AvalancheCharacteristicsandRatingsofPowerMOSFETGiovanniPriviteraProduct&ApplicationengineeringPowerMOSFETDivisionSTMicroelectronicsCataniaItaly#nS1.1 IntroductionBackinthemidSOs,powerMOSFETmanufacturersstartedtoclaimanewoutstandingfeature:theAvalancheRuggednessSuddenly,newfamiliesofdevicesevolved,allwiththis“newfeature.Theimplementationwasquitesimple:tlieverticalMOSFETstructurehasanintegralbodydraindiode,whichcannotbeeliminatedSo,bychangingsomeprocessandlayoutparameters,itispossibletoguaranteetheuseoftheclampingcapabilityofthisdiodeinwithstandingaccidentalvoltage/powersurgesbeyondthenominaldrainsourcevoltage,Ofcoursetlieconfusionaboutthemeaningofruggedness,andhowtoratetinsinthedatasheetwassohuge,coupledwiththepoortlieoreticalknowledgeofit.Despitethis,allPowerMOSFETmanufacturersstartedtoproduceavalancherateddevices,proposedatasheetratings(altliougliimperfect),toprotectthemselvesandtheendusers,fromthisincompleteknowledge.Today,knowledgeondevice*sbehaviordiinngavalancheconditionsisenhancedAlotof却plicationnotesandpaperswereissuedwithdifferentapproachestoexplainratingsandavalanchebehaviorThescopeofthisnoteistobneflyreviewtheMOSFETphysicsonavalanchetosupplydesignerswithtoolsandhintstodealwithavalancheissues.1.2 MOSFETfundamentalsThebasicandsimplifiedverticalstructureofaMOSFETisdrawninFigure1.TheactualMOSFETisaninfiniteparallelofthesemicroscopic1structuresthatworktogethersharingthesameDRAIN,withalltheGATEsconnectedtogetherbyadeposedpolysiliconmeshandalltheSOURCESlinkedbytlietopmetal.SourceGateDrainFigure1MOSFETverticalstructureandparasiticelementsFigure1referstothewellknownSTpatentedhiglivoltageMOSFETstructure,MESHOVERLAYexceptsomeprocessoptimizationoftheshapeofthebody-drainjunctionandotherimportantimprovementsintlieMESHoverlaydesignTheconceptofthisverticalstnicturecouldbeconsideredvalidalsoforvanousoldercellularorotherteclinologiesDuringonstate,wlulethegatesourcevoltageisabovethethreshold,tlieconductioncurrentislocalizedinthedramandintheregionbelowthegate(channel).DuringoffstatetlieVoltagedropacrossdrainandsourceissustainedbythePNjunctionatreversebias,andaverysmallcurrent(leakage)flowstlirouglithejunctionIftlievoltageincreasestoomuchandtheelectricalfieldreachesthecriticalvalue,thejunctiongoestobreakdown,andcurrentstartstoflowthroughthebodyregion.So,ifanovervoltageisappliedtothejunction,acurrentflowstliroughitwhiletheMOSFETlimitstheactualdrain一sourcebreakdownvoltageThebreakdownmechanismitself,isnotdestructiveforap-njunctionHowever,overheatingcausedbythelargebreakdowncurrentandhiglibreakdownvoltagedamagesthePNjunctionunlesssufficientheatsinkingisprovided.LookingatthestructureoftheMOSFET,onecanseethatthePNjunctionisnotasimplenorperfectdiodeThediodeoftheMOSFETisthecollectorbasejunctionofaBJT(BipolarJunctionTransistor,alsocalledtheparasitictransistor)madebytheN+regionofsource,P/P*regionofthebodyandN*regionofthedrain,withthebaseshortedtotheemitterbythe丘ontmetal.ThecapabilityofaMOSFETtowithstandavalancheconditiontakesintoaccounttliesetwoconcerns.Infact,twokindsoffailureanse:onerelatedtocurrent,andtheothertopowerdissipation.Intheformer,failureiscausedbytlielatchingoftheparasiticbipolarduetothecurrentthatflowsthroughitsbaseresistance,multipliedbythegainTliesecondisreachedwhenthetemperatureofthejunctionrisestoacriticalvaluetliatprovokestheformationofhotspotswithaveragetemperaturesabout650Candpeakofapproximately1000Ccausedbyregenerativethermalrunaway,causingtheextremelyrapiddestructionoftliedevice1.2.1FailureModeDescriptionshasbeentliefirststepfortheimprovementoftheMOSFET,followedbyothermoresubtleoptimizations.ThepowerthatisdissipatedintheMOSFETcausesanincreaseinjunctiontemperature.Iftlietemperatureincreasestoacriticalvaluesetbythepropertyofthesilicon2,thefailure,withoutthecontnbutionoftheparasiticbipolar,occursbecauseofthecreationofthermallygeneratedearnersintheepitaxyal/bulkregionandsothecreationofhotspots.Thecriticaltemperaturetohavethisphenomenonisbeyondthemaximumjunctiontemperatureofthedevicesandisrelatedtotheintrinsictemperatureofdopedsilicon,towhichtheconcentrationofthebulkequalstheoneofthethermalgeneratedcarriers.Thetemperatureincreasedunngavalanchephenomena,duetothermalcapacitanceofthesilicon,isnotinstantaneous.Hence,tinskindoffailureshouldbedistinguished&omthatcausedbycurrentasthedeviceholdsthebreakdownvoltageforafinitetimebeforedestruction.1.3 TestingtheAvalancheRuggedness#Aspreviouslydiscussed,theintegraldiodeofaMOSFETistliecollectorbasejunctionoftheparasitictransistor.Ifthecurrentflowslaterallytliroughregionp,theincreaseinthevoltagedropacrosstheemitterbaseresistancecausestheBJTtoturn-on.Tlieinitialavalanchecurrentisconcentratedmainlyinthediodelocalizedinthedeepzoneofpassoonastliecurrentgrows,itbeginstointerestalsothep,lighterdoped,regions.Since,bydesign,thevalueoflateralresistanceRpisluglierthantheoneoftheverticalresistanceoftheheavydopedp+region,andthecurrentisconcentratedintheregionp+,sotheBJTshouldnotturnonAsso