Frédéric Christophoul

Patrice Baby

Jean-Claude Soula

Michel Rosero

José Burgos

Les ensembles fluviatiles néogènes du bassin subandin d'Équateur et implications dynamiques

The Neogene uvial systems of the Ecuadorian foreland basin and dynamic inferences.

fl

A sedimentological study of the Neogene continental in ll of the Subandean foreland basin of Ecuador led us to de ne an evolution of the uvial fi fi fl sy stem from an alluvial plain to an alluvial fan with an increasing slope in the same time as the drainage changed from mostly longitudinal to transverse. Combined with the data presently available on palaeotopography, exhumation, tectonic evolution and geomorphology, these results enable us to infer that, in contrast with the other Subandean foreland basins of Bolivia and Peru, the progradation of the Neogene alluvial fans proceeded by an overall expansion, associated with a relatively small tectonic shortening and not as a result of the development of successive thrust-related depocentres. This also indicates that the surrection of the Cordillera progressed in Ecuador throughout the Neogene.

uvial sedimentology / alluvial fans / Subandean basin / Neogene / Ecuador fl

Résumé -Une étude sédimentologique du remplissage continental néogène du bassin subandin d 'Équateur nous a permis de mettre en évidence le fait que le système uviatile ait fl évolué d'une plaine alluviale à un éventail alluvial à pente de plus en plus forte, en même temps que le drainage passait de longitudinal à transverse. Combinés avec les données actuellement disponibles sur la paléotopographie, l'exhumation, l'évolution tectonique et la géomorphologie, ces résultats montrent qu'au contraire des bassins d'avant-pays subandins de Bolivie et d u Pérou, la progradation des cônes alluviaux néogènes s'accomplissait par une expansion d'ensemble dans u n contexte de raccourcissement faible, plutôt qu'à la suite du développement de dépocentres successifs, contrôlés par des chevauchements. Ils montrent aussi que la surrection de la cordillère s'est poursuivie durant tout le Néogène.

Introduction

The sedimentary signature of structural events in foreland basins has been widely described for the past ten years [START_REF] Burbank | Causes of t he recent Hi m al ayan upl i ft deduced from deposi t ed pat t erns in t he Ganges basin[END_REF][START_REF] Delfaud | Reconstitution des ét apes de la surrect ion des Andes d'Équat eur à part ir de l 'i nterprétati on des mi néraux argi l eux cont enus dans les bassi ns i nt ramont agneux (Mi ocène à Act uel )[END_REF][START_REF] Jordan | Retroarc forel and and related basi ns[END_REF], particularly in the Alpine Molasse basin [START_REF] Homewood | Dynamics of the Molasse basin of western Switzerland[END_REF][START_REF] Schl Unegger | Crust al t hi ckeni ng and crustal ext ensi on as cont rol s on the evoluti on of t he drainage network of t he cent ral Swiss Alps bet ween 30 Ma and t he present : const rai nt s from t he st rat i graphy of t he Nort h Al pi ne Forel and Basi n and t he st ruct ural evol ut i on of t he Al ps[END_REF][START_REF]Si ncl ai r,Tect onost rat i graphic model for under l led fi peri pheral forel and basi ns: an Al pine perspect ive[END_REF]. The authors agree that the uvial systems at rst de-fl fi velo ped with an axial drainage, which changed to a transverse drain age [START_REF] Burbank | Causes of t he recent Hi m al ayan upl i ft deduced from deposi t ed pat t erns in t he Ganges basin[END_REF][START_REF] Homewood | Dynamics of the Molasse basin of western Switzerland[END_REF][START_REF] Jordan | Retroarc forel and and related basi ns[END_REF][START_REF]Si ncl ai r,Tect onost rat i graphic model for under l led fi peri pheral forel and basi ns: an Al pine perspect ive[END_REF] as a result of either thrust sheets stacking near the inner margin of the basin or postcompressional erosional unloading [5,[START_REF] Burbank | Causes of t he recent Hi m al ayan upl i ft deduced from deposi t ed pat t erns in t he Ganges basin[END_REF][START_REF] Hel Ler | Twophase strati graphic response model of foreland-basi n sequences[END_REF][START_REF] Jordan | Retroarc forel and and related basi ns[END_REF][START_REF] Si | Vert i cal versus hori zont al mot i ons i n t he Al pi ne orogenic wedge: st rat i graphi c response in t he forel and basi n[END_REF].

1029

Comptes Rendus Geoscience, 334 (2002), 1029-1037

The Neogene fluvial systems of the Ecuadorian Foreland basin and dynamic inferences

In th is paper, we study the Ecuadorian Subandean foreland basin (Oriente Basin ) [START_REF] Baby | St yle and ti mi ng of deformat i on in t he Ori ent e of Ecuador[END_REF][START_REF] Dock | Geología del Ecuador[END_REF][START_REF] Hungerbühl Er | Neogene st rat i graphy and Andean geodynam i cs of sout her Ecuador[END_REF][START_REF] Tschopp | Oil expl orat i ons in t he Ori ent e ofEcuador[END_REF] (Fig. 1), wh ich was lled by uvial deposits since the end of the Oligocene fi fl by means of lithofacies and architectural element analyses using Miall's nomenclature [START_REF] Mial L | The Geology of Fluvi al Deposi t s:Sedi m ent ary Faci es[END_REF] and p alaeocurrent measu remen ts. Th e r esults of th is stu dy will be th en comb in ed with the d ata available on palaeoaltitudes, exhumation, tectonic evo lution, and geomorphology, and compared with th e o th er Subandean basins of th e central Andes.

S tratigrap hy

Th e stratig rap hy of th e d ep o sits of th e Ecu ad orian Oriente basin is rather well known , although biostratigraphic markers are scarce [START_REF] Dock | Geología del Ecuador[END_REF][START_REF] Tschopp | Oil expl orat i ons in t he Ori ent e ofEcuador[END_REF]. Four formations have been recognised: th e Chalcana Formatio n of Upper Oligocene th rough Miocene age [START_REF] Baby | St yle and ti mi ng of deformat i on in t he Ori ent e of Ecuador[END_REF][START_REF] Decelles | Foreland basin systems[END_REF], the Arajuno Formatio n of Upper Mio cene age [START_REF] Baby | St yle and ti mi ng of deformat i on in t he Ori ent e of Ecuador[END_REF][START_REF] Dock | Geología del Ecuador[END_REF][START_REF] Rosero | Anal í sis t ectono-sedimentaria y i nterpretaci ón geodí nami ca de l as secuenci as Ol i gocenas y Neogenas de la Cuenca Oriente Ecuat oriana, thèse[END_REF][START_REF] Tschopp | Oil expl orat i ons in t he Ori ent e ofEcuador[END_REF], the Ch ambira Fo rmatio n of Upper Mio cene through Plio cene age, and th e Mesa/Mera Fo rmations [START_REF] Dock | Geología del Ecuador[END_REF][START_REF] Tschopp | Oil expl orat i ons in t he Ori ent e ofEcuador[END_REF] of Pleistocene age [START_REF] Baby | Mapa Geologico de la Cuenca Ori ente[END_REF][START_REF] Li | Forest changes in t he Amazon Basin duri ng t he l ast gl aci al maximum[END_REF][START_REF] Rosero | Anal í sis t ectono-sedimentaria y i nterpretaci ón geodí nami ca de l as secuenci as Ol i gocenas y Neogenas de la Cuenca Oriente Ecuat oriana, thèse[END_REF][START_REF] Tschopp | Oil expl orat i ons in t he Ori ent e ofEcuador[END_REF]. The Cu raray Formation [START_REF] Dock | Geología del Ecuador[END_REF][START_REF] Tschopp | Oil expl orat i ons in t he Ori ent e ofEcuador[END_REF] is the easterly equivalent of the Arajuno [START_REF] Baby | Mapa Geologico de la Cuenca Ori ente[END_REF] and Chambira [START_REF] Rosero | Anal í sis t ectono-sedimentaria y i nterpretaci ón geodí nami ca de l as secuenci as Ol i gocenas y Neogenas de la Cuenca Oriente Ecuat oriana, thèse[END_REF] Formations. All th ese formations were deposited with in a single d epocentre, east of the Pastaza depression (Fig . 1). Maximum subsid ence rates for the Ch alcana, Arajuno and Chambira Formations are 86, 162, and 336 mMa -1 , respectively.

S ed imentology of u vial d ep osits fl

Chalcana Fo rmation

The Chalcana Formatio n (Fig. 2a) mostly crops out along th e Río Aguarico. The main part of th e formation is made of reddish sh ales intercalated with rare ne-grained fi and thin sandstonebeds. No sedim entary structures have been observed. The lithofacies [START_REF] Mial L | The Geology of Fluvi al Deposi t s:Sedi m ent ary Faci es[END_REF]are Fr and P in the sh ales and Sh and St in the sandstones. These lithofacies can be grouped in architectural elements FF ( oodplain fl nes) and CR (crevasse channels).Near-symmetricalchan-fi nels with lithofacies St are occasionally observed. Such an asso ciation may characterise the inter-distribu tary (interriver) part of a low-sinuosity sandy meanderingor anasto mo sed r iver sy stem [2 2], similar to th e presen t-d ay Amazonian plain [START_REF] Rosero | Anal í sis t ectono-sedimentaria y i nterpretaci ón geodí nami ca de l as secuenci as Ol i gocenas y Neogenas de la Cuenca Oriente Ecuat oriana, thèse[END_REF]. Palaeocurrent measurements (Fig. 3A) sh ow two main d irections NW-SE and north-south, wh ich may be interpreted as transverse river systems perpendicular to the Andean protocordillera, debouching into river sy stems p arallel to it.

Arajuno Fo rmation

The Arajuno Fo rmatio n (Fig. 2b) mostly crops out along th e Río Nap o. It is essen tially made of n e-to co arse-fi grained sandstones and conglomerates(lithofacies St, Sh, Gh, Gp an d Gs) in terb ed d ed with m in or siltsto n e b eds (Fl, P). These lithofacies can be groupedinto architectural elements LA ( lateral accretion) and DA (downstream accretion), which indicates migrating sandy bars. These lithofacies and architectural elements can be interpreted as the d istal p art of gravel-wandering rivers. Palaeocurrent measurements (Fig. 3B) indicate two drainage directions: WNW-ESE to west-east and north-south, as in the Chalcana Formation. The longitudinal drain age axis, however, appears now to have been sh ifted eastward.

Chambira Formation

The Chambira Formatio n crops out along th e Pastaza depression andthe Río Napo (Fig. 2c). Most of th e formatio n is made up of quartz pebbles-bearin g conglomerates included in a quartz-richargillaceousmatrix [START_REF] Tschopp | Oil expl orat i ons in t he Ori ent e ofEcuador[END_REF]. The basal part contains lithofacies Gt and Gms. The upper part is composed of conglomerates gradin g u p to sandsto n es and siltsto n es ( lith o facies Gt, Gh, St, Sr an d Fr ). These assemblages can be in terpreted as the expression of high-energy gravel-braid ed rivers with frequent mud ows fl [START_REF] Mial L | The Geology of Fluvi al Deposi t s:Sedi m ent ary Faci es[END_REF]. Palaeocurrents (Fig. 3C) indicate main ow direc-fl tions, ranging from SW-NE to east-west, i.e., tr ansverse drainage.

Cura ray Formation

The Curaray Fo rmation appears as the easterly equivalent of both the Arajuno and Chambira formations [START_REF] Baby | Mapa Geologico de la Cuenca Ori ente[END_REF]. Th is fo rmation is m ade up of san dy to silty tid alites ( lith ofacies St), co n tain in g cro co d ilian s an d m arin e tu r tles [START_REF] Dock | Geología del Ecuador[END_REF], which indicates an estuarine env ironment. As commonly observed in estuarine environments, no marked p referred palaeo ows are observed. fl

Mesa/Mera Formation

The Mesa/Mera Fo rmation crops out along th e Río Pastaza and Aguarico (Fig. 2d). Near the apex of the present-day Pastaza fan, this formationcomprises coarse conglomerateswith lithofacies Gmm to Gmg (debris ow fl deposits), wh ere volcanic pebbles( > 85%), in cluded in a tuffaceous matrix , are predominant. Boulders of granite and gneiss( > 1 m) are also observed [START_REF] Tschopp | Oil expl orat i ons in t he Ori ent e ofEcuador[END_REF]. Eastward, the conglomerates are less coarse and form stacked sh allow sy mmetrical ch an n els with lith o facies Gt ( Fig . 2d ). Th ese deposits are interpreted as representing gravel-braided r ivers lack in g mu d ow depo sits in th eir d istal p art, sim ilar fl to the present Río Pastaza. Palaeocurrent data (Fig. 3D) indicate an east-west mean ow direction in the west, fl d iverg in g to SW-NE an d NW-SE in th e east. To g ether, these d ata indicate a typical alluvial fan drainage system similar to that observed today.

D iscu ssion and con clu sions

Overall, th e sedimento logical study of th e uvial fl dep o sits con stitu tin g th e Neo g en e in ll fi of the Ecu ad orian Subandean Basin reveals that the uvial fl sy stem evolved from an alluvial plain (Ch alcana Formation) to an alluvial fan (Formations Ch ambira and Mesa/Mera), in the same tim e as the drainage changed from mostly longitudinal to transverse, i.e., fromunder lled to over lled fi fi se n su Jordan [START_REF] Jordan | Retroarc forel and and related basi ns[END_REF]. When considering that climate remained fairly wet du r in g th is p erio d [1 5], th e in crease in stream power recorded by this evolution can be ascribed to an increase in slope as, for ex ample, in the Alpine molasses [START_REF] Homewood | Dynamics of the Molasse basin of western Switzerland[END_REF]. This agrees with Delfaud et al. [START_REF] Delfaud | Reconstitution des ét apes de la surrect ion des Andes d'Équat eur à part ir de l 'i nterprétati on des mi néraux argi l eux cont enus dans les bassi ns i nt ramont agneux (Mi ocène à Act uel )[END_REF], who demonstrated that the Co rdillera was less than 1000 m h igh at the en d of the Oligocene, and with Sp ikings et al. [2 8 ], wh o observed h ig h exhumatio n rates in the Early and Late Miocene, wh ich show that the ratio uplift/erosion was positive (p re-stead y state). A similar in crease in stream p ower with tim e h as been recognised in the Neogene Subandean basin of Boliv ia [START_REF] Hort On | DeCel l es, Modern and anci ent uvi al fl megafans in t he forel and basi n syst em of t he Cent ral Andes, Sout hern Bolivi a: implications for drainage net work evolution of fol d-thrust belt s[END_REF]. However, in contrast with the Subandean basins of Bolivia, Peru, and Co lo mbia, which formed several d ep o cen tres prog r essively in teg r ated in to th e orogenic wedge durin g the eastward p ropagation of the th rust system, the deposition of the Neogene in Ecuador occurred in a single depocentre, with a moderate subsidence rate, at least un til th e Late Pleisto cen e. Th e presen t-d ay up lift of th e Ecuadorian Subandean Basin evidenced by th e strong in cision of th e pre-Holocene formations studied herein by the present-day rivers [START_REF] Baby | Mapa Geologico de la Cuenca Ori ente[END_REF][START_REF] Räsänen | Recent and anci ent uvi al fl deposi t ionalsyst ems in t he Amazon forel and basi n, Peru[END_REF] indicates that the uplift seems to exaggerate today, causing th e Subandean zone to pass from the domain of long-term deposition to that of predomin ant incision, wh ile the Subandean basins of Bo livia, Peru and Colombia continue to accumulatesediments. Again, th e overall shortening of th e Subandean zone was considerably smaller in Ecuador [START_REF] Christophoul | Stratigraphic responses to a maj or t ect oni c event in a Foreland Basin: t he Ecuadorian Ori ente Basi n from Eocene to Oli gocene t i m es[END_REF][START_REF] Hungerbühl Er | Neogene st rat i graphy and Andean geodynam i cs of sout her Ecuador[END_REF] th an in Bolivia and Peru [START_REF] Gi L Rodri Guez | St ruct ure et cont rôle paléogéographique de la zone subandine péruvi enne[END_REF][START_REF]Transfert s sédi m ent ai res Mi o-Pl i ocène et évol ut i on géodynami que dans l es bassi ns Hual l aga et M arañon (Pérou, haut bassi n de l 'Amazone)[END_REF] and Colombia [START_REF] Cooper | Basi n devel opment and t ect oni c hi st ory of the Llanos basin, Eastern Cordillera and Middle Magdalena Valley, Col ombi a[END_REF], and no ev idences of the growth of th e Subandean front are recorded in the d eposits. The evo lu tio n ofthe drainagepattern and the eastward progradatio n o f the alluvial fans revealed by th e present study [START_REF] Baby | The Hi gh Amazoni an Basi n : t ect oni c cont rol and m ass bal ance[END_REF] thus appears to h ave been a result of an expansion of the fans unrelated to either a forward migration of th e o rogenic front or an in crease in tectonic shortenin g in the inner margin of the basin, as commonly adm itted [1 6, 20 , 27 ]. An o th er orig in , su ch as ero sio n al unloading [START_REF] Burbank | Causes of t he recent Hi m al ayan upl i ft deduced from deposi t ed pat t erns in t he Ganges basin[END_REF] or a d eep-seated process related to the special situation of Ecuador, is thus to be researched(study in progress).

Introduction

L a signature sédimentaire des phénomènes structuraux dans les bassins d'avant-pays a été largem ent décrite au cours de ces dernières années [START_REF] Burbank | Causes of t he recent Hi m al ayan upl i ft deduced from deposi t ed pat t erns in t he Ganges basin[END_REF][START_REF] Delfaud | Reconstitution des ét apes de la surrect ion des Andes d'Équat eur à part ir de l 'i nterprétati on des mi néraux argi l eux cont enus dans les bassi ns i nt ramont agneux (Mi ocène à Act uel )[END_REF][START_REF]Transfert s sédi m ent ai res Mi o-Pl i ocène et évol ut i on géodynami que dans l es bassi ns Hual l aga et M arañon (Pérou, haut bassi n de l 'Amazone)[END_REF][START_REF] Jordan | Retroarc forel and and related basi ns[END_REF][START_REF] Schl Unegger | Crust al t hi ckeni ng and crustal ext ensi on as cont rol s on the evoluti on of t he drainage network of t he cent ral Swiss Alps bet ween 30 Ma and t he present : const rai nt s from t he st rat i graphy of t he Nort h Al pi ne Forel and Basi n and t he st ruct ural evol ut i on of t he Al ps[END_REF][START_REF]Si ncl ai r,Tect onost rat i graphic model for under l led fi peri pheral forel and basi ns: an Al pine perspect ive[END_REF]. L es diff érents auteurs s'entendentsur le fait que, dans l'histoire de leur m ise en place, les systèm es uviatiles se développent dans un premier temps avec fl un drainage parallèle à la chaîne ( drainage axial) [START_REF] Homewood | Dynamics of the Molasse basin of western Switzerland[END_REF][START_REF]Si ncl ai r,Tect onost rat i graphic model for under l led fi peri pheral forel and basi ns: an Al pine perspect ive[END_REF], ou stade sous-alim enté sensu Jordan [START_REF] Jordan | Retroarc forel and and related basi ns[END_REF], pour ensuite évoluer vers un systèm e à drainage perpendiculaire à l a chaîne ( drainage transve rsal pour B urbank [START_REF] Burbank | Causes of t he recent Hi m al ayan upl i ft deduced from deposi t ed pat t erns in t he Ganges basin[END_REF], stade suralim enté pour Jordan [START_REF] Jordan | Retroarc forel and and related basi ns[END_REF]). Le s phénom ènes fréquem m ent invoqués pour expliquer ce type d'évolution sont : (1) une exion continentale [5], sui-fl vie d'un sur-rem plissage, résultant d'un em pilement de structures chevauchantes progressive m e nt érodées en bord de bassin [START_REF] Si | Vert i cal versus hori zont al mot i ons i n t he Al pi ne orogenic wedge: st rat i graphi c response in t he forel and basi n[END_REF], im pliquant l'approfondissem ent du bassin et éventuellem ent la m igration du dépocentre vers l'intérieur de l'orogène [START_REF] Jordan | Retroarc forel and and related basi ns[END_REF] ou vers le craton [START_REF] Decelles | Foreland basin systems[END_REF] ; (2) une surrection générale de l'orogène, due à l'érosion post-tectonique des structures chevauchantes [START_REF] Burbank | Causes of t he recent Hi m al ayan upl i ft deduced from deposi t ed pat t erns in t he Ganges basin[END_REF], avec la création d'un foresag en position distale [START_REF] Hel Ler | Twophase strati graphic response model of foreland-basi n sequences[END_REF].

L e rétro-bassin d'avant-pays des Andes équatoriennes (bassin O riente [START_REF] Dock | Geología del Ecuador[END_REF][START_REF] Hungerbühl Er | Neogene st rat i graphy and Andean geodynam i cs of sout her Ecuador[END_REF][START_REF] Tschopp | Oil expl orat i ons in t he Ori ent e ofEcuador[END_REF], Fig. 1) est le siège d'une sédimentation uviatile depuis la n de l'O li-fl fi gocène [START_REF] Dock | Geología del Ecuador[END_REF][START_REF]Si ncl ai r,Tect onost rat i graphic model for under l led fi peri pheral forel and basi ns: an Al pine perspect ive[END_REF] et m ontre à l'heure actuelle un éventail alluvial. L es dépôts néogènes qui rem plissent ce bassin af eurent essentiellement dans la zone subandine fl et présentent de bonnes conditions de préserva tion, du fait de la faible déform ation qui affecte le bassin O riente [START_REF] Baby | St yle and ti mi ng of deformat i on in t he Ori ent e of Ecuador[END_REF][START_REF] Christophoul | Stratigraphic responses to a maj or t ect oni c event in a Foreland Basin: t he Ecuadorian Ori ente Basi n from Eocene to Oli gocene t i m es[END_REF].

L' objet de cet article est de caractériser les paléoréseaux uviatiles d'un point de vue sédim entologie fl defaciès [START_REF] Mial L | The Geology of Fluvi al Deposi t s:Sedi m ent ary Faci es[END_REF], paléocourants et géom étrie des corps sédim entaires, pour proposer un modèle d'évolution en relation avec les données structurales, sédim entaires et de paléoaltitudes connues, tant dansles bassins O riente et adjacents [START_REF] Delfaud | Reconstitution des ét apes de la surrect ion des Andes d'Équat eur à part ir de l 'i nterprétati on des mi néraux argi l eux cont enus dans les bassi ns i nt ramont agneux (Mi ocène à Act uel )[END_REF][START_REF] Gi L Rodri Guez | St ruct ure et cont rôle paléogéographique de la zone subandine péruvi enne[END_REF][START_REF]Transfert s sédi m ent ai res Mi o-Pl i ocène et évol ut i on géodynami que dans l es bassi ns Hual l aga et M arañon (Pérou, haut bassi n de l 'Amazone)[END_REF][START_REF] Hoorn | Fluvial paleoenvironments in the intracratonic Amazonas Basi n (Earl y Mi ocene-Earl y Mi ddl e Mi ocene, Col ombi a)[END_REF][START_REF] Hoorn | Andean Tect onics as a cause for changing drainage patt erns in Mi ocene Northern Sout h Ameri ca[END_REF] que dans la cordillère orientale des A ndes équatoriennes [START_REF] Gi L Rodri Guez | St ruct ure et cont rôle paléogéographique de la zone subandine péruvi enne[END_REF][START_REF] Spi Kings | Lowtemperat ure t hermochronology of the northern Cordillera Real , Ecuador: tectonic insights for zircon and apatite ssion track analysis[END_REF].

Stratigraphie

L a stratigraphie du Néogène du bassin O riente est aujourd'hui bien décrite. Le découpage stratigraphique retenu est en grande partie repris de ceux de Tschopp [START_REF] Tschopp | Oil expl orat i ons in t he Ori ent e ofEcuador[END_REF] et de B aldock [START_REF] Dock | Geología del Ecuador[END_REF], précisé par l'IRD-P etroproducción [START_REF] Baby | Mapa Geologico de la Cuenca Ori ente[END_REF]. Il com prend quatre formations continentales distinctes, séparées par des discordances angulaires (F ig. 1). L a form ation C halcana est d'âge O ligocène supérieur à M iocène [START_REF] Baby | The Hi gh Amazoni an Basi n : t ect oni c cont rol and m ass bal ance[END_REF][START_REF] Decelles | Foreland basin systems[END_REF]. La form ation A rajuno est d'âge M iocène supérieur [START_REF] Baby | The Hi gh Amazoni an Basi n : t ect oni c cont rol and m ass bal ance[END_REF][START_REF] Dock | Geología del Ecuador[END_REF][START_REF] Rosero | Anal í sis t ectono-sedimentaria y i nterpretaci ón geodí nami ca de l as secuenci as Ol i gocenas y Neogenas de la Cuenca Oriente Ecuat oriana, thèse[END_REF][START_REF] Tschopp | Oil expl orat i ons in t he Ori ent e ofEcuador[END_REF]. La form ation C hambira est d'âge M iocène supérieur à P liocène. La form ation Mesa/Mera est d'âge P leistocène supérieur [START_REF] Li | Forest changes in t he Amazon Basin duri ng t he l ast gl aci al maximum[END_REF]. L a form ation C uraray [START_REF] Dock | Geología del Ecuador[END_REF][START_REF] Tschopp | Oil expl orat i ons in t he Ori ent e ofEcuador[END_REF] est l'équivalent latéral m arin des form ations A rajuno [START_REF] Baby | Mapa Geologico de la Cuenca Ori ente[END_REF] et C ham bira [START_REF] Rosero | Anal í sis t ectono-sedimentaria y i nterpretaci ón geodí nami ca de l as secuenci as Ol i gocenas y Neogenas de la Cuenca Oriente Ecuat oriana, thèse[END_REF]. ; i sopaques d'après Rosero [START_REF] Rosero | Anal í sis t ectono-sedimentaria y i nterpretaci ón geodí nami ca de l as secuenci as Ol i gocenas y Neogenas de la Cuenca Oriente Ecuat oriana, thèse[END_REF]). À droi t e, colonne st rat i graphi que des format i ons ét udi ées.

Sédimentologie des ensembles uviatiles fl

Fi g. 1. Top left, location map of the studi ed area. Bot t om left, simpli ed tect oni c map of fi t he Subandean basi n of Ecuador (modi ed fi from Baby et al. [START_REF] Baby | St yle and ti mi ng of deformat i on in t he Ori ent e of Ecuador[END_REF]; isopachs aft er Rosero [START_REF] Rosero | Anal í sis t ectono-sedimentaria y i nterpretaci ón geodí nami ca de l as secuenci as Ol i gocenas y Neogenas de la Cuenca Oriente Ecuat oriana, thèse[END_REF]). Ri ght, st rat i graphiccol umn of t he st udi ed format i ons.

de la « dépression Pastaza » (Figs. 1 et 3). Les vitesses de subsidence m axim ales pour les formations C halcana, A rajuno et C hambira sont respectivem ent de 86, 162 et 336 mMa -1 . L es form ations seront décrites selon la nom enclature de Miall [START_REF] Mial L | The Geology of Fluvi al Deposi t s:Sedi m ent ary Faci es[END_REF].

1. Formation Ch alcana

L a formation C halcana af eure essentiellement fl le long du río A guarico (F ig. 2a). L a granulom étrie est généralem ent très ne. fi L'essentiel de la coupe est constitué d'argilites rouges à racines (lithofaciès F r), qui présentent de fréquentes m arm orisations, caractérisant des paléosols (lithofaciès P). Les niveaux de grès présents le long de la coupe sont en général ns fi à m oyens et très rarem ent grossiers. L es niveaux les plus ns ne présentent pas de structure sédim entaire. fi L es niveaux de grès moyens montrent le plus souvent une lamination plane parallèle (S h) ou des strati ca-fi tions obliques en auge (S t). On peut noter, dans les argilites, des niveaux de grès ns disposés en lentilles fi plan convexes (chenaux de crevasses). Ces lithofaciès peuvent être groupés en élém ents architecturaux de type FF (épandage de crevasse, plaine d'inondation) et C R (chenaux de crevasse). L es dépôts de la form ation C halcana sont ainsi dom inés par des corps sédim entaires, correspondant à des assem blages de plaine d'inondation. Q uelques chenaux sym étriques peu profonds à rem plissage n (C H ) s'intercalent fi dans ces dépôts de plaine d'inondation. C e type d'assem blage suggère des cours d'eau de type m éandriform e sableux ou anastom osé [START_REF] Mial L | The Geology of Fluvi al Deposi t s:Sedi m ent ary Faci es[END_REF], à fa ible sinuosité, c om me l'indique la présence de chenaux sym étriques. L' ensem ble semble montrer une plaine alluviale semblable à la plaine am azonienne actuelle [START_REF] Räsänen | Recent and anci ent uvi al fl deposi t ionalsyst ems in t he Amazon forel and basi n, Peru[END_REF]. L'analyse des paléocourants en plusieurs points (F i g. 3A ) de la zone subandine m ontre deux m axim a: NW-S E dans la zone subandine etnord-sud auvoisinage dufront subandin. Ce s directions traduisent un drainage perpendiculaire à la proto-cordillère [START_REF] Christophoul | Stratigraphic responses to a maj or t ect oni c event in a Foreland Basin: t he Ecuadorian Ori ente Basi n from Eocene to Oli gocene t i m es[END_REF] se déversant, vers l'est, dans un axe de drainage parallèle à la chaîne (F ig. 3A).

