 January 2008 (UTC)

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Hello,

The developer team at Wikimedia is making some changes to how accounts work, as part of our on-going efforts to provide new and better tools for our users like cross-wiki notifications. These changes will mean you have the same account name everywhere. This will let us give you new features that will help you edit and discuss better, and allow more flexible user permissions for tools. One of the side-effects of this is that user accounts will now have to be unique across all 900 Wikimedia wikis. See the announcement for more information.

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Yours, Keegan Peterzell Community Liaison, Wikimedia Foundation 23:28, 17 March 2015 (UTC)

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title: User talk:Aran~enwikiversity


text:
United States currency/$20 bill

Overview
The United States twenty-dollar bill ($20) is a denomination of United States currency. U.S. President Andrew Jackson is currently featured on the front side of the bill, while the White House is featured on the reverse side.

The $20 bill is sometimes also nicknamed a "double-sawbuck" because it is twice the value of a $10 bill, which is nicknamed a "sawbuck" due to the resemblance the Roman numeral for ten (X) bears to the legs of a sawbuck, although this usage is far less common today than it was in the early 20th century. It may also be referred to as a "yuppie food stamp", since the $20 bill is often the main currency dispensed by ATM's.

The $20 bill is the primary banknote (and in most cases, the only banknote, as it would increase costs to make ATM's compatible with other bills) dispensed by ATM's in the United States as well as the primary bill used for withdrawals or cashing checks. It is, therefore, the highest denomination note most commonly used by Americans on an everyday basis.

The Bureau of Engraving and Printing says the "average circulation life" of a $20 bill is two years before it is replaced due to wear. Approximately 22% of all notes printed today are $20 bills. Twenty-dollar bills are delivered by Federal Reserve Banks in violet straps.

Pre-Federal Reserve History

 * 1861: A demand note with Lady Liberty holding a sword and shield on the front, and an abstract design on the back. The back is printed in green.
 * 1862: A note that is very similar, the first $20 United States note. The back is different, with several small variations extant.
 * 1863: A gold certificate $20 note with an Eagle vignette on the face. The reverse has a $20 gold coin and various abstract elements. The back is orange.
 * 1865: A national bank note with "The Battle of Lexington" and "Columbia Leading a Procession" on either side of the face and obligation text conspicuously in the middle. The reverse features "The Baptism of Pocahontas" in black, and a green border.
 * 1869: A new United States note design with Alexander Hamilton on the left side of the front and Victory holding a shield and sword. The back design is green.
 * 1875: As above, except with a different reverse.
 * 1878: A silver certificate $20 note with a portrait of Stephen Decatur on the right side of the face. The back design is black.
 * 1882: A new gold certificate with a portrait of James Garfield on the right of the face. The back is orange and features an eagle.
 * 1882: A new national bank note. The front is similar, but the back is different and printed in brown.
 * 1886: A new silver certificate $20 note with Daniel Manning on the center of the face.
 * 1890: Twenty-dollar (Treasury) "Coin" Notes with John Marshall's image on the left of the face were issued. This type had a large white TWENTY while featuring an abstract design that occupied almost the entire note.
 * 1891: The reverse of the $20 Treasury Note was changed because of people thinking they were "busy" which would make it too easy to counterfeit. More open space was added into the design.
 * 1902: A new national bank note. The front design features Hugh McCulloch, and the back has a vignette of an allegorical America.
 * 1905: A new gold certificate $20 note with George Washington on the center of the face. The back design is orange.
 * 1918: A federal reserve bank note with Grover Cleveland on the front, and a back design similar to the 1914 Federal Reserve Note.

Federal Reserve History
Jackson first appeared on the twenty dollar bill in 1928. It is not clear the reason the bill was switched from Grover Cleveland to Andrew Jackson. According to the U.S. Treasury, "Treasury Department records do not reveal the reason that portraits of these particular statesmen were chosen in preference to those of other persons of equal importance and prominence." The placement of President Andrew Jackson on the $20 bill may be an historical irony; as president, Jackson vehemently opposed both the National Bank and paper money and made the goal of his administration the destruction of the National Bank. In his farewell address to the nation, he cautioned the public about paper money.

