研究生入学考试

研究生入学考试第1页/共37页Wewillusethep-n-ptransistorformostillustration.Themainadvantageofthep-n-pfordiscussingtransistoractionisthatholeflowandcurrentareinthesamedirection.Oncethesebasicideasareestablishedforthep-n-pdevice,itissimpletorelatethemtothemorewidelyusedthen-p-ntransistor.第2页/共37页7.1FundamentalsofBJToperationTounderstandhowthecurrentthroughtwoterminalscanbecontrolledbysmallchangesinthecurrentatathirdterminal,let’sconsiderthereverse-biasedp-njunction第3页/共37页Thereversecurrentdependsontherateatwhichminoritycarriersaregeneratedintheneighborhoodofthejunction.Thereversecurrentisindependentofthereversebias.TheholecurrentdependsonhowoftenminorityholesaregeneratedbyEHPcreation,notuponhowfastaparticularholeissweptacrossthedepletionlayerbythefield.第4页/共37页Howtoincreasethereversecurrent?opticalexcitationwithlighth>Egholeinjectiondevice:Injectholesatpredeterminedrateintothensideofthejunction.Thecurrentfromntopwilldependontheholeinjectionrate,andwillbeessentiallyindependentofthebiasvoltage第5页/共37页Aconvenientholeinjectiondeviceisaforward-biasedp+-njunction.Inp+-njunction,thecurrentisdueprimarilytoholesinjectedfromthep+intothenregion.Ifwemakeuseofthesamensideforbothforwardandreverse-biasedjunction,wecanobtainthep+-n-pstructure.Inthisstructure,injectionofholesfromp+-njunctionintothenregionsuppliestheminoritycarrierholestoparticipateinthereversecurrentthroughthen-pjunction.Toenablethep+-n-pstructuretonormallyoperate,itisimportantthattheinjectedholesdonotrecombineinthenregionbeforetheycandiffusetothedepletionlayerofthereversebiasedjunction.So,wemustmakethenregionnarrowerthantheholediffusionlength,Wb<<Lp.forward-biasedreverse-biased第6页/共37页第7页/共37页InjectedholeslosttorecombinationinthebaseHolesreachingthereverse-biasedcollectorjunctionThermallygeneratedelectronsandholesmakingupthereversesaturationcurrentofthecollectorjunctionElectronssuppliedbythebasecontactforrecombinationwithholesElectronsinjectedacrosstheforward-biasedemitterjunction第8页/共37页Thebasecurrentoriginatesfromthefollowingthreemechanisms:Theremustbesomerecombinationofinjectedholeswithelectronsinthebase,evenwithWb<<Lp.Theelectronslosttorecombinationmustberesuppliedthroughthebasecontact.Someelectronswillbeinjectedfromntopintheforwardbiasedemitterjunction,eveniftheemitterisheavilydopedcomparedtothebase.TheseelectronsmustalsobesuppliedbyIB.Someelectronsaresweptintothebaseatthereverse-biasedcollectorjunctionduetothermalgenerationinthecollector.ThissmallcurrentreducesIBbysupplyingelectronstothebase.第9页/共37页7.2AmplificationwithBJTSomeassumption:(1)Neglectingthesaturationcurrentatthecollector;(2)Neglectingtherecombinationinthedepletion(transition)region.Signals:dc+ac,atlowfrequency第10页/共37页TheemitterinjectionefficiencyThecurrenttransferratioTheamplificationfactor

TheBasetransportfactoriC=BiEp第11页/共37页ItremainstobeshownthatthecollectorcurrentiCcanbecontrolledbyvariationsinthesmallcurrentiB.Infact,wecanshowfromspacechargeneutralityargumentsthatiBcanindeedbeusedtodeterminethemagnitudeofiC.Assumetheemitterinjectionefficiency=1;Thecollectorsaturationcurrent=0iE=iEp,andiBonlyconsistsoftheelectroncurrentsuppliedbythebasecontactforrecombinationwithholes,i.e.theaverageelectronwaitspsecondsforrecombination.第12页/共37页t:thetransittimeforaaverageexcessholefromemittertocollectorp:theaverageholelifetimeLp:theholediffusionlengthBecausethebasewidthWb<<Lp,t<<p第13页/共37页Whiletheaverageelectronwaitsp

secondsforrecombination,manyindividualholescanenterandleavethebaseregion,eachwithanaveragetransittimet.Inparticular,foreachelectronenteringfromthebasecontact,p/t

holescanpassfromemittertocollectorwhilemaintainingspacechargeneutrality.Thus第14页/共37页Iftheelectronsupplytothebase(iB)isrestricted,thetrafficofholesfromtheemittertobaseiscorrespondinglyreduced.Thiscanbearguedbysupposingthattheholeinjectiondoescontinuedespitetherestrictiononelectronsfromthebasecontact.Theresultswouldbeanetbuildupofpositivechargeinthebaseandalossofforwardbias(andthereforealossofholeinjection)attheemitterjunction.Clearly,thesupplyofelectronsthroughiBcanbeusedtoraiseorlowertheholeflowfromemittertocollector(iC).第15页/共37页第16页/共37页Example7-1Solution:(a)Insteadystatethereareexcesselectronsandholesinthebase.ThechargeintheelectrondistributionQnisreplacedeverypseconds.iB=Qn/pThechargeintheholedistributionQpiscollectedeverytseconds.iC=Qp/t

Forspacechargeneutrality,Qn=Qp第17页/共37页(b)Insteadystate,第18页/共37页*7.3BJTfabrication第19页/共37页第20页/共37页第21页/共37页7.4Minoritycarrierdistributionsandterminalcurrent(少数载流子分布和端电流）第22页/共37页Assumptions:Holesdiffusefromemittertocollector;driftisnegligibleinthebaseregion.Theemittercurrentismadeupentirelyofholes;theemitterinjectionefficiency=1.Thecollectorsaturationcurrentisnegligible.Theactivepartofthebaseandthetwojunctionsareofuniformcross-sectionalareaA;currentflowinthebaseisessentiallyone-dimensionalfromemittertocollector.Allcurrentandvoltagesaresteadystate.第23页/共37页7.4.1SolutionoftheDiffusionEquationinthebaseregionVEB>>kT/qVCB<<0Excessholeconcentrationsattheedgeoftransitionregions,ΔpEandΔpC?第24页/共37页Oncetheminoritycarriersreachattheedgeoftransitionregions,theywilldiffusionandproduceadistributionδpinthebase.Theholedistributioninbaseregionisdifferentfromthatinap-njunction.第25页/共37页Diffusionequation:Boundaryconditions:第26页/共37页第27页/共37页Forcollectorjunctionstrongreverse-biased,pC