2. Formation A raju no

L a formation Arajuno af eure surtout fl sur le río N apo et ses af uents (F ig. fl 2b). La coupe est pour l'essentiel com posée de grès ns à grossiers et fi de conglom érats. D es argilites rouges, intercalées, sont moins fréquentes que dans la formation C halcana. L es corps conglom ératiques présentent des lithofaciès de type Gh (grains jointifs à litage horizontal), m ais plus souvent Gt et Gp (strati cations obliques en auge et fi planes), et constituent des rem plissages de chenaux (type C H ) sym étriques et peu profonds. L a com position de ces conglomérats m ontre la prédom inance des galets dequartz( > 90% ). L es grès sont égalem ent quartzeux et en général grossiers. Le s lithofaciès les plus fréquents sont St et Sh et se trouvent au sein de barres sableuses m igrant latéralem ent et longitudinalement (L A et DA ). L es argilites complétant la coupe sont m assives (F sm ), généralem ent rouges à lie de vin, montrant peu d'indices de pédogenèse. L' ensem ble peut être interprété com me la signature d'un réseau de cours d'eau diva gant de type G ravel Wandering [START_REF] Mial L | The Geology of Fluvi al Deposi t s:Sedi m ent ary Faci es[END_REF], dans la m esure où la sym étrie des chenaux semble peu com patible avec un type méandriform e et où les élém ents caractéristiques de m igration d'intrados de m éandres sont absents. La forte proportion de grès laisse supposer qu'il s'agit de lapartie distale d'un systèm e uviatile de ce type, qui présente fl classiquem ent des faciès plus grossiers. Le système de la form ation A rajuno peut ainsi représenter un réseau hydrographique présentant une pente plus forte que celui de la form ation C halcana et inclus dans un cône alluvial.

L es données de paléocourants montrent une direction de drainage transverse (ouest-est ; N W-SEà ouest-est) et localem ent longitudinale (tributaires la-téraux?) à l'ouest et longitudinale (nord-sud) à l'est (F ig.3B ).C e dispositif est com parable à celui de la form ation C halcana, mais l'axe de drainage, parallèle à la chaîne, se déplace vers l'est.

Form ation Ch am b ira

L a formation C hambira (F ig.2c) af eure dans la fl zone subandine, au niveau et au nord de la dépression Pastaza et le long du río Napo (F ig. 1). La strati cation est fi renversée sous le chevauchem ent bordier du granite jurassique d'Abitagua; son pendage devient norm al et de plus en plus faible vers l'est et le haut, montrant ainsi une disposition en discordance progressive. L es conglom érats contiennent des clastes de quartz, d'origine lonienne à plus de 90%, fi dans une matrice argilo-gréseuse très riche en quartz. Les clastes de granite sont absents, ce qui traduit l'im portante altération du granite d'A bitagua. La base de la coupe est faite de bancs de conglom érats m assifs. L es lithofaciès sont essentiellem ent de type Gt. Certains niveaux se présentent sous la form e de galets em ballés dans une m atrice argileuse (G m g), indiquant des coulées de débris. Le r este de la coupe est constitué d'une succession de séquences de com blem ent de chenaux, incluant des faciès Gt, Gh, St et Sr, ave c parfois, en som m et, de séquences de lithofaciès F r. Ces agencements caractérisent un système uviatile à haute éner-fl gie, avec de fréquentes coulées boueuses. L e fa ible déve loppem entdes faciès de plaine d'inondation indique aussi un systèm e à fort ux sédim entaire régu-fl lier. Ces associations de lithofaciès peuvent être interprétées com me caractéristiques de cours d'eau en tresses de type Gravel-B ed B raided [START_REF] Mial L | The Geology of Fluvi al Deposi t s:Sedi m ent ary Faci es[END_REF]. Les paléocourants (F ig. 3C) indiquent une direction S W-NEà ouest-est.

4. Form ation C u raray

L a Formation C uraray est l'équivalent latéral, vers l'est, des form ations C ham bira et A rajuno [START_REF] Baby | The Hi gh Amazoni an Basi n : t ect oni c cont rol and m ass bal ance[END_REF][START_REF] Rosero | Anal í sis t ectono-sedimentaria y i nterpretaci ón geodí nami ca de l as secuenci as Ol i gocenas y Neogenas de la Cuenca Oriente Ecuat oriana, thèse[END_REF] mais n'af eure que de manière très discontinue.

fl Elle est form ée de tidalites gréso-silteuses à séquences granodécroissantes (lithofaciès St), contenant une macrofaune (crocodiliens, tortues m arines), qui perm et de dé nir un environnement fi estuarien [START_REF] Dock | Geología del Ecuador[END_REF]. Les données de paléocourants montrent des directions très variables, traduisant probablem ent la présence de courants marins m ultidirectionnels.

5. Form ation Mesa/Mera

L a formation Mesa/Mera af eure de m anière dis-fl continue le longdes ríos Pastaza et Aguarico, où elleconstitue deuxcônes alluviaux distincts. P rès de l'apex du cône Pastaza actuel, le matériel est conglom ératique grossier et les coulées de débris sont fré-quentes (lithofaciès Gmm et Gmt). Les clastes sont m ajoritairem ent des roches volcaniques (> 85% ), mais aussi du quartz( ∼ 10% ) et des roches m étamorphiques( ∼ 5% ). Des blocs granitiques et gneissiques (> 1 m) ont été localem ent observés [START_REF] Dock | Geología del Ecuador[END_REF], ce qui laisse supposer que l'apex du cône n'a pas bougé. Vers la partie diatale du cône, les bancs de conglom érats grossiers sont d'épaisseur métrique, avec des lithofaciès G t. L es clastes de même composition qu'à l'ouest sont ici bien roulés, avec une sphéricité élevée. C es dépôts m ontrent un empilem ent de chenaux sym étriques peu profonds (2-3 m pour une largeur de 10 à 20 m) et les coulées boueuses sont absentes. Ce type d'agencement peut être interprété com me résultant de cours d'eau en tresses de type Gravel-braided [START_REF] Mial L | The Geology of Fluvi al Deposi t s:Sedi m ent ary Faci es[END_REF], caractérisés par des dépôts grossiers et deschenaux faiblem ent incisants, sem blables à ceux déposés par les ríos Pastaza et Aguarico actuels. L es paléocourants montrent un m axim um ouest-estprès dufront subandin. Vers l'est, les directions se disposent en éventail, à l'im age du cône Pastaza actuel [START_REF] Baby | The Hi gh Amazoni an Basi n : t ect oni c cont rol and m ass bal ance[END_REF].

Discussion et conclusion

L a caractérisation sédimentologique des dépôts du bassin subandin d'É quateur m ontre tout d'abord que, durant la période considérée, les systèm es uviatiles fl évoluent d'une plaine alluviale (form ation C halcana) à un cône alluvial sem blable au cône actuel, avec des cours d'eau à courant de plus en plus puissant. Si l'on considère que le clim at de la zone est resté très hum ide (cf. discussion in [START_REF] Hort On | Erosi onal cont rol on t he geomet ry and ki nemati cs of thrust belt devel opment in t he cent ral Andes[END_REF]), on peut interpréter l'accroissem ent de la puissance du courant comme le résultat d'une augm entation de la pente m oyenne des cônes uviatiles, à l'im age des m olasses alpines [START_REF] Homewood | Dynamics of the Molasse basin of western Switzerland[END_REF]. fl C ette hypothèse est en grande partie con rmée par fi les travaux sur la m inéralogie des argiles [START_REF] Delfaud | Reconstitution des ét apes de la surrect ion des Andes d'Équat eur à part ir de l 'i nterprétati on des mi néraux argi l eux cont enus dans les bassi ns i nt ramont agneux (Mi ocène à Act uel )[END_REF], qui ont montré qu'à la n de l'O ligocène, fi la protocordillère était en grande partie érodée (m oins de 800 m). L'exhum ation m ise en évidence par les traces de s-fi sion [START_REF] Hungerbühl Er | Neogene st rat i graphy and Andean geodynam i cs of sout her Ecuador[END_REF][START_REF] Spi Kings | Lowtemperat ure t hermochronology of the northern Cordillera Real , Ecuador: tectonic insights for zircon and apatite ssion track analysis[END_REF], m ais aussi par la présence, pour la prem ière fois, de clastes de granite dans M esa/Mera, accom pagnait donc une surrection supérieure à l'érosion (« stade pré-stationnaire»). L es paléocourants montrent que l'augmentation de la pente s'accom pagnait d'une progradation vers l'est des cônes alluviaux. En même temps, Le réseau hydrographique passait progressivement d'un drainage transversal, débouchant dans un systèm e de drainage parallèle aux Andes (formations Chalcana et Arajuno) et peut-être à la ligne de rivage (formation Curaray), caractéristique d'un bassin d'avant-pays sous-alimenté sensu Jordan [START_REF] Jordan | Retroarc forel and and related basi ns[END_REF], à un drainage strictem ent transversal (formation C hambira), caractéristique d'un bassin suralim enté, aboutissant directem ent à une dépression marine (form ation Curaray), elle-mêm e recouverte par l'expansion du cône alluvial (formation Mesa/Mera). Le soulèvem ent du bassin subandin d'É quateur,révélé par la forte incision des form ations décrites ici par les cours d'eau actuels [START_REF] Baby | Mapa Geologico de la Cuenca Ori ente[END_REF][START_REF] Räsänen | Recent and anci ent uvi al fl deposi t ionalsyst ems in t he Amazon forel and basi n, Peru[END_REF], montre que la surrection sem ble actuellem ent s'exagérer, en faisant passer le front subandin du domaine de dépôt à long terme au dom aine d'incision dom inante.

R eplacée dans le contexte de l'ensemble des bassins d'avant-pays des Andes centrales, l'évolution enregistrée par les dépôts néogènes dubassin équatorien diffère fondam entalem ent de celle des bassins subandins de Bolivie, du P érou et de C olom bie.En effet, les dépôts néogènes de Bolivie, du Pérou et de Colom bie sont répartis en plusieurs dépocentres, formés successive m e nt au cours de la propagation vers l'est de chevauchem ents et ont été progressive m entintégrés au prism e orogénique [START_REF] Baby | Mapa Geologico de la Cuenca Ori ente[END_REF][START_REF] Cooper | Basi n devel opment and t ect oni c hi st ory of the Llanos basin, Eastern Cordillera and Middle Magdalena Valley, Col ombi a[END_REF][START_REF] Gi L Rodri Guez | St ruct ure et cont rôle paléogéographique de la zone subandine péruvi enne[END_REF][START_REF]Transfert s sédi m ent ai res Mi o-Pl i ocène et évol ut i on géodynami que dans l es bassi ns Hual l aga et M arañon (Pérou, haut bassi n de l 'Amazone)[END_REF][START_REF] Hort On | DeCel l es, Modern and anci ent uvi al fl megafans in t he forel and basi n syst em of t he Cent ral Andes, Sout hern Bolivi a: implications for drainage net work evolution of fol d-thrust belt s[END_REF]. L'étude sédim entologique ci-dessus montre qu'en É quateur, au contraire, la sédim entation s'est produite dans un unique dépocentre, qui s'est formé dès la n de l'O li-fi gocène [START_REF] Christophoul | Stratigraphic responses to a maj or t ect oni c event in a Foreland Basin: t he Ecuadorian Ori ente Basi n from Eocene to Oli gocene t i m es[END_REF], avec un raccourcissem ent considérablem ent plus faible et une vitesse de subsidence m odérée pour un bassin suralim enté. L'évolution d'un drainage longitudinal vers undrainage transversal accom pagnant l'augm entation de pente et la progradation vers l'est du cône alluvial traduit i ci l'expansion du cône et la surrection de l'orogène, sans migration notable vers l'avant du front orogénique et sans augm entation importante du raccourcissem ent tectonique sur la m arge interne du bassin com me com m uném entadm is [START_REF] Hort On | DeCel l es, Modern and anci ent uvi al fl megafans in t he forel and basi n syst em of t he Cent ral Andes, Sout hern Bolivi a: implications for drainage net work evolution of fol d-thrust belt s[END_REF][START_REF] Jordan | Retroarc forel and and related basi ns[END_REF][START_REF]Si ncl ai r,Tect onost rat i graphic model for under l led fi peri pheral forel and basi ns: an Al pine perspect ive[END_REF]. U ne autre origine, com me un allègement par érosion (erosional unloading) [START_REF] Burbank | Causes of t he recent Hi m al ayan upl i ft deduced from deposi t ed pat t erns in t he Ganges basin[END_REF] ou un m écanisme profond, en relation avec la situation particulière de l'É quateur, doit être recherchée (travaux en cours).

L

  es dépôts du bassin Oriente af eurent fl essentiellement sur le anc est des deux culminations suban-fl dines (Napo au nord et Cutucú au sud) et plus à l'est, grâce à des incisions pratiquées par les principaux cours d'eau (río Napo, Pastaza et Aguarico). Elles se sont déposées avecununique dépocentre situé à l'est

Fi g. 1 .

 1 En haut àgauche, localisation de la région étudi ée. En bas à gauche, carte st ruct urale si m pli ée fi du bassi n subandin d'Équat eur (m odi é fi d'après Baby et al. [1]

Fig. 2 .

 2 Fig. 2. Col onnes sédi ment ol ogi ques des format i ons du bassi n subandi n d'Équat eur. a. Format i on Chal cana. b. Format i on A raj uno. c. Format i ons Chambi ra et C uraray. d. Format i on M esa/ M era. Éléments archi t ecturaux d'après Miall [19] : FF, dépôt s de pl ai ne d'inondati on; DA, barre d'accrét ion l ongi t udi nal e ; LA, barre d'accrét i on l at éral e ; CH, fond de chenal ; GB, b arre de grav i ers ; HO,chenal de con uence ; fl SG, débris ow fl .

Fig. 2 .

 2 Fig. 2. Sedi ment ol ogi c col umns of t he format i ons of t he Ecuadori an Subandean Basi n. a. Chal cana Format i on. b. Araj uno Format i on. c. Chambi ra and Curaray Form at i ons. d. M esa/ M era Format i on. Archit ect uralel ement s,aft er Miall [19]: FF, oodpl ai n nes, fl fi DA, downst ream accret i on macroforms, LA, l at eral accret i on macroforms, CH, channel s, GB,Gravel Bar, HO, Hol l ow Scour, SG, Sediment Gravity ow. fl

Fig. 3 .

 3 Fig. 3. Cart es de pal éocourant s et i sopaques des format i ons du bassi n subandi n d'Équat eur. A. Formation Chalcana. Vitesse de subsidence maximale de 86 mMa -1 . B. Format i on Araj uno/ Curaray. Vit esse de subsi dence m axi m al e de 162 m Ma -1 . C. Format i ons Chambi ra/ Curaray. Vi t esse de subsidence m axi m ale de 336 m Ma -1 D. Format i on M esa/ M era. Les èches i ndi quent l es pal éo-écoul ement s. La li gne bri sée en fl A, B et C i ndi que l 'axe de drainage principal . Les front i ères de l 'Équat eur sont i ndiquées en tiret és.

Fig. 3 .

 3 Fig. 3. Pal aeocurrent and i sopach maps of t he format i ons of t he Ecuadori an Subandean Basi n. A. Chal cana Format i on. M axi mum subsi dence rat e of 86 mMa -1 . B. Araj uno/ Curaray Format i ons. M axi mum subsi dence rat e of 162 m Ma -1 . C. Chambi ra/ Curaray Format i on. M axi mum subsi dence rat e of 336 m Ma -1 . D. M esa/ M era Format i on. Arrows i ndi cat e pal aeo ows. Broken li nes in fl A, B,andC i ndi cat e drai nage axi s. The Ecuadori an border is del i neat ed by i nt errupt ed li nes.

Remerciements. Cestravaux ont été menés dans le cadre de la conventi on de coopérat i on IRD-Pet roproduccí on et du proj et INSUn o 99 PNSE 59. Nous remerci ons aussi J. Del faud et W. Wi nkl er pour l eur revue const ruct ive.

sédimentologie uviatile / cônes alluviaux


Member, IEEE Cyril Buttay

Christophe Raynaud

Member, IEEE Hervé Morel

Gabriel Civrac

Marie-Laure Locatelli

Member, IEEE Florent Morel

Thermal stability of silicon-carbide power diodes

Keywords: Silicon carbide, Power electronics, High temperature techniques, Schottky diodes, P-i-n diodes

Silicon carbide (SiC) power devices can operate at much higher junction temperature than those made of silicon. However, this does not mean that SiC devices can operate without a good cooling system. To demonstrate this, the model of a Merged PiN Schottky (MPS) SiC diode is presented, and its parameters are identified with experimental measurements. This model is then used to study the ruggedness of the diode regarding the thermal run-away phenomenon. Finally, it is shown that where a purely unipolar diode would be unstable, the MPS structure brings increased stability.

I. INTRODUCTION

W IDE BANDGAP power devices, especially those made using silicon carbide (SiC), offer very good performance regarding high temperature electronics. The ubiquitous silicon devices are indeed limited to 150 to 200 °C, depending on their breakdown voltage (see figure 1), whereas SiC devices successfully operating at much higher junction temperature have been reported. For example, an inverter operating at 250°C (resulting in an ever higher junction temperature) is described in [START_REF] Bergogne | Towards an airborne high temperature SiC inverter[END_REF]. In [START_REF] Funaki | Power Conversion With SiC Devices at Extremely High Ambient Temperatures[END_REF], the authors present a converter with a SiC JFET and a Schottky-Barrier Diode (SBD) operating at 450°C.

Applications of high-temperature power devices include aircraft, space, oil and gas exploration [START_REF] Buttay | State of the art of High Temperature Power Electronics[END_REF], where power systems are expected to operate in an elevated ambient temperature. These devices are also interesting in milder environments, because they should require less cooling. This latter approach is described in [START_REF] Hornberger | A High-Temperature Multichip Power Module (MCPM) Inverter utilizing Silicon Carbide (SiC) and Silicon on Insulator (SOI) Electronics[END_REF]: using a power module designed for 250°C in a 150°C environment allows for the use of a much smaller heatsink. Si-based devices indeed offer less headroom between the ambient and maximum junction temperatures, requiring very efficient cooling. This is of great importance, as the thermal management system is one of the bulkiest and heaviest parts of a converter.