 * 1914: Began as a large-sized note with a portrait of Grover Cleveland on the face, and, on the back, a steam locomotive approaching from the left, and a steamship approaching from the right.
 * 1928: Switched to a small-sized note with a portrait of Andrew Jackson on the face and the south view of the White House on the reverse. The banknote is redeemable in gold or silver (at the bearer's discretion) at any Federal Reserve Bank.
 * 1934: The "redeemable in gold" clause was taken off the obverse of the $20 Federal Reserve Note due to the U.S. withdrawing from the gold standard.
 * 1943: A special emergency series, with brown serial numbers and "HAWAII" overprinted on both the front and the back, is issued. These notes are designed to circulate on the islands, and be deemed invalid in the event of a Japanese invasion.
 * 1948: The White House picture was updated to reflect renovations to the building itself as well as the passage of time. Most notably, the trees are larger.
 * 1950: The $20 FRN was changed. Most notably, the treasury seal, gray word TWENTY, the serial numbers, and the Federal Reserve Seal were made smaller, furthermore, the Federal Reserve seal had spikes added around it and the serial numbers were moved and changed in font.
 * 1963: The $20 bill recieved a makeover when WILL PAY TO THE BEARER ON DEMAND was removed and IN GOD WE TRUST was added on the reverse. The acts (authorizing the national motto, and taking U.S. currency off of silver backing) may have been the reason for these changes. Also, the obligation was changed to THIS NOTE IS LEGAL TENDER FOR ALL DEBTS, PUBLIC AND PRIVATE.
 * 1969: The $20 bill accepts the new treasury seal with wording in English instead of Latin.
 * 1977: A new type of serial-number press results in a slightly different font. The old presses are gradually retired, and old-style serial numbers appear as late as 1981 for this denomination.
 * 1990: Anti-counterfeiting features are added: microprinting around the portrait, and a plastic strip embedded in the paper.
 * September 24, 1998: Received a completely new appearance to further deter counterfeiting; the picture of the White House was changed to the north side view. A larger, off-center portrait of Jackson was used on front, and several anti-counterfeiting features were added, including color-shifting ink, microprinting, and a watermark. The plastic strip now reads "USA 20" and glows green under a black light.
 * October 9, 2003: Still another new appearance with light background shading in green and yellow, and no oval around Andrew Jackson's portrait (background images of eagles, etc. were also added to the front); the back is the same view of the White House, but without the oval around it.

Many tiny, faint "20"s are scattered on the back in yellow as a "EURion constellation" to prevent photocopying. The series date is 2004 and the signatures are Marin-Snow.

Related Small Sized Notes
The 1928 design was made both as a Federal Reserve Note and also a Gold Certificate. The design work is identical, but the obligations are different. Both are black on the front, and green on the back, but the Federal Reserve Note has green serial numbers, and the Gold Certificate has yellow. There is also a Series of 1929 National Bank Note and Federal Reserve Bank Note, which have the same back design and a similar front design.

Unmade Bills
Several unmade twenty-dollar bills are known. Most are similar in design to the ones described above, with the only difference being certain obligations. For example, a silver-certificate variety of the 1928 style Federal Reserve Note is known in the proof stage, with blue serial numbers. More interesting examples include a 1923 Federal Reserve Note, with Grover Cleveland as on the Series of 1914, but different border elements, an 1873 National Bank note whose design is unknown, and also an 1896 Silver Certificate.

title: United States currency/$20 bill


text:
United States currency/$50 bill

Overview
The United States fifty-dollar bill ($50) is a denomination of United States currency. U.S. President Ulysses S. Grant is currently featured on the obverse, while the U.S. Capitol is featured on the reverse. All $50 bills issued today are Federal Reserve Notes.

The Bureau of Engraving and Printing says the "average life" of a $50 bill in circulation is 55 months before it is replaced due to wear. Approximately 5% of all notes printed today are $50 bills. They are delivered by Federal Reserve Banks in brown straps.

A $50 bill is sometimes called a "Grant" based on the use of Ulysses S. Grant's portrait on the bill.

Large Size Note History
(≈ 7.4218 × 3.125 in ≅ 189 × 79 mm)

