FieldEffectTransistors（FETs）场效应晶体管（FET）教学课件

FieldEffectTransistors(FETs)Inafieldeffecttransistor,currentflowthroughasemiconductorchanneliscontrolledbytheapplicationofanelectricfield(voltage)perpendiculartothedirectionofcurrentflow.WeconsidertheMOSFET(MOST)-themetaloxidesemiconductor(fieldeffect)transistor.FieldEffectTransistors(FETsFieldEffectTransistors(FETs)InadepletionmodeMOSFETachannelisbuiltinsothatconductionoccurswithnocontrolvoltageapplied.Weconsiderann-channelenhancementmodedevicewhichhasno‘built-in’conductionchannel.FieldEffectTransistors(FETsN-channelenhancementmodeMOSFET(schematic)Oxide(SiO2)n+n+p-typesiliconGate(G)Drain(D)Source(S)N-channelenhancementmodeMOSSymbolforN-channelenhancementmodeMOSFETNoteconnectiontosubstrateshownSymbolforN-channelenhancemeSymbolforN-channelenhancementmodeMOSFETNoteconnectiontosubstrateshownGSDSymbolforN-channelenhancemeN-channelenhancementmodeMOSFET:outlineofoperationVGS=0,littleornocurrentcanflowSD,backtobackp-njunctions.AsapositiveVGSisapplied,holesinthep-regionarerepelled.AsVGSincreasesfurther,electronsareattractedfromthesubstratetowardsthepositivegate.Aninversionlayerofmobileelectronsformsnearthesiliconsurface.N-channelenhancementmodeMOSN-channelenhancementmodeMOSFETwithinversionlayerSource(S)Oxide(SiO2)n+n+p-typesiliconGate(G)Drain(D)Inversionlayer(conductionchannel)N-channelenhancementmodeMOSN-channelenhancementmodeMOSFET:outlineofoperationThisinversionlayeriseffectivelyn-type.Electronscancarrycurrentinancontinuousn-typepath(orchannel)fromsourcetodrain.TheconductionchannelformswhenVGSattainsathresholdvoltage,VTN-channelenhancementmodeMOSN-channelenhancementmodeMOSFETThevalueofVTisdeterminedbythedeviceprocessandthelevelofthep-type(substrate)doping.Notethatann-channeldeviceisformedwithap-typesubstrate.N-channelenhancementmodeMOSN-channelenhancementmodeMOSFETAsVGSincreasesfurther,moreelectronsaredrawnintotheinversionlayerandthechannelresistancedecreases(Thedeviceoperatesasavoltagecontrolledresistor).However……N-channelenhancementmodeMOSN-channelenhancementmodeMOSFETThevoltagebetweengateandchannelvariesfromVGSatthesourceendtoVGS-VDSatthedrainend.Thusasthevoltageatthedrainend,VDS,isincreasedtheeffectivegate-channelvoltage(verticalfield)isdecreased.N-channelenhancementmodeMOSN-channelenhancementmodeMOSFETThiscausesthecarrierdensityatthedrainendoftheinversionlayertodecrease.Thecurrentlevelsofforsaturates.TheFETisthensaidtohaveentereditssaturationregion(ThisexpressionhadadifferentmeaninginourdiscussionofBJTs).N-channelenhancementmodeMOSN-channelenhancementmodeMOSFETTheboundarywheresaturationstartsoccursat(VGS–VDS)=VT.i.e.VDS=(VGS–VT)IdeallywewouldwantafurtherincreaseinVDStohavenoeffectonthedraincurrentIDN-channelenhancementmodeMOSN-channelenhancementmodeMOSFETInpracticeasVDSisincreasedfurtherIDalsoincreases.ThisisduetoareductionintheeffectivechannellengthwithVDS.Wenote,fordesignpurposes,thatthegatecurrent(IG)iseffectivelyzero.(Wehaveaninsulatingoxideregion)N-channelenhancementmodeMOSN-channelDepletionmodeMOSFETInadepletionmodeMOSTachannelisbuiltintothedevice.ConductionoccursatVDS=0.(ThethresholdvoltageVTisnegative)N-channelDepletionmodeMOSFESymbolforN-channeldepletionmodeMOSFETConnectiontosubstrateshown.ThesolidlineindicatesthataphysicalchannelexistsatVGS=0.SymbolforN-channeldepletionP-channeldevicesThesearewidelyused,particularlyinComplementaryMOS(C-MOS)circuits.Electronmobilityislargerthanholemobilityson-channeldevicesarefaster,i.e.theyhaveahigherfrequencyresponse.P-channeldevicesThesearewidPlotsandEquations,Enhancementmode,VT2VVDS=VGS-VTPlotsandEquations,EnhancemeFETPlotsandequationsTheFETisavoltagecontrolleddevice.The‘controlparameter’isVGS–comparewithIBintheBJT.FETPlotsandequationsTheFETLooseequivalencesFETDrainSourceGateBJTCollectorEmitterBaseLooseequivalencesFETBJTPlotsandEquationsVDS=VGS-VTSaturationregionPlotsandEquationsVDS=VGS-PlotsandEquationsVDS=VGS-VT‘Triode’regionPlotsandEquationsVDS=VGS-TransferCharacteristicGivestherelationshipbetweenIDandVGSinthesaturation(constantcurrent)regime.e.g.foranenhancementmodedeviceVTVGSIDTransferCharacteristicGivestTransferCharacteristicThisI-VcurveinthesaturationregioncanbeapproximatedbytheparabolaThisequationisveryimportant!TheID-VGSrelationshipisnotalinearone!TransferCharacteristicThisI-TransferCharacteristicKisadeviceparameterwhosevalueistypically0.25mA/V2TransferCharacteristicKisaTransferCharacteristicDetailedanalysisshowsthatL=channellength,W=channelwidth,e=electronmobilityandCoxisthecapacitanceperunitareaformedbythethegate-oxide-substrateTransferCharacteristicDetaileTransferCharacteristicWecouldallowforchannellengthmodulationbyincludingasmalllineardependenceofIDonVDSinthesaturationregion.TransferCharacteristicNon-saturation(triode)regionNon-saturation(triode)regionNon-saturation(triode)regionNon-saturation(triode)regionSmallSignalModelInthesaturationregionwecanmodelthesquarelawdevicebyalinearsmallsignalequivalentcircuit.(Althoughthedevicecharacteristicisaparabolaweapproximateitbyastraightlineforsmallsignals)SmallSignalModelInthesaturSmallsignalmodelSmallsignalmodelFETSmallSignalEquivalentCircuitgmvgsvdsSDidG

vgsIg

0FETSmallSignalEquivalentCiFETSmallSignalEquivalentCircuitChannellengthmodulationmakestheoutputresistanceinsaturationfinitegmvgsvdsSDidG

vgsig

0r0FETSmallSignalEquivalentCiBiasCircuitA:VoltageDividerBiasRememberIG=0VDDGNDRDR1R2RSVGGID=ISIS=IDIGGDSBiasCircuitA:VoltageDivideBiasCircuitASolveVGS=VGG–IDRSwithtogive:BiasCircuitASolveVGS=VGGBiascircuitABiascircuitABiascircuitAThisisaquadraticequationforID.UsuallyonesolvesforIDthenforVGSanddeterminesthesmallsignaltransconductancegmasBiascircuitAThisisaquadraBiascircuitsWenotethatgmisdeterminedbythed.c.biasconditioninmuchthesamewayasthedynamicresistancerwasfortheBJT.BiascircuitsWenotethatgmiBiascircuitB(DrainFeedbackBias)VDDGNDRDRG~100MIG=0IDIS~IDB