However, a recent paper [START_REF] Sheng | Maximum Junction Temperatures of SiC Power Devices[END_REF] has shown that SiC devices are sensitive to thermal runaway. The author claims that SiC devices could not be able to operate realistically above 200°C, and that they require an efficient cooling system. The paper shows theoretically that exceeding these limits results in an unstable situation, where temperature builds up in the device because the power dissipated is not properly removed. As the junction temperature of the device increases, its power January 20, 2012 The authors are with the Université de Lyon, F-69622 France, CNRS UMR 5005, INSA Lyon, Laboratoire Ampère, bât. L. de Vinci, 21, avenue Capelle, F-69621, Villeurbanne. Marie-Laure LOCATELLI is with the LAPLACE, UMR CNRS 5213, Université Paul Sabatier, bât. 3R3, 118, route de Narbonne, F-31062 Toulouse, cedex 9 [START_REF] Pérez | Temperature Dependence of 4H-SiC JBS and Schottky Diodes after High Temperature Treatment of Contact Metal[END_REF] Ti/Ni 350-500°C solid state reaction losses increase as well, eventually yielding to destruction. This phenomenon is called thermal runaway. Some of the findings of [START_REF] Sheng | Maximum Junction Temperatures of SiC Power Devices[END_REF] were confirmed in [START_REF] Pyo | Junction temperature dynamics of power MOSFET and SiC diode[END_REF], for a SiC SBD operating in a room-temperature ambient. However, as new diode structures are now available on the market, and because these diodes could operate in a high-ambient temperature environment, it appears that further studies are needed to assess the extent of the issue on these structures. This is the object of the present paper.

The first section is dedicated to a presentation of the mechanisms that trigger the thermal runaway. Then, the device under study, a SiC diode, is presented along with its model. The experimental identification of the parameters of this model is described in the third section. Finally, using this model, the conditions of thermal runaway are investigated and discussed.

II. THEORETICAL LIMITS

A. Maximum Operating Temperature of Power Devices

Silicon Carbide remains solid up to 2730°C. However, the actual maximum operating temperature of a SiC device is much lower. One of the limits is the Semiconductor Thermal Runaway (STR) described in [START_REF] Raynaud | Comparison of high voltage and high temperature performances of wide bandgap semiconductors for vertical power devices[END_REF]: Above a certain temperature, the intrinsic carrier density becomes higher than the maximum doping level sustaining a given voltage. So at the STR, the material is no longer able to block its rated voltage. The device becomes more and more conductive as the temperature increases, and the temperature increases as more current flows through the device (runaway phenomenon). It can be seen from figure 1 that a 1200 V (4H-SiC) device has a maximum operating temperature of around 1500 K (1230°C).

Furthermore, a SiC-based device actually requires some other materials, such as metals for the contacts, and some passivating materials. For example, the device that is used in this paper has a thick layer (a few micrometers) of Aluminium on top of the anode for compatibility with the wirebonding process. It also uses polyimide for the secondary passivation. The maximum temperature these materials can sustain are summarized in Tab. I.

Finally, during the manufacturing process, the dies are submitted to a rapid thermal annealing step to improve the metal/semiconductor contact. Depending on the annealing temperature, either Schottky or ohmic contacts are formed: for example, it has been shown that a Ti/Ni Schottky contact (annealed at 350°C) can become ohmic if annealed at 500°C [START_REF] Pérez | Temperature Dependence of 4H-SiC JBS and Schottky Diodes after High Temperature Treatment of Contact Metal[END_REF].

Therefore, it can be said that in the case of a Ti/Ni SiC-Schottky diode, the maximum operating temperature must be lower than 500°C. Note that this does not mean that some ageing phenomenon will not occur at a lower temperature. Mechanisms such as chemical species diffusion or polyimide degradation are known to be activated at a lower temperature [START_REF] Johnson | Power Device Packaging Technologies for Extreme Environments[END_REF], especially in presence of oxygen. This is confirmed by tests in our research group (to be published), which showed that the behaviour of a Ti/Ni Schottky diode degrades rapidly after being stored a few hours at 350°C, even under vacuum. However, it should be noticed that this does not constitute an intrinsic limitation of SiC devices, as other metals can be selected as the Schottky contact (such as tungsten [START_REF] Lundberg | CVD-based tungsten carbide schottky contacts to 6H-SiC for very high-temperature operation[END_REF]), or bipolar devices can be used instead of unipolar. Note that ageing phenomena are beyond the scope of this paper.

B. Conditions of the Device Thermal Runaway

The cross-section of a power module is shown in figure 2. With such a module, the power dissipated by the semiconducting die (on top) is transferred through the various layers of the module to the heatsink (bottom) and then to the ambient medium (in the case of figure 2, this is the surrounding air). For other packaging solutions (for example in the case of discrete components), the composition of the stack may differ, but the cooling technique is the same.

As stated by the first law of thermodynamics, the variation of the internal energy in the device (U ) is:

dU dt = P -Q (1)
Where P is the electrical power dissipated in the device and Q is the heat flux removed from the device. At the electrical steady state, in conduction mode, P = I × V , with V , the voltage across the device being a function of I and T j , the junction temperature. I is the current flowing through the device. Therefore, P is itself a function of I and T j only:

P = I × V (I, T j ) = P (I, T j ) (2)
At the thermal steady state, dU dt = 0, so from (1) P = Q, which simply means that the cooling system removes all the energy supplied to the device. If we consider a very simple model for the cooling system, based on conduction only, the heat flux removed is equal to:

Q = T j -T A R T h = ∆T R T h (3)
With R T h the thermal resistance between the junction of the device and its environment (ambient temperature), and T A the ambient temperature.

If we consider slow thermal transients at the electrical steady state (quasi-static conditions), (1) can be rewritten using (2) and (3):

dU dt = P (I, T j ) - T j -T A R T h (4)
For a given, constant I, this equation can be expressed (in the neighbourhood of an initial temperature T j0 ) as the following linearization:

dU dt = P (T j0 ) + ∂P ∂T j (T j -T j0 ) - T j -T A R T h (5)
dU dt = P (T j0 ) + ∂P ∂T j ε - T j0 -T A R T h - ε R T h (6)
with ε = T j -T j0 . As stated above, T j0 corresponds to a steady state if dU (Tj0) dt = 0, that is, if

P (T j0 ) = T j0 -T A R T h (7)
In figure 3, this corresponds to the intersects between the device (dashes) and cooling (plain) characteristics. This figure represents the power P (T j ) dissipated by an imaginary device (at a constant current) depending on its junction temperature, and the cooling capability (

Tj -T A R T h ) of its associated thermal management system.
T j0 is a point of stable equilibrium if a small variation in junction temperature ε yields an opposite variation in internal energy (negative feedback), that is if

sgn ∂P ∂T j ε - ε R T h = -sgn (ε) (8)
Fig. 3. Conditions of thermal equilibrium: when the device is in region A, it tends to heat up (because its power dissipation is higher than the cooling capabilities). When in region B, it tends to cool down.

In figure 3, this corresponds to the lower intersect only, the higher intersect being an unstable equilibrium.

Let us consider that the cooling system line divides the (P,T) domain in two regions (A and B). When a point of the device characteristic is located in region B (that is, under the cooling system line), that means that the device dissipates less power than the cooling system can extract. Therefore, the system tends to cool down. On the contrary, when a point is located in region A, the system tends to warm up. Graphically, that means that a point in region A will move along the device characteristic towards the right, until it reaches an intersect with the cooling system line (or until it fails), whereas it will move to the left if it is in region B.

Therefore, it must be noticed that both intersects in figure 3 have different properties: the lower one corresponds to a stable equilibrium (attractor). The higher intersect, in the opposite, is an unstable equilibrium. If the device experiences even a small cooling down, it will get attracted by the lower intersect. If it gets a bit hotter, it enters region A and will warm indefinitely (that is, until it fails due to the limits described in section II-A). This corresponds to the so-called device thermal runaway (DTR) [START_REF] Stout | Beyond the Data Sheet: Demistifying Thermal Runaway[END_REF]. As can be seen, a device is at risk of thermal runaway if its characteristics enters in region A as the temperature increases. This can happen either because the device characteristic is always above the cooling system line, in which case there is no equilibrium point at all, or as described in figure 3: here instability occurs only above a certain point (the "point of no return" in the figure).

From (3), it can be seen that the ambient temperature T A corresponds to the x-intercept of the cooling system line in figure 3. This means that if T A increases, the cooling system line shifts towards the right, as illustrated in figure 4(c). Eventually, one might end up in the situation where the device characteristic is completely in region A, meaning that there is no longer any stable equilibrium possible (T A3 in figure 4(c), which corresponds to figure 4(b)).

Finally, please note that the device characteristic is plotted in figure 3 for a given I only. As we will see below, it is important to ensure that the system remains thermally stable, even if the device experiences an increase in current.

After this introduction to the DTR, we will present in the next section an analytical model for the SBD and MPS diodes. This model will then be used for a stability study.

III. MPS DIODE MODELLING

Si-based diodes have to rely on a bipolar structure above 100 to 200 volts. In the opposite, due to the much better performance of their base material, SiC Schottky diodes can be designed for voltages up to several kilovolts [START_REF] Baliga | Power Semiconductor Devices[END_REF]. The Schottky Barrier (unipolar) Diodes offer several advantages over their bipolar counterparts, such as very low switching losses or simple parallel-connection [START_REF] Friedrichs | Compact Power Electronics due to SiC Devices[END_REF]. This explains why bipolar SiC diodes are only considered for very high voltage applications (several kilovolts) [START_REF] Morisette | Theoretical Comparison of SiC PiN and Schottky Diodes Based on Power Dissipation Considerations[END_REF].

However, pure SBDs have shown a lack of ruggedness, especially when submitted to current overload or avalanche. Diodes based on a Merged PiN Schottky structure have been proposed [START_REF] Hilsenbeck | Investigations on surge current capability of SiC Schottky diodes by implementation of new pad metallizations[END_REF], [START_REF] Treu | A Surge Current Stable and Avalanche Rugged SiC Merged pn Schottky Diode Blocking 600V Especially Suited for PFC Applications[END_REF] to overcome this issue.

Compared to other PiN-Schottky mixed structures such as the JBS (Junction Barrier Schottky), the MPS diode is designed so that the bipolar features are not involved during normal operation [START_REF] Baliga | Power Semiconductor Devices[END_REF]. They are only active during current or voltage overloads. Reliability is therefore improved without the large reverse recovery currents associated with bipolar diodes.

The MPS diodes are supposed to behave as pure SBDs (under normal conditions). Therefore, the existing models of these devices do not consider a simultaneous conduction of the Schottky and bipolar junctions [START_REF] Zhu | Analytical Modeling of High-Voltage 4H-SiC Junction Barrier Schottky (JBS) Rectifiers[END_REF]. However, such a model is required in the present study, as the devices can be brought outside of their nominal operating conditions.

As describing a complete model for the MPS diodes would be beyond the scope of this paper, we will just focus on the onstate. Taking into account the switching losses would indeed require a lot more parameters, most of them being dependent not on the diode itself, but on the external circuit. The conduction losses in a forward-biased diode only depend on its forward current and voltage, and on its junction temperature. Furthermore, it has been shown that the conduction mode is the main source of losses for the SBDs [START_REF] Morisette | Theoretical Comparison of SiC PiN and Schottky Diodes Based on Power Dissipation Considerations[END_REF].

Therefore, in the remaining of the paper, we will use the model described in figure 5: the MPS diode is considered as a parallel assembly of a SBD and a PiN diode. For both elements, only the forward behaviour will be modelled.

A. SBD Model

The model used for the SBD is described in [START_REF] Raynaud | Barrier height determination of SiC Schottky diodes by capacitance and currentvoltage measurements[END_REF], with some modifications added here to take into account the series resistance of the diode.

I SBD = I sat exp qV d nkT -1 (9)
With V d the voltage bias applied to the metal-semiconductor junction, n the ideality factor, and I sat given by:

I sat = SA * T 2 exp - qΦ e kT (10)
S being the surface of the diode, A * the Richardson constant (see [START_REF] Raynaud | Barrier height determination of SiC Schottky diodes by capacitance and currentvoltage measurements[END_REF] for more details) and Φ e the effective barrier height.

The voltage across the diode, V , is equal to

V = V d + R S (T )I SBD (11)
Where R S is the series resistance of the SBD, and it represents the resistance of the epitaxial layer, as well as that of the SiC substrate. It is considered to have the following dependence on temperature:

R S (T ) = R S300 T 300 K ( 12
)
Where R S300 is the series resistance at 300 K, and K is a temperature coefficient. Merging (10) into ( 9) and ( 12) into (11) yields:

   I SBD =SA * T 2 exp -qΦe kT exp qV d nkT -1 V =V d + R S300 T 300 K I SBD (13)

B. PiN Diode Model

Equation ( 9) has been found not to be a good model for a PiN diode operating in high-injection [START_REF] Masszi | Computer modeling and comparison of different rectifier (M-S, M-S-M, p-n-n+) diodes[END_REF]. This was confirmed in the present case, as trying to identify the parameters of ( 9) to the experimental data resulted in aberrant values. Better models exist but have more parameters [START_REF] Strollo | A new SPICE model of power P-I-N diode based on asymptotic waveform evaluation[END_REF], making them more difficult to identify. Therefore, it was decided to use a

I P iN = β(V -α(T )) 2 (14)
with β a constant and α(T ) a linear function of temperature:

α(T ) = α 0 + α 1 T (15)
Note that this model is only valid for V > α(T ) (that is, above the threshold voltage of the diode).

IV. EXPERIMENTAL IDENTIFICATION

A. Experimental Setup

The devices under test are SiCED 1200 V un-packaged diodes, with a die area of 2.7 x 2.7 mm 2 (batch ref #1024). They are designed as 15 A-nominal-current diodes. The tested diodes are mounted directly on the heating chuck of a vacuum probe station (see figure 6) using a thin layer of silver adhesive. The temperature is monitored using a K-type thermocouple. Our tests showed a temperature accuracy of ±2 K or ±1% across the surface of the chuck. All the tests are performed under vacuum, to prevent the devices from oxidizing.

The electrical measurements are performed using a Keithley Source and Measure Unit (SMU) 2602A, which can source as much as 10 A in pulse mode. A 4-probe setup is used (two probes on the top of the device, one on the chuck, the fourth being the chuck itself), resulting in a measurement accuracy better than 1%. Thanks to this 4-probe setup, the probe-todie contact resistance has no influence on the measurements. The absence of current focusing effect was confirmed by a series of acquisitions, where the probes where placed in different locations of the die metal pad, with different contact pressures. All the results where identical. The silver adhesive thickness was also found to have a negligible effect on the measurements.

The pulse width is set to 900 µs with a very low duty-cycle (<1%) to reduce self-heating while ensuring good measurement accuracy. With such a configuration, the average power dissipated by the device under test remains very low. Some tests showed that with this pulsed measurement, the results were not dependant on the adhesive thickness, nor on the pulse duration, providing it was shorter than 1 ms.

B. Model identification

The identification process is summarized in figure 7. As it can be seen, the 8 parameters of the MPS model are identified in 4 different steps (always using a least-square method). The details of this process are described below.

The experimental data are visible in figure 8 (plain lines). To identify the model of figure 5(b), we consider two regions:

• below a forward voltage of 2 V, all the curves show a knee, followed by a straight line. This correspond to a unipolar diode (the straight line corresponds to the unmodulated series resistance R S ), so we use this region to identify the SBD model.

• above this voltage, a "bend" is visible. This corresponds to the PiN diode conducting some of the forward current. In this region both diodes of figure 5(b) are carrying current. For each temperature (corresponding to each single curve in figure 8), the parameters of the models are identified (step 1). Then the parameters which are temperature-dependent (such as I sat or R S ) are plotted versus the temperature, and their

V.K -1 β 0.89 Ω -1 V -1
temperature-dependence is itself identified (steps 2 and 3 for the SBD model, step 4 for the PiN). This results in a diode model of the form V = f (I, T ) (or I = g(V, T )). More details on the process are given below: 1) SBD model: From equations ( 9) and ( 11), it can be written:

I = I sat exp q(V -R S I) nkT -1 (16)
This model has two variables (V and I) and 3 parameters (n, I sat and R S ). The parameter values are identified for each curve of figure 8 (step 1 in Fig. 7), and the corresponding models are plotted with a dashed line. It can be seen that a very good agreement was found, as experimental and calculated curves match very well at voltage below 2 V (where only the SBD is active).

2) PiN model: As written above, over a certain voltage, the current starts flowing through both the SBD and the PiN diode junctions. Therefore, I P iN = I -I SBD , with I SBD being calculated (not measured) using the model of the SBD (identified in section IV-B1).

Once I P iN is calculated, the identification of the parameters of ( 14) (step 4 in Fig. 7) is pretty straightforward, and gives satisfying results (see figure 9).

3) Electro-thermal model of the forward-biased MPS diode: Once a complete (SBD and PiN) model has been identified for each temperature in the experimental data, the temperaturedependent parameters can be identified. This is the case for I sat and R S (equations ( 10) and ( 12), steps 2 and 3 in Fig. 7), and α (15) (step 4). Figures 10 and11 represent the results of the fitting process for I sat and R S respectively. The table II summarizes all the parameters of the complete, temperature-dependent, MPS model. The experimental and simulated characteristics present a satisfying matching, as can be seen in figure 12.

V. DISCUSSION

The models described above are of the form V = f (I, T ). As P = I × V , it becomes possible to calculate P as P = I × f (I, T ). This equation is plotted, for various values of I in figures 13(a) and (b). In the former, only the SBD part of the diode is considered (the PiN model is removed), while in the latter, the whole MPS model is used. Also in figure 13 the characteristics of 5 cooling systems are plotted. Their R T h values span between 1 K.W -1 (efficient cooling system) and 8 K.W -1 (reasonably efficient). These characteristics are given for two ambient temperatures (T A ): 300 K (27°C) and 473 K (200 °C).

From these figures, the effect of the PiN diode is obvious: a pure SBD has a characteristic similar to the example in figure 3. For example, the 14 A curve and the 4 K/W characteristic (the one obtained for T A = 300K) have two points of equilibrium, one of them unstable. Also, it can be seen that a cooling system with a thermal resistance of 4 K/W is simply not sufficient to cool the diode operating at 16 A, even with an ambient temperature of 300 K. At an ambient of 473 K, 4 K/W is no longer enough to prevent device thermal runaway as soon as the forward current exceeds 10 A.

The characteristic of the MPS (figure 13(b)), in the opposite, does not show a similar quadratic shape. All the cooling system curves only have one intersect, offering a single, stable,

[W] 1 K /W 2 K / W 4 K / W 8 K /W 1 K /W 2 K / W 4 K / W 8 K /W 2.0 A 4.0 A 6.0 A 8.0 A 10.0 A 12.0 A 14.0 A 16.0 A (b)
Fig. 13. Power that would be dissipated by the pure-Schottky Barrier Diode (a), depending on the temperature and the forward current; same plot for the MPS diode (b). Four thermal resistances (1, 2, 4, and 8 K/W) are superimposed on both graphs, for two ambient temperatures: 300 and 473 K (200°C). As the models were only identified with measurements up to 10 A, results higher than this value are displayed with a dotted line.

equilibrium. Moreover, as the device characteristics are much flatter, it can be estimated that even at T A = 473 K, a cooling system with a R T h of around 3 K/W should be sufficient to allow operation of the diode up to 16 A (providing the diode can operate in the long term at 700 K, which is not the case with our sample).

Using the same diode model, another way of presenting the data is visible in figure 14: each point of a curve represents the stable equilibrium point reached for a given forward current. The curves are plotted for various R T h (same values as figure 13), and for two ambient temperature values (300 and 473 K, the y-intersects). Once again, two dataset are shown: one for a pure SBD (figure 14(a)) and one for the complete MPS (figure 14(b))

As can be seen, with the SBD, in many cases a thermal equilibrium could not be found. For a thermal resistance of 1 K/W (and 2 K/W for an ambient of 300 K), the diode

Rth=1.0 K/W Rth=2.0 K/W Rth=4.0 K/W Rth=8.0 K/W (b)
Fig. 14. Steady-state junction temperature versus forward current, and for two ambient temperatures: 300 K and 473 K for the SBD (a) and for the MPS diode (b). The ambient temperature can be read as the temperature at I=0. As the models were only identified with measurements up to 10 A, results higher than this value are displayed with a dotted line.

would remain thermally stable over its entire current range. For higher thermal resistances, however, a thermal runaway could be triggered at current levels lower than the nominal current of the diode. With the MPS (figure 14(b)), no such thermal runaway was found. However, it is interesting to note that both datasets are pretty similar, especially at the lowest temperatures. This is consistent with the static characteristic in figure 8 which shows a very good agreement between the SBD and the MPS characteristics at low temperature and/or lower current.

The bipolar (PiN) diode characteristic has a negative temperature coefficient (NTC), which means that its losses tend to decrease as the temperature increases. This tends to stabilize the unipolar (SBD) diode, which exhibits the opposite behaviour (positive temperature coefficient, PTC). The transition between PTC and NTC is therefore an important parameter for the thermal stability of the diode, which can be adjusted at the design stage [START_REF] Friedrichs | Unipolar SiC power devices and elevated temperature[END_REF]. Maximum thermal resistance [K/W] Tamb=300 K, SBD Tamb=300 K, MPS Tamb=473 K, SBD Tamb=473 K, MPS Fig. 15. Maximum value of R thja versus direct continuous current. Above these curves, the device is either in runaway condition, or at a junction temperature exceeding 783K (the lowest temperature listed in table I). As the models were only identified with measurements up to 10 A, results higher than this value are displayed with a dotted line.

Another approach is to plot the maximum thermal resistance value that will yield to DTR for each forward current value (figure 15). A designer who wants to use this diode can use this graph to choose the appropriate cooling system.

Finally, it must be noted that in this study, the thermal conductivity of SiC was considered constant. This is a coarse approximation, as this conductivity (hence the cooling performance) tends to decrease dramatically as the temperature rises [START_REF] Janke | Nonlinear thermal characteristics of silicon carbide devices[END_REF]. As a result, thermal runaway can be expected to occur even sooner than calculated. SiC, however, is only a small fraction of the power module thermal stack (figure 2), so a more detailed study would be needed to assess the influence of this non-linearity.

VI. CONCLUSION

After the first generation of SiC SBDs was commercially introduced in 2001, it was found that they were prone to failure during current or voltage surges. The following generations were designed to be more robust by using a MPS structure.

In this paper we have shown that this MPS structure also makes the diodes more thermally stable, overcoming the limitations of pure SBD devices. Some of the findings in [START_REF] Sheng | Maximum Junction Temperatures of SiC Power Devices[END_REF] were confirmed, such as the existence of a risk of thermal runaway with some SiC power devices. It was also found that for a SBD operating in a mild ambient (300 K), this thermal runaway could occur at junction temperatures as low as 200°C.

However, the junction temperature is not a design variable. It is the consequence of the power dissipated in the device, of the performance of its cooling system, and of the ambient temperature. Therefore, we focused on these parameters to provide the engineer with guidelines for the use of SiC diodes. It was shown that a MPS diode does not exhibit a thermal runaway behaviour. It was also demonstrated that, providing that the thermal resistance is kept low enough (depending on the ambient temperature and the maximum current level in the diode), no runaway will occur even with a SBD, as shown in Fig. 15.

In some applications SiC devices are preferred in order to lower the size of the thermal management system (resulting in a higher R T h and therefore a higher T j ). For these applications, it must be stressed that pure SBD should be avoided, as they are at risk of thermal runaway. MPSs offer a much robust behaviour, although sufficient cooling must be provided to keep the nominal junction temperature at a level compatible with long-term operation.

A similar study is ongoing to evaluate the risks of thermalrunaway on other SiC devices such as the JFET or the BJT. He is currently with the laboratory Ampère and the École Centrale, Lyon, France as an associate professor. His research interests include power electronics and electromagnetic compatibility.

Fig. 1 .

 1 Fig.1. Theoretical semiconductor runaway temperature as a function of the breakdown voltage, for various semiconductor materials[START_REF] Raynaud | Comparison of high voltage and high temperature performances of wide bandgap semiconductors for vertical power devices[END_REF]

Fig. 2 .

 2 Fig. 2. Cross-section of a power module

Fig. 5 .

 5 Fig. 5.Internal structure of the MPS diode, as described in[START_REF] Friedrichs | Compact Power Electronics due to SiC Devices[END_REF] (a) (terminations not shown), and its equivalent model when forward-biased (b).

Fig. 4 .

 4 Fig. 4. Three particular cases: (a) only one intersect (unconditionally stable), (b) no intersect (no equilibrium), and (c) a change in stability (from stable to unstable) due to an increase in ambient temperature (T A1 to T A3 ).

Fig. 6 .

 6 Fig. 6. Picture of the vacuum probe station (with top cover removed) showing the diode under test, directly bonded to the temperature-controlled chuck and the three probes (the fourth is directly connected to the chuck)

Fig. 7 .

 7 Fig. 7. Diagram of the model identification process. The identification steps are labeled 1 to 4.

Fig. 9 .Fig. 10 .

 910 Fig.9. PiN diode forward characteristics (plain line) and the model fitted (dashed line). The plain line is calculated as the difference between the MPS diode measured characteristic and the SBD model (i.e. the difference between the plain and the dashed lines in figure8).

Fig. 11 .Fig. 12 .

 1112 Fig. 11. Values of the R S parameter versus temperature for the SBD model, fitted with equation (12) (dashed line).

  Cyril Buttay (M'04) received the Engineer and Ph.D. degrees from the "Institut des Sciences Appliquées" (INSA) in Lyon, France, in 2001 and 2004 respectively. From 2005 to 2007 he was a research associate within the Electrical Machines and Drives research team of the University of Sheffield (UK) and the Power Electronics Machines and Control group of the university of Nottingham. Since 2008, he has been a scientist with the French Centre National de Recherche Scientifique (CNRS), working within the laboratoire Ampère, Lyon, France. He currently works on high-temperature power electronics, with a special focus on packaging-related issues. He is also interested in assessing the reliability of SiC power devices operating in harsh environment. Christophe Raynaud was born in Vienne, France, in 1969. He received the engineering degree at INSA de Lyon in 1992, and the Ph.D. degree in 1995. His Ph.D. studies were devoted to characterizations of Silicon Carbide bipolar and Schottky diodes, and MOS devices at the Materials Physics Laboratory (now INL). He also worked for 3 years on submicronic Silicon-based MOSFET, and EEPROMs, especially in charge pumping measurements, in modeling quantum effects in ultra-thin oxide MOS, and studying the EEPROM aging, in collaboration with ST Microelectronics. Since 1999, he has been an Assistant Professor with INSA de Lyon. His research activities at Ampère are focused in high voltage devices, in the area of failure analysis, and also Optical Beam Induced Current measurements, especially silicon carbide based devices but also other wide bandgap devices. He has written or coauthored more than 40 articles in refereed journal and more than 50 articles in international conference proceedings. Hervé Morel (S'07), was born in Reims, France in 1959. He received the Engineer and PhD degrees from Ecole Centrale de Lyon in 1982 and 1984 respectively. In 1985, he joined the CNRS as Associated Scientist. He is currently Senior Scientist at the Université de Lyon, INSA Lyon, Ampère Lab. His research area includes power semiconductor device characterization and modeling, CAE of Power Electronic System Integration, multi-physics modeling based on bond graphs. He is particularly involved in the design of high temperature power electronics for the More Electric Aircraft and high voltage power electronics for smartgrids. He is the manager of SEEDS, the French academic networks on Electrical Engineering. Gabriel Civrac received the M.S. and Ph.D. degrees in microelectronics from the University of Toulouse, France, in 2005 and 2009, respectively. In 2009, he joined the Ampère laboratory (University of Lyon, France) to work with the EPI (Power Electronic and Integration) group. His researches focus on the use of wide band gap materials for the design of high voltage electronic devices, including their fabrication and electrical characterization. Marie-Laure Locatelli received the Engineering degree in Electrical Engineering in 1988, and the Ph.D. degree in Integrated Electron Devices in 1993, both from the Applied Sciences National Institute (INSA) of Lyon (France). As a Research Associate of the National Center of Scientific Research (CNRS) since 1993, she had been working at CEGELY (today Ampère) laboratory in Lyon for 8 years, on the study of silicon power device high temperature limitations, and on the study of new silicon carbide (SiC) power devices. Her activities there had been dealing with design, technology and electrical characterizations of SiC high voltage and high temperature components. Since 2001 up to now, she has been working at LAPLACE in Toulouse. Her research topic is the study of insulating dielectric materials suitable for high temperature and/or high voltage wide band gap semiconductor dies (SiC, GaN and Diamond), for improved power electronics systems performance and reliability. The current temperature range of investigation is above 200°C up to 400°C. She is co-author of 58 communications in international scientific reviews, 53 papers in international conferences, 2 chapters of book. Florent Morel (S'06-M'07) received the Agrégation in electrical engineering from the École Normale Supérieure (ENS), Cachan, France, in 2002. Then he received the M.S. and Ph.D. degrees in electrical engineering from the Institut National des Sciences Appliquées (INSA), Lyon, France, in 2004 and 2007, respectively. His Ph.D. thesis deals with predictive control, permanent magnet synchronous machines, voltage source inverters and matrix converters.

TABLE I TEMPERATURE

 I LIMITS FOR VARIOUS ELEMENTS OF A SIC DIE.

		material	max. temp.	cause
	Semiconductor	SiC	2730°C	sublimation
	Schottky metal	Ti	1668°C	melting point
	Top metallization	Al	660°C	melting point
	secondary passivation polyimide	500-620°C	decomposition
	contact anneal.

ACKNOWLEDGMENT

The authors would like to thank the National Fundation for Research in Aeronautics and Space (FRAE) for their financial support of this work.