 * 1861: Three-year fifty dollar Interest Bearing Notes were issued that paid a cent of interest per day. These notes were not primarily designed to circulate, and were payable to the original purchaser of the dollar bill. The obverse of the note featured a Bald Eagle.
 * 1862: The first circulating $50 bill was issued.
 * 1863: Both one and two year Interest Bearing Notes were issued that paid 5% interest. The one-year Interest Bearing Notes featured a vignette of Alexander Hamilton to the left and an allegorical figure representing loyalty to the right. The two-year notes featured allegorical figures of loyalty, and justice. The vignette of Hamilton used for the 1-year IBNs was unlike the portrait currently used for the $10 bill.
 * 1864: Compound Interest Treasury Notes were issued that were intended to circulate for three years and paid 6% interest compounded semi-annually. The obverse is similar to the 1863 one-year Interest Bearing Note.
 * 1865: Three-year Interest Bearing Notes were issued again with a slightly different Bald Eagle and border design on the obverse.
 * 1869: A new $50 United States Note was issued with a portrait of Henry Clay on the right and an allegorical figure holding a laurel branch on the left of the obverse.
 * 1870: Fifty Dollar National Gold Bank Notes were issued specifically for payment in gold coin by 2 national gold banks. The obverse featured vignettes of George Washington crossing the Delaware River and at Valley Forge; the reverse featured a vignette of U.S. gold coins.
 * 1874: Another new $50 United States Note was issued with a portrait of Benjamin Franklin on the left and allegorical figure of Lady Liberty on the right of the obverse.
 * 1878: The first $50 Silver Certificate was issued with a portrait of Edward Everett. The reverse was printed in black ink, unlike any other government issued dollar bill made.
 * 1880: The 1878 Silver Certificate was slightly revised.
 * 1882: The first $50 Gold Certificate with a portrait of Silas Wright was issued. The reverse was printed in orange ink and featured a Bald Eagle perched atop an American flag.
 * 1891: The obverse of the $50 Silver Certificate was slightly revised and the reverse was completely changed.
 * 1891: Fifty-dollar Treasury or "Coin Notes" were issued and given for government purchases of silver bullion from the silver mining industry. The note featured a portrait of William H. Seward, the back had a large ornate design that occupied almost the entire note.
 * 1913: A new $50 Gold Certificate with a portrait of Ulysses Grant was issued. The style of the area below Grant's portrait was later used on small-sized notes.
 * 1914: The first $50 Federal Reserve Note was issued with a portrait of Ulysses S. Grant on the obverse and an allegorical figure of Panama between a merchant and battle ship on the reverse.
 * 1918: Federal Reserve Bank Notes (not to be confused with Federal Reserve Notes) were issued by the Federal Reserve Bank of St Louis. The obverse was similar to the 1914 Federal Reserve Notes, except for large wording in the middle of the bill and a portrait with no border on the left side of the bill. The note was an obligation of the St. Louis Federal Reserve Bank and could only be redeemed there.

Small Size Note History
 (6.14 × 2.61 in ≅ 156 × 66 mm)

 * 1929: Under series of 1928, all U.S. currency was changed to its current size. All variations of the $50 bill would carry the same portrait of Ulysses S. Grant, same border design on the obverse, and the same reverse with a vignette of the U.S. Capitol. The $50 bill was issued as a Federal Reserve Note with a green seal and serial numbers and as a Gold Certificate with a golden seal and serial numbers.
 * 1933: As an emergency response to the Great Depression, additional money was pumped into the American economy through Federal Reserve Bank Notes issued under series of 1929. This was the only small-sized $50 bill that had a different border design on the obverse. The serial numbers and seal on it were brown.
 * 1934: The redeemable in gold clause was removed from Federal Reserve Notes due to the U.S. withdrawing from the gold standard.
 * 1950: Many minor aspects on the obverse of the $50 Federal Reserve Note were changed. Most noticeably, the treasury seal, gray word FIFTY, and the Federal Reserve Seal were made smaller; also, the Federal Reserve seal had spikes added around it.
 * 1966: WILL PAY TO THE BEARER ON DEMAND was removed from the obverse and IN GOD WE TRUST was added to the reverse of the series 1963 A $50 Federal Reserve Note. Also, the obligation was changed to its current wording, THIS NOTE IS LEGAL TENDER FOR ALL DEBTS PUBLIC AND PRIVATE.
 * 1969: The $50 bill began using the new treasury seal with wording in English instead of Latin.
 * 1991: The first new-age anti-counterfeiting measures were introduced under series 1990 with microscopic printing around Grant's portrait and a plastic security strip on the left side of the bill.
 * October 27, 1997: Major design changes were implemented under series 1996 to further deter counterfeiters. Included were an enlarged and off-center portrait, an enlarged and updated view of the U.S. Capitol on the reverse, a security thread which glows yellow under ultraviolet light, a numeric 50 which shifts color from black to green when tilted, and a watermark of Grant. Also, for those with vision limitations, a large dark 50 was added to the bottom left corner of the reverse. The Federal Reserve seal was also changed to a unified Federal Reserve System seal and an additional prefix letter was added to the beginning of the bill's serial number.
 * September 28, 2004: A revised design was implemented with the first use of multiple colors since the 1905 $20 Gold Certificate. Around the new border-less portrait of Ulysses Grant appears a subtle, stylized blue and red background image of the American Flag. A small silver-blue star was also added to the lower right of Grant's portrait. All previous series 1996 security features were included, although the color-shifting numeric 50 now shifts from copper to green. The oval border and fine lines surrounding the U.S. Capitol on the reverse have been removed and replaced with sky and clouds. The new design also seems to have the "EURion constellation" on the back to prevent photocopying of the bill.
 * 2005: U.S. Representative John Kline introduced legislation to replace Grant's image on the $50 bill with an image of President Ronald Reagan.

km 16:35, 22 March 2008 (UTC)

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title: United States currency/$50 bill