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1、AvalancheCharacteristicsandRatingsofPowerMOSFETGiovanniPriviteraProduct&ApplicationengineeringPowerMOSFETDivisionSTMicroelectronicsCataniaItaly#nS1.1 IntroductionBackinthemidSOs,powerMOSFETmanufacturersstartedtoclaimanewoutstandingfeature:theAvalancheRuggednessSuddenly,newfamiliesofdevicesevolved,al
2、lwiththis“newfeature.Theimplementationwasquitesimple:tlieverticalMOSFETstructurehasanintegralbodydraindiode,whichcannotbeeliminatedSo,bychangingsomeprocessandlayoutparameters,itispossibletoguaranteetheuseoftheclampingcapabilityofthisdiodeinwithstandingaccidentalvoltage/powersurgesbeyondthenominaldra
3、insourcevoltage,Ofcoursetlieconfusionaboutthemeaningofruggedness,andhowtoratetinsinthedatasheetwassohuge,coupledwiththepoortlieoreticalknowledgeofit.Despitethis,allPowerMOSFETmanufacturersstartedtoproduceavalancherateddevices,proposedatasheetratings(altliougliimperfect),toprotectthemselvesandtheendu
4、sers,fromthisincompleteknowledge.Today,knowledgeondevice*sbehaviordiinngavalancheconditionsisenhancedAlotof却plicationnotesandpaperswereissuedwithdifferentapproachestoexplainratingsandavalanchebehaviorThescopeofthisnoteistobneflyreviewtheMOSFETphysicsonavalanchetosupplydesignerswithtoolsandhintstodea
5、lwithavalancheissues.1.2 MOSFETfundamentalsThebasicandsimplifiedverticalstructureofaMOSFETisdrawninFigure1.TheactualMOSFETisaninfiniteparallelofthesemicroscopic1structuresthatworktogethersharingthesameDRAIN,withalltheGATEsconnectedtogetherbyadeposedpolysiliconmeshandalltheSOURCESlinkedbytlietopmetal
6、.SourceGateDrainFigure1MOSFETverticalstructureandparasiticelementsFigure1referstothewellknownSTpatentedhiglivoltageMOSFETstructure,MESHOVERLAYexceptsomeprocessoptimizationoftheshapeofthebody-drainjunctionandotherimportantimprovementsintlieMESHoverlaydesignTheconceptofthisverticalstnicturecouldbecons
7、ideredvalidalsoforvanousoldercellularorotherteclinologiesDuringonstate,wlulethegatesourcevoltageisabovethethreshold,tlieconductioncurrentislocalizedinthedramandintheregionbelowthegate(channel).DuringoffstatetlieVoltagedropacrossdrainandsourceissustainedbythePNjunctionatreversebias,andaverysmallcurre
8、nt(leakage)flowstlirouglithejunctionIftlievoltageincreasestoomuchandtheelectricalfieldreachesthecriticalvalue,thejunctiongoestobreakdown,andcurrentstartstoflowthroughthebodyregion.So,ifanovervoltageisappliedtothejunction,acurrentflowstliroughitwhiletheMOSFETlimitstheactualdrain一sourcebreakdownvoltag
9、eThebreakdownmechanismitself,isnotdestructiveforap-njunctionHowever,overheatingcausedbythelargebreakdowncurrentandhiglibreakdownvoltagedamagesthePNjunctionunlesssufficientheatsinkingisprovided.LookingatthestructureoftheMOSFET,onecanseethatthePNjunctionisnotasimplenorperfectdiodeThediodeoftheMOSFETis
10、thecollectorbasejunctionofaBJT(BipolarJunctionTransistor,alsocalledtheparasitictransistor)madebytheN+regionofsource,P/P*regionofthebodyandN*regionofthedrain,withthebaseshortedtotheemitterbythe丘ontmetal.ThecapabilityofaMOSFETtowithstandavalancheconditiontakesintoaccounttliesetwoconcerns.Infact,twokin
11、dsoffailureanse:onerelatedtocurrent,andtheothertopowerdissipation.Intheformer,failureiscausedbytlielatchingoftheparasiticbipolarduetothecurrentthatflowsthroughitsbaseresistance,multipliedbythegainTliesecondisreachedwhenthetemperatureofthejunctionrisestoacriticalvaluetliatprovokestheformationofhotspo
12、tswithaveragetemperaturesabout650Candpeakofapproximately1000Ccausedbyregenerativethermalrunaway,causingtheextremelyrapiddestructionoftliedevice1.2.1FailureModeDescriptionshasbeentliefirststepfortheimprovementoftheMOSFET,followedbyothermoresubtleoptimizations.ThepowerthatisdissipatedintheMOSFETcauses
13、anincreaseinjunctiontemperature.Iftlietemperatureincreasestoacriticalvaluesetbythepropertyofthesilicon2,thefailure,withoutthecontnbutionoftheparasiticbipolar,occursbecauseofthecreationofthermallygeneratedearnersintheepitaxyal/bulkregionandsothecreationofhotspots.Thecriticaltemperaturetohavethispheno
14、menonisbeyondthemaximumjunctiontemperatureofthedevicesandisrelatedtotheintrinsictemperatureofdopedsilicon,towhichtheconcentrationofthebulkequalstheoneofthethermalgeneratedcarriers.Thetemperatureincreasedunngavalanchephenomena,duetothermalcapacitanceofthesilicon,isnotinstantaneous.Hence,tinskindoffai
15、lureshouldbedistinguished&omthatcausedbycurrentasthedeviceholdsthebreakdownvoltageforafinitetimebeforedestruction.1.3 TestingtheAvalancheRuggedness#Aspreviouslydiscussed,theintegraldiodeofaMOSFETistliecollectorbasejunctionoftheparasitictransistor.Ifthecurrentflowslaterallytliroughregionp,theincrease
16、inthevoltagedropacrosstheemitterbaseresistancecausestheBJTtoturn-on.Tlieinitialavalanchecurrentisconcentratedmainlyinthediodelocalizedinthedeepzoneofpassoonastliecurrentgrows,itbeginstointerestalsothep,lighterdoped,regions.Since,bydesign,thevalueoflateralresistanceRpisluglierthantheoneoftheverticalresistanceoftheheavydopedp+region,andthecurrentisconcentratedintheregionp+,sotheBJTshouldnotturnonAsso
