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1、Slide8 1 Chapter8BipolarJunctionTransistors Since1970 thehighdensityandlow poweradvantageoftheMOStechnologysteadilyerodedtheBJT searlydominance BJTsarestillpreferredinsomehigh frequencyandanalogapplicationsbecauseoftheirhighspeedandhighpoweroutput Question Whatisthemeaningof bipolar ModernSemiconduc
2、torDevicesforIntegratedCircuits C Hu Slide8 2 8 1IntroductiontotheBJT ICisanexponentialfunctionofforwardVBEandindependentofreverseVCB ModernSemiconductorDevicesforIntegratedCircuits C Hu Slide8 3 Common EmitterConfiguration Question WhyisIBoftenpreferredasaparameteroverVBE ModernSemiconductorDevices
3、forIntegratedCircuits C Hu Slide8 4 SemiconductorDevicesforIntegratedCircuits C Hu 8 2CollectorCurrent tB baserecombinationlifetimeDB baseminoritycarrier electron diffusionconstant Boundaryconditions Slide8 5 Itcanbeshown GB s cm4 isthebaseGummelnumber 8 2CollectorCurrent ModernSemiconductorDevicesf
4、orIntegratedCircuits C Hu Slide8 6 Atlow levelinjection inverseslopeis60mV decade High levelinjectioneffect 8 2 1HighLevelInjectionEffect AtlargeVBE Whenp NB inverseslopeis120mV decade ModernSemiconductorDevicesforIntegratedCircuits C Hu Slide8 7 8 3BaseCurrent SomeholesareinjectedfromtheP typebasei
5、ntotheN emitter Theholesareprovidedbythebasecurrent IB b ModernSemiconductorDevicesforIntegratedCircuits C Hu Slide8 8 IsalargeIBdesirable Why 8 3BaseCurrent ModernSemiconductorDevicesforIntegratedCircuits C Hu Slide8 9 8 4CurrentGain HowcanbFbemaximized Common emittercurrentgain bF Common basecurre
6、ntgain Itcanbeshownthat ModernSemiconductorDevicesforIntegratedCircuits C Hu Slide8 10 EXAMPLE CurrentGainABJThasIC 1mAandIB 10mA WhatareIE bFandaF Solution Wecanconfirm and ModernSemiconductorDevicesforIntegratedCircuits C Hu Slide8 11 8 4 1EmitterBandgapNarrowing Emitterbandgapnarrowingmakesitdiff
7、iculttoraisebFbydopingtheemitterveryheavily ModernSemiconductorDevicesforIntegratedCircuits C Hu Slide8 12 TofurtherelevatebF wecanraiseniBbyusinganepitaxialSi1 hGehbase Withh 0 2 EgBisreducedby0 1eVandniE2by30 x 8 4 2Narrow BandgapBaseandHeterojuncionBJT ModernSemiconductorDevicesforIntegratedCircu
8、its C Hu Slide8 13 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 EmitterBandgapNarrowingandSiGeBase ModernSemiconductorDevicesforIntegratedCircuits C Hu S
9、lide8 14 Ahigh performanceBJTtypicallyhasalayerofAs dopedN poly siliconfilmintheemitter bFislargerduetothelargeWE mostlymadeoftheN poly silicon Adeepdiffusedemitterjunctiontendstocauseemitter collectorshorts 8 4 3Poly SiliconEmitter ModernSemiconductorDevicesforIntegratedCircuits C Hu Slide8 15 Whyd
10、oesonewanttooperateBJTsatlowICandhighIC WhyisbFafunctionofVBCintherightfigure bF Fromtoptobottom VBC 2V 1V 0V 8 4 4GummelPlotandbFFall offatHighandLowIc Hint SeeSec 8 5andSec 8 9 SCRBEcurrent ModernSemiconductorDevicesforIntegratedCircuits C Hu Slide8 16 8 5Base WidthModulationbyCollectorVoltage Out
11、putresistance LargeVA largero isdesirableforalargevoltagegain IB3 IC V CE 0 V A VA EarlyVoltage IB2 IB1 ModernSemiconductorDevicesforIntegratedCircuits C Hu Slide8 17 Howcanwereducethebase widthmodulationeffect 8 5Base WidthModulationbyCollectorVoltage ModernSemiconductorDevicesforIntegratedCircuits
12、 C Hu Slide8 18 Thebase widthmodulationeffectisreducedifwe A Increasethebasewidth B Increasethebasedopingconcentration NB or C Decreasethecollectordopingconcentration NC Whichoftheaboveisthemostacceptableaction 8 5Base WidthModulationbyCollectorVoltage ModernSemiconductorDevicesforIntegratedCircuits
13、 C Hu Slide8 19 8 6Ebers MollModel TheEbers MollmodeldescribesboththeactiveandthesaturationregionsofBJToperation ModernSemiconductorDevicesforIntegratedCircuits C Hu Slide8 20 ICisdrivenbytwotwoforces VBEandVBC WhenonlyVBEispresent Nowreversetherolesofemitterandcollector WhenonlyVBCispresent bR reve
14、rsecurrentgainbF forwardcurrentgain 8 6Ebers MollModel ModernSemiconductorDevicesforIntegratedCircuits C Hu Slide8 21 Ingeneral bothVBEandVBCarepresent Insaturation theBCjunctionbecomesforward biased too VBCcausesalotofholestobeinjectedintