FieldEffectTransistors(FETs)Inafieldeffecttransistor,currentflowthroughasemiconductorchanneliscontrolledbytheapplicationofanelectricfield(voltage)perpendiculartothedirectionofcurrentflow.WeconsidertheMOSFET(MOST)-themetaloxidesemiconductor(fieldeffect)transistor.精品文檔FieldEffectTransistors(FETs)InadepletionmodeMOSFETachannelisbuiltinsothatconductionoccurswithnocontrolvoltageapplied.Weconsiderann-channelenhancementmodedevicewhichhasno‘built-in’conductionchannel.精品文檔N-channelenhancementmodeMOSFET(schematic)Oxide(SiO2)n+n+p-typesiliconGate(G)Drain(D)Source(S)精品文檔SymbolforN-channelenhancementmodeMOSFETNoteconnectiontosubstrateshown精品文檔SymbolforN-channelenhancementmodeMOSFETNoteconnectiontosubstrateshownGSD精品文檔N-channelenhancementmodeMOSFET:outlineofoperationVGS=0,littleornocurrentcanflowSD,backtobackp-njunctions.AsapositiveVGSisapplied,holesinthep-regionarerepelled.AsVGSincreasesfurther,electronsareattractedfromthesubstratetowardsthepositivegate.Aninversionlayerofmobileelectronsformsnearthesiliconsurface.精品文檔N-channelenhancementmodeMOSFETwithinversionlayerSource(S)Oxide(SiO2)n+n+p-typesiliconGate(G)Drain(D)Inversionlayer(conductionchannel)精品文檔N-channelenhancementmodeMOSFET:outlineofoperationThisinversionlayeriseffectivelyn-type.Electronscancarrycurrentinancontinuousn-typepath(orchannel)fromsourcetodrain.TheconductionchannelformswhenVGSattainsathresholdvoltage,VT精品文檔N-channelenhancementmodeMOSFETThevalueofVTisdeterminedbythedeviceprocessandthelevelofthep-type(substrate)doping.Notethatann-channeldeviceisformedwithap-typesubstrate.精品文檔N-channelenhancementmodeMOSFETAsVGSincreasesfurther,moreelectronsaredrawnintotheinversionlayerandthechannelresistancedecreases(Thedeviceoperatesasavoltagecontrolledresistor).However……精品文檔N-channelenhancementmodeMOSFETThevoltagebetweengateandchannelvariesfromVGSatthesourceendtoVGS-VDSatthedrainend.Thusasthevoltageatthedrainend,VDS,isincreasedtheeffectivegate-channelvoltage(verticalfield)isdecreased.精品文檔N-channelenhancementmodeMOSFETThiscausesthecarrierdensityatthedrainendoftheinversionlayertodecrease.Thecurrentlevelsofforsaturates.TheFETisthensaidtohaveentereditssaturationregion(ThisexpressionhadadifferentmeaninginourdiscussionofBJTs).精品文檔N-channelenhancementmodeMOSFETTheboundarywheresaturationstartsoccursat(VGS–VDS)=VT.i.e.VDS=(VGS–VT)IdeallywewouldwantafurtherincreaseinVDStohavenoeffectonthedraincurrentID精品文檔N-channelenhancementmodeMOSFETInpracticeasVDSisincreasedfurtherIDalsoincreases.ThisisduetoareductionintheeffectivechannellengthwithVDS.Wenote,fordesignpurposes,thatthegatecurrent(IG)iseffectivelyzero.(Wehaveaninsulatingoxideregion)精品文檔N-channelDepletionmodeMOSFETInadepletionmodeMOSTachannelisbuiltintothedevice.ConductionoccursatVDS=0.(ThethresholdvoltageVTisnegative)精品文檔SymbolforN-channeldepletionmodeMOSFETConnectiontosubstrateshown.ThesolidlineindicatesthataphysicalchannelexistsatVGS=0.精品文檔P-channeldevicesThesearewidelyused,particularlyinComplementaryMOS(C-MOS)circuits.Electronmobilityislargerthanholemobilityson-channeldevicesarefaster,i.e.theyhaveahigherfrequencyresponse.精品文檔PlotsandEquations,Enhancementmode,VT2VVDS=VGS-VT精品文檔FETPlotsandequationsTheFETisavoltagecontrolleddevice.The‘controlparameter’isVGS–comparewithIBintheBJT.精品文檔LooseequivalencesFETDrainSourceGateBJTCollectorEmitterBase精品文檔PlotsandEquationsVDS=VGS-VTSaturationregion精品文檔PlotsandEquationsVDS=VGS-VT‘Triode’region精品文檔TransferCharacteristicGivestherelationshipbetweenIDandVGSinthesaturation(constantcurrent)regime.e.g.foranenhancementmodedeviceVTVGSID精品文檔TransferCharacteristicThisI-VcurveinthesaturationregioncanbeapproximatedbytheparabolaThisequationisveryimportant!TheID-VGSrelationshipisnotalinearone!精品文檔TransferCharacteristicKisadeviceparameterwhosevalueistypically0.25mA/V2精品文檔TransferCharacteristicDetailedanalysisshowsthatL=channellength,W=channelwidth,e=electronmobilityandCoxisthecapacitanceperunitareaformedbythethegate-oxide-substrate精品文檔TransferCharacteristicWecouldallowforchannellengthmodulationbyincludingasmalllineardependenceofIDonVDSinthesaturationregion.精品文檔Non-saturation(triode)region精品文檔Non-saturation(triode)region精品文檔SmallSignalModelInthesaturationregionwecanmodelthesquarelawdevicebyalinearsmallsignalequivalentcircuit.(Althoughthedevicecharacteristicisaparabolaweapproximateitbyastraightlineforsmallsignals)精品文檔Smallsignalmodel精品文檔FETSmallSignalEquivalentCircuitgmvgsvdsSDidG

vgsIg

0精品文檔FETSmallSignalEquivalentCircuitChannellengthmodulationmakestheoutputresistanceinsaturationfinitegmvgsvdsSDidG

vgsig

0r0精品文檔BiasCircuitA:VoltageDividerBiasRememberIG=0VDDGNDRDR1R2RSVGGID=ISIS=IDIGGDS精品文檔BiasCircuitASolveVGS=VGG–IDRSwithtogive:精品文檔BiascircuitA精品文檔BiascircuitAThisisaquadraticequationforID.UsuallyonesolvesforIDthenforVGSanddeterminesthesmallsignaltransconductancegmas精品文檔BiascircuitsWenotethatgmisdeterminedbythed.c.biasconditioninmuchthesamewayasthedynamicresistancerwasfortheBJT.精品文檔BiascircuitB(DrainFeedbackBias)VDDGNDRDRG~100MIG=