功率半导体器件基本原理第04章精.docx

功率半导体器件基本原理第04章精.docx

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功率半导体器件基本原理第04章精

Chapter4

SchottkyRectifiers

ASchottkyrectifierisformedbymakinganelectricallynonlinearcontactbetweenametalandthesemiconductordriftregion.TheSchottkyrectifierisanattractiveunipolardeviceforpowerelectronicapplicationsduetoitsrelativelylowon-statevoltagedropanditsfastswitchingbehavior.Ithasbeenwidelyusedinpowersupplycircuitswithlowoperatingvoltagesduetotheavailabilityofexcellentdevicesbaseduponsilicontechnology.Inthecaseofsilicon,themaximumbreakdownvoltageofSchottkyrectifiershasbeenlimitedbytheincreaseintheresistanceofthedriftregion.Commerciallyavailabledevicesaregenerallyratedatbreakdownvoltagesoflessthan100V.Novelsiliconstructuresthatutilizethecharge-couplingconcepthaveallowedextendingthebreakdownvoltagetothe200Vrange.1,2

ManyapplicationsdescribedinChap.1requirefastswitchingrectifierswithlowon-statevoltagedropthatcanalsosupportover500V.ThemuchlowerresistanceofthedriftregionforsiliconcarbideenablesdevelopmentofsuchSchottkyrectifierswithveryhighbreakdownvoltages.3Thesedevicesnotonlyofferfastswitchingspeedbutalsoeliminatethelargereverserecoverycurrentobservedinhigh-voltagesiliconP-i-Nrectifiers.ThisreducesswitchinglossesnotonlyintherectifierbutalsointheIGBTsusedwithinthepowercircuits.4

Inthischapter,thebasicstructureofthepowerSchottkyrectifierisfirstintroducedtodefineitsconstituentelements.Thischapterthenprovidesadiscussionofthebasicprinciplesofoperationofthemetal–semiconductorcontact.Thecurrenttransportmechanismsthatarepertinenttopowerdevicesareelucidatedforboththeforwardandreversemodeofoperation.Inthefirstquadrantofoperation,thethermionicemissionprocessisdominantforpowerSchottkyrectifiers.Inthethirdquadrantofoperation,theinfluenceofSchottkybarrierloweringhasastrongimpactontheleakagecurrentforsilicondevices.Inthecaseofsiliconcarbidedevices,theinfluenceoftunnelingcurrentmustalsobetakenintoaccountwhenperformingtheanalysisofthereverseleakagecurrent.

B.J.Baliga,FundamentalsofPowerSemiconductorDevices,doi:

10.1007/978-0-387-47314-7_4,

©SpringerScience+BusinessMedia,LLC2008

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Thetradeoffbetweenreducingpowerdissipationintheon-stateandtheoff-stateforSchottkyrectifiersisalsoanalyzedinthischapter.Thistradeoffrequirestakingintoaccountthemaximumoperatingtemperaturefortheapplication.ThepowerdissipationintheSchottkyrectifierisshowntodependuponthebarrierheightaswellasthedutycycle.

4.1PowerSchottkyRectifierStructure

Thebasicone-dimensionalstructureofthemetal–semiconductororSchottkyrectifierstructureisshowninFig.4.1togetherwithelectricfieldprofileunderreversebiasoperation.Theappliedvoltageissupportedbythedriftregionwithatriangularelectricfielddistributionifthedriftregiondopingisuniform.Themaximumelectricfieldoccursatthemetalcontact.Thedeviceundergoesbreakdownwhenthisfieldbecomesequaltothecriticalelectricfieldforthesemiconductor.

Schottkyrectifierbythetransportofelectronsoverthemetal–semiconductorcontactandthroughthedriftregionaswellasthesubstrate.Theon-statevoltagedropisdeterminedbythevoltagedropacrossthemetal–semiconductorinterfaceandtheohmicvoltagedropintheresistanceofthedriftregion,thesubstrate,anditsohmiccontact.

Attypicalon-stateoperatingcurrentdensitylevels,thecurrenttransportisdominatedbymajoritycarriers.Consequently,thereisinsignificantminoritycarrierstoredchargewithinthedriftregioninthepowerSchottkyrectifier.ThisenablesswitchingtheSchottkyrectifierfromtheon-statetothereverse-blockingoff-stateinarapidmannerbyestablishingadepletionregionwithinthedrift

SchottkyRectifiers169

region.ThefastswitchingcapabilityoftheSchottkyrectifierenablesoperationathighfrequencieswithlowpowerlosses,makingthisdevicepopularforhighfrequencyswitch-modepowersupplyapplications.Withtheadventofhigh-voltageSchottkyrectifiersbaseduponsiliconcarbide,theyareexpectedtobeutilizedinmotorcontrolapplicationsaswell.

4.2Metal–SemiconductorContact

Nonlinearcurrenttransportacrossametal–semiconductorcontacthasbeenknownforalongtime.Thepotentialbarrierresponsibleforthisbehaviorwasascribedtothepresenceofastablespace-chargelayerbyWalterSchottkyin1938.Inthissection,theprinciplesfortheformationofarectifyingcontactbetweenametalandanN-typesemiconductorregionaredescribed.ThisenablesrelatingtheSchottkybarrierheightbetweenthemetalandthesemiconductortotheirfundamentalproperties.

TheenergybanddiagramforametalandanN-typesemiconductorisshowninFig.4.2whentheyareisolatedfromeachother.Ingeneral,thepositionoftheFermilevelinthemetalandthesemiconductorwillhavedifferentenergyvalues.Intheexampleshowninthefigure,theFermilevelinthesemiconductorliesabovetheFermilevelforthemetal.Theworkfunctionforthemetal（ΦM）isdefinedastheenergyrequiredtomoveanelectronfromtheFermilevelpositioninthemetal（EFM）toastateofrestinfreespaceoutsidethesurfaceofthemetal.Inthesamemanner,theworkfunctionforthesemiconductor（ΦS）isdefinedastheenergyrequiredtomoveanelectronfromtheFermilevelpositioninthesemiconductor（EFS）toastateofrestinfreespaceoutsidethesurfaceofthesemi-conductor.SincenoelectronsarelocatedattheFermilevelpositioninthesemiconductor,itisusefultodefineanelectronaffinityforthesemiconductor（χS）

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astheenergyrequiredtomoveanelectronfromthebottomoftheconductionbandinthesemiconductor（EC）toastateofrestinfreespaceoutsidethesurfaceofthesemiconductor.Theworkfunctionandelectronaffinityforthesemiconductorarerelatedby

ΦS=χS+（EC−EFS）.（4.1）

ThepotentialdifferencebetweentheFermilevelinthesemiconductorandtheFermilevelinthemetaliscalledthecontactpotential（VC）whichisgivenby

qVC=（EFS−EFM）=ΦM−ΦS=ΦM−（χS+EC−EFS）.（4.2）

Whenanelectricalconnectionisprovidedbetweenthemetalandthesemiconductor,electronsaretransferredfromthesemiconductortothemetaldue

SchottkyRectifiers171

totheirgreaterenergyuntilthermalequilibriumisestablished.Thistransferofelectronscreatesanegativechargeinthemetalandapositivechargewithinadepletionregionformedatthesemiconductorsurface.TheresultingbandstructureisillustratedinFig.4.3forthecaseofaseparationdbetweenthemetalandthesemiconductorsurfaces.Whenthemetalandthesemiconductorsurfacesarebroughtintocontactbyreducingtheseparationdtozero,thebandstructureforthemetal–semiconductorcontactisobtainedasillustratedinFig.4.4.Theentirecontactpotentialisnowsupportedwithinthedepletionregionformedatthesurfaceofthesemiconductor.Thisvoltageisthereforealsoreferredtoasthebuilt-inpotential（Vbi）ofthemetal–semiconductorcontact.

TheSchottkybarrierheight（ΦBN）isrelatedtothebuilt-inpotentialby

ΦBN=qVbi+（EC−EFS）.

ΦBN=ΦM−χS,（4.3）AnotherusefulrelationshipforobtainingtheSchottkybarrierheightis（4.4）

becausethesepropertiesforthematerialsareknown.Thebuilt-inpotentialcreatesazero-biasdepletionregionwithinthesemiconductorwhosewidthisgivenby

W0=

4.3ForwardConduction（4.5）

Currentflowacrossthemetal–semiconductorjunctioncanbeproducedbytheapplicationofanegativebiastotheN-typesemiconductorregion.ThisproducesashiftintheenergybandstructureasillustratedinFig.4.5.Currentflowacrosstheinterfacethenoccursmainlyduetomajoritycarriers–electronsforthecaseofanN-typesemiconductor.Thecurrenttransportacrossthecontactcantakeplaceviafourbasicprocesses5thatareschematicallyshowninthefigure:

1.Thetransportofelectronsfromthesemiconductorintothemetaloverthe

potentialbarrier（referredtoasthethermionicemissioncurrent）

2.Thetransportofelectronsbyquantummechanicaltunnelingthroughthe

potentialbarrier（referredtoasthetunnelingcurrent）

3.Thetransportofelectronsandholesintothedepletionregionfollowedby

theirrecombination（referredtoastherecombinationcurrent）

4.Thetransportofholesfromthemetalintotheneutralregionofthe

semiconductorfollowedbyrecombination（referredtoastheminoritycarriercurrent）

Inthecaseofpowerrectifiers,thedopingconcentrationinthesemiconductormustberelativelylowtosupportthereversebias（orblocking）voltage.Thisspreadsthedepletionregionoverasubstantialdistance.Consequently,thepotentialbarrieris

172FUNDAMENTALSOFPOWERSEMICONDUCTORDEVICES

（a）

EFMbi-qVFSECEFS

FEV

METALSEMICONDUCTOR

Fig.4.5Energybanddiagramforametal–semiconductorjunctionaftertheapplicationofaforwardbiasvoltage（electronsareshownascirclesandholesareshownassquares）

notsharpenoughtoallowsubstantialcurrentviathetunnelingprocess.Therecombinationcurrentinthespace-chargeregionisobservableonlyatverylowon-statecurrentlevels.ThecurrenttransportduetotheinjectionofholesisusuallynegligibleunlesstheSchottkybarrierheightislarge.InpowerSchottkyrectifiers,thebarrierheightisintentionallyreducedtolowertheon-statevoltagedropmakingtheminoritycarriercurrentsmall.Consequently,thecurrentflowviathethermionicemissionprocessisthedominantcurrenttransportmechani