最新BJT双极型晶体管课件PPT.ppt 95页

'BJT双极型晶体管 8.1IntroductiontotheBJTICisanexponentialfunctionofforwardVBEandindependentofreverseVCB.ModernSemiconductorDevicesforIntegratedCircuits(C.Hu)NPNBJT:N+PNECBVBEVCBEmitterBaseCollector2 Common-EmitterConfigurationQuestion:WhyisIBoftenpreferredasaparameteroverVBE?ModernSemiconductorDevicesforIntegratedCircuits(C.Hu)3 8.3BaseCurrentSomeholesareinjectedfromtheP-typebaseintotheN+emitter.Theholesareprovidedbythebasecurrent,IB.pE"nB"WEWB(b)emitterbasecollectorcontactIEICelectronflow–+holeflowIB(a)contactModernSemiconductorDevicesforIntegratedCircuits(C.Hu)7 IsalargeIBdesirable?Why?8.3BaseCurrentemitterbasecollectorcontactIEICelectronflow–+holeflowIB(a)contactModernSemiconductorDevicesforIntegratedCircuits(C.Hu)Forauniformemitter,8 8.4CurrentGainHowcanbFbemaximized?Common-emittercurrentgain,bF:Common-basecurrentgain:ItcanbeshownthatModernSemiconductorDevicesforIntegratedCircuits(C.Hu)9 EXAMPLE:CurrentGainABJThasIC=1mAandIB=10mA.WhatareIE,bFandaF?Solution:WecanconfirmandModernSemiconductorDevicesforIntegratedCircuits(C.Hu)10 8.4.1EmitterBandgapNarrowingEmitterbandgapnarrowingmakesitdifficulttoraisebFbydopingtheemitterveryheavily.ModernSemiconductorDevicesforIntegratedCircuits(C.Hu)ToraisebF,NEistypicallyverylarge.Unfortunately,largeNEmakes(heavydopingeffect).SinceniisrelatedtoEg,thiseffectisalsoknownasband-gapnarrowing.DEgEisnegligibleforNE<1018cm-3,is50meVat1019cm-3,95meVat1020cm-3,and140meVat1021cm-3.11 TofurtherelevatebF,wecanraiseniBbyusinganepitaxialSi1-hGehbase.Withh=0.2,EgBisreducedby0.1eVandniE2by30x.8.4.2Narrow-BandgapBaseandHeterojuncionBJTModernSemiconductorDevicesforIntegratedCircuits(C.Hu)12 AssumeDB=3DE,WE=3WB,NB=1018cm-3,andniB2=ni2.WhatisbFfor(a)NE=1019cm-3,(b)NE=1020cm-3,and(c)NE=1020cm-3andaSiGebasewithDEgB=60meV?(a)AtNE=1019cm-3,DEgE50meV,(b)AtNE=1020cm-3,DEgE95meV(c)EXAMPLE:EmitterBandgapNarrowingandSiGeBaseModernSemiconductorDevicesforIntegratedCircuits(C.Hu)13 Ahigh-performanceBJTtypicallyhasalayerofAs-dopedN+poly-siliconfilmintheemitter.bFislargerduetothelargeWE,mostlymadeoftheN+poly-silicon.(Adeepdiffusedemitterjunctiontendstocauseemitter-collectorshorts.)N-collectorP-baseSiO2emitterN+-poly-Si8.4.3Poly-SiliconEmitterModernSemiconductorDevicesforIntegratedCircuits(C.Hu)14 WhydoesonewanttooperateBJTsatlowICandhighIC?WhyisbFafunctionofVBCintherightfigure?bFFromtoptobottom:VBC=2V,1V,0V8.4.4GummelPlotandbFFall-offatHighandLowIcHint:SeeSec.8.5andSec.8.9.SCRBEcurrentModernSemiconductorDevicesforIntegratedCircuits(C.Hu)15 8.5Base-WidthModulationbyCollectorVoltageOutputresistance:LargeVA(largero)isdesirableforalargevoltagegainIB3ICVCE0VAVA:EarlyVoltageIB2IB1ModernSemiconductorDevicesforIntegratedCircuits(C.Hu)16 Howcanwereducethebase-widthmodulationeffect?8.5Base-WidthModulationbyCollectorVoltageN+PNemitterbasecollectorVCEWB3WB2WB1xn"}VCE1