電子器件-9-BJT-3課件

Chapter7.5:GeneralizedBiasingPresentedby:LeiWang7.5GeneralizedbiasingNormalbiasing(normalactivemode):theemitterjunctionisforward-biasedandthecollectorisreversebiased.27.5GeneralizedbiasingThecollectorsaturationcurrentisnegligible.Theactivepartofthebaseandthetwojunctionsareofuniformcross-sectionalareaA;currentflowinthebaseisessentiallyone-dimensionalfromemittertocollector.Theemitterjunctionisforwardbiased;thecollectorjunction

isreversebiased.37.5GeneralizedbiasingGeneralizedbiasing?E-Mequations(J.J.Ebers&J.L.Moll1954)J.J.Ebers(1921-1957)J.J.EbersAwardwasestablishedin1971byIEEEElectronDeviceSocietyForoutstandingtechnicalcontributionstoelectrondevices.IEEEInternationalElectronDeviceMeeting,USA57.5.1Thecoupled-diodemodelTheemitterandcollectorjunctionsarebothforwardbiased;whatwillhappen？67.5.1Thecoupled-diodemodel77.5.1Thecoupled-diodemodel+97.5.1Thecoupled-diodemodelForthesymmetricaltransistorForNormalMode107.5.1Thecoupled-diodemodelIE’-ICI,IC’-IEI,PE’PC,PC’PE(=0),Forinvertedmode117.5.1Thecoupled-diodemodel(7-18)becomes:137.5.1Thecoupled-diodemodelFortheasymmetricaltransistorForcomparison,Where,IESistheemittersaturationcurrentwiththecollectorjunctionshortcircuited.147.5.1Thecoupled-diodemodelFortheasymmetricaltransistorForcomparison,Where,ICSisthecollectorsaturationcurrentwiththeemitterjunctionshortcircuited.157.5.1Thecoupled-diodemodel177.5.1Thecoupled-diodemodelICOisthemagnitudeofthecollectorsaturationcurrentwiththeemitterjunctionopen(IE=0).IEOisthemagnitudeoftheemittersaturationcurrentwiththecollectorjunctionopen(IC=0).187.5.1Thecoupled-diodemodel197.5.2ChargeControlAnalysisTheemitterinjectionefficiency=1;Thecollectorsaturationcurrent=0t:thetransittimeforaaverageexcessholefromemittertocollectorp:theaverageholelifetimeLp:theholediffusionlength217.5.2ChargeControlAnalysisBecausethebasewidthWb<<Lp,t<<pInsteadystatethereareexcesselectronsandholesinthebase.ThechargeintheelectrondistributionQnisreplacedeverypseconds.iB=Qn/pThechargeintheholedistributionQpiscollectedeverytseconds.iC=Qp/t

Forspacechargeneutrality,Qn=Qp227.5.2ChargeControlAnalysis237.5.2ChargeControlAnalysisTheeffectsoftimedependenceofstoredchargecanbeincludedinaboveequationsbyaddingtherateofchangeofstoredchargetoeachoftheinjectioncurrentsIENandICI.257.5.2ChargeControlAnalysisTimedependenceofstoredchargexxx+xDiffusionlaw:thenumberofchargesthroughthecross-sectionperunitareaperunittime.Thenetincreaseinholechargewithinthetotalvolumeofdifferentialelementperunittimeduetodiffusion.ThenetincreaseinholeparticleperunitvolumeperunittimeAtpointx,thenetdecreaseinholeparticleperunitvolumeperunittimeduetorecombination.Atpointx,thenetincreaseinholeparticleperunitvolumeperunittime.Atpointx,thechangeofholeconcentrationwithtime.theContinuityequation(連續(xù)性方程）(diffusionequation)267.5.2ChargeControlAnalysisxxx+x277.6CharacteristicsofBJTCommon-emitter,IB~VEB

297.6CharacteristicsofBJT(2)OutputCharacteristicsCommon-base,IC~VCB

VCB>0,i.e.thecollectorjunctionispositivebiased.Atsomepoint,Ic=0saturationVEB=0Cut-off307.6CharacteristicsofBJTVCB<<-kT/qCommon-emitter,IC~VCE

