功率MOSFET-基础-Power-MOSFET-Basics

(MOSFET)isbasedontheoriginalfield-effecttransistorintroducedinthe70s.Figure1showsthedeviceschematic,transfercharacteristicsanddevicesymbolforaMOSFET.TheinventionofthepowerMOSFETwaspartlydrivenbythelimitationsofbipolarpowerjunctiontransistors(BJTs)which,untilrecently,wasthedeviceofchoiceinpowerelectronicsapplications.Althoughitisnotpossibletodefineabsolutelytheoperatingboundariesofapowerdevice,wewilllooselyrefertothepowerdeviceasanydevicethatcanswitchatleast1A.Thebipolarpowertransistorisacurrentcontrolleddevice.Alargebasedrivecurrentashighasone-fifthofthecollectorcurrentisrequiredtokeepthedeviceintheONstate.Also,higherreversebasedrivecurrentsarerequiredtoobtainfastturn-off.DespitetheveryadvancedstateofmanufacturabilityandlowercostsofBJTs,theselimitationshavemadethebasedrivecircuitdesignmorecomplicatedandhencemoreexpensivethanthepowerMOSFET.SourceContactFieldOxideGateOxideGateMetallizationDrainContactn*Drainp-SubstrateChanneln*SourcetoxlVGSVT00ID(a)(b)IDDSB(ChannelorSubstrate)SG(c)Figure1.PowerMOSFET(a)Schematic,(b)TransferCharacteristics,(c)DeviceSymbol.AnotherBJTlimitationisthatbothelectronsandholescontributetoconduction.PresenceofholeswiththeirhighercarrierlifetimecausestheswitchingspeedtobeseveralordersofmagnitudeslowerthanforapowerMOSFETofsimilarsizeandvoltagerating.Also,BJTssufferfromthermalrunaway.Theirforwardvoltagedropdecreaseswithincreasingtemperaturecausingdiversionofcurrenttoasingledevicewhenseveraldevicesareparalleled.PowerMOSFETs,ontheotherhand,aremajoritycarrierdeviceswithnominoritycarrierinjection.TheyaresuperiortotheBJTsinhighfrequencyapplicationswhereswitchingpowerlossesareimportant.Plus,theycanwithstandsimultaneousapplicationofhighcurrentandvoltagewithoutundergoingdestructivefailureduetosecondbreakdown.PowerMOSFETscanalsobeparalleledeasilybecausetheforwardvoltagedropincreaseswithincreasingtemperature,ensuringanevendistributionofcurrentamongallcomponents.However,athighbreakdownvoltages(200V)theon-statevoltagedropofthepowerMOSFETbecomeshigherthanthatofasimilarsizebipolardevicewithsimilarvoltagerating.Thismakesitmoreattractivetousethebipolarpowertransistorattheexpenseofworsehighfrequencyperformance.Figure2showsthepresentcurrent-voltagelimitationsofpowerMOSFETsandBJTs.Overtime,newmaterials,structuresandprocessingtechniquesareexpectedtoraisetheselimits.20001500100050001101001000MaximumCurrent(A)HoldoffVoltage(V)BipolarTransistorsMOSFigure2.Current-VoltageLimitationsofMOSFETsandBJTs.DrainMetallizationDrainn+Substrate(100)n-EpiLayerChannelsn+pn+p+BodyRegionp+DriftRegionGSDSourceGateOxidePolysiliconGateSourceMetallizationFigure3.SchematicDiagramforann-ChannelPowerMOSFETandtheDevice.Figure3showsschematicdiagramandFigure4showsthephysicaloriginoftheparasiticcomponentsinann-channelpowerMOSFET.TheparasiticJFETappearingbetweenthetwobodyimplantsrestrictscurrentflowwhenthedepletionwidthsofthetwoadjacentbodydiodesextendintothedriftregionwithincreasingdrainvoltage.TheparasiticBJTcanmakethedevicesusceptibletounwanteddeviceturn-onandprematurebreakdown.ThebaseresistanceRBmustbeminimizedthroughcarefuldesignofthedopinganddistanceunderthesourceregion.ThereareseveralparasiticcapacitancesassociatedwiththepowerMOSFETasshowninFigure3.CGSisthecapacitanceduetotheoverlapofthesourceandthechannelregionsbythepolysilicongateandisindependentofappliedvoltage.CGDconsistsoftwoparts,thefirstisthecapacitanceassociatedwiththeoverlapofthepolysilicongateandthesiliconunderneathintheJFETregion.Thesecondpartisthecapacitanceassociatedwiththedeple