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1、6.012-ElectronicDevicesandCircuitsLecture12-MOSFETBasics-Outline•AnnouncementsGC+SiO2vGCOnwebsite-2write-upsonMOSFETmodels–vBC<0•Review-MOSCapacitorn+(n-MOSexample)–vBCp-Si+AccumulationDepletionInversionvGCVFB0VTBFlat-bandvoltage:VFB≡vGBsuchthatf(0)=fp-Si;VFB=fp-Si–fmThresholdvoltage:VT
2、≡vGCsuchthatf(0)=–fp-Si;VT(vBC)=VFB–2fp-Si+[2eSiqNA(
3、2fp-Si
4、–vBC)]1/2/Cox*(vatthreshold)GCInversionlayersheetchargedensity:qN*=–Cox*[vGC–VT(vBC)]•TheMOSFET-qualitativeexplanationDefinitionofstructure:cross-section(Example:n-channelMOSFET)Gateaction(creatingachannel)andchannelcurrent(dri
5、ft)•Quantitativemodeling-theGradual-ChannelApproximationThegateandsubstratecurrents:iG(vGS,vDS,vBS),iB(vGS,vDS,vBS)Thedraincurrent:iD(vGS,vDS,vBS)1.thein-planeproblem(relatingvoltagedropalongchanneltochannelcharge)2.thenormalproblem(relatinggatevoltagetochannelcharge)3.thefulldraincurre
6、ntexpression(quadraticapproximation)CharacteristicsandregionsofoperationClifFonstad,10/03Lecture12-Slide1GC+SiO2vGC–Ann-channelvBC<0n+–MOSFETcapacitorvBCp-Si+ReviewingtheresultsofLecture11BFlat-bandvoltage:V≡vatwhichf(0)=fFBGBp-SiV=f-fFBp-SimThresholdvoltage:V≡vatwhichf(0)=-f+vTGCp-SiBC
7、11/2V(v)=V-2f+2eqN2f-vTBCFBp-Si*{SiA[p-SiBC]}CoxAccumulationDepletionInversionvGCV†FB0VT**Inversionlayersheetchargedensity:q=-C[v-V(v)]NoxGCTBC**Accumulationlayersheetchargedensity:q=-C[v-V)]PoxGBFBClifFonstad,10/03Lecture12-Slide2†Ann-channelMOSFETGSvGS+vDSD–iGiDn+n+p-SivBS+iBBClifFons
8、tad,10/03Lecture12-Slide3Ann-channelMOSFETshowinggradualchannelaxesGSvGS+vDSD–iGiDn+0n+xy0Lp-SivBS+iBBExtentintoplane=WClifFonstad,10/03Lecture12-Slide4GradualChannelApproximationi-vModeling(n-channelMOSusedastheexample)Restrictvoltageranges:vBS≤0,vDS≥0,soiG(vGS,vDS,vBS)≈0,iB(vGS,vDS,vB
9、S)≈0Theissueisthedraincurrent(andonlywhenv>V,otherwisei=0):GSTD•Thein-planeproblem:Lookingatelectronmotioninthechannelwewritethedraincurrentas**dvcsi=-Ws(y)q(y)=-Wmq(y)(atmoderateE-fields)DeynendyThiscanbeintegratedfromy=0toy=L,andvCS=0tovCS=vDS,togetiD(vGS,vDS,vBS),butfirstwen