VCB=0,saturationIB=0,Cutoff317.6CharacteristicsofBJTExampleAssumethatap-n-ptransistorisdopedsuchthattheemitterdopingistentimesthatinthebase,theminoritycarriermobilityintheemitterisone-halfthatinthebase,andthebasewidthisone-tenththeminoritycarrierdiffusionlength.Thecarrierlifetimesareequal.Calculateαandβforthistransistor.327.6CharacteristicsofBJTSolution:Wehaveknown,pp/nn=10,μnp/μpn=1/2,Wb/Lpn=1/10337.7Otherimportanteffects7.7.1DriftintheBaseRegionOneimportantresultofagradedbaseregionisthatabuilt-inelectricfieldexistsfromemittertocollector.Withinthebaseregion,thenetdopingconcentrationdecreasesalongaprofileThiselectricfieldwilladdadriftcomponentstothetransportofholesacrossthebase.347.7.1DriftintheBaseRegionAtequilibrium,inthebaseregion357.7.1DriftintheBaseRegionSincethisfieldaidsthetransportofholesacrossthebaseregionfromemittertocollector,thetransittimeisreducedbelowthatofacomparableuniformbasetransistor.Thisshorteningoftransittimecanbeveryimportantinhighfrequencydevices.367.7.2BaseNarrowingBase–WidthModulation，EarlyEffect（基區(qū)寬度調(diào)制效應(yīng)，Early效應(yīng)）ThedecreaseinWbcausesβtoincrease.

WhenVCBisincreasedfarenough,itispossibletodecreaseWbtotheextentthatthecollectordepletionregionfillstheentirebasepunchthrough.Atthispoint,Thetransistoractionislost.377.7.2BaseNarrowingAsaresult,thecollectorcurrentIcincreaseswithcollectorvoltageratherthanstayingconstantaspredictedfromthesimpletreatment.TheslopeintroducedbyEarlyeffectisalmostlinearwithIc.387.7.3AvalancheBreakdownBVCBO

：thebreakdownvoltagewhenIE=0inthecommon-basecaseBVCEO

：thebreakdownvoltageforIB=0inthecommon-emittercaseBVCEO<<BVCBOThereisastrongInfluenceofcarriermultiplicationoverafairlybroadrangeofcollectorvoltage.

397.7.3AvalancheBreakdownForcommon-basecase,IE=0,IC=MICOForcommon-emittercase,IB=0,IC=IE=MICO/(1-MαN)BVCEO<<BVCBOMαN~1Thebreakdownvoltageisdefinedastheinversebiasedwhenthereversecurrentreachesthegivenvalue.407.7.3AvalancheBreakdownPhysically,inthecommon-emittercase(forIB=0)whenanionizingcollisionoccursinthecollectorjunctiondepletionregion,asecondaryholeandelectronarecreated.Theprimaryandsecondaryholesandelectronsaresweptintothecollectorandthebaserespectively.Thesupplyofelectronsinthebaseisincreased,theholeinjectionattheemittermustincreasetomaintainspacechargeneutrality.AnincreasedholesinjectionfromtheemittercausesanincreasedmultiplicationcurrentatthecollectorJunction;thisinturnincreasestherateatwhichsecondaryelectronsaresweptintothebase,callingformoreholeinjection.417.7.4InjectionLevel;ThermalEffectIdeally，wehaveassumedαandβareindependentofcarrierinjectionlevel.Actually?TheemitterinjectionefficiencyThecurrenttransferratioTheamplificationfactor

TheBasetransportfactoriC=BiEPInjectionLevel427.7.4InjectionLevel;ThermalEffectForverylowinjection,therecombinationinthejunctionsmustbeconsidered.Thisisparticularlyimportantintheemitterjunction,whereanyrecombinationtendstodegradetheemitterinjectionefficiencyγ.Therefore,α,βshoulddecreaseforlowvaluesofIC,causingthecurvesofthecollectorcharacteristicstobespacedmorecloselyforlowcurrentsthanforhighercurrents.AsIcisincreasedbeyondthelowinjectionlevel,α,βincreases.Forveryhighinjectionlevel,α,βfallagain.Why?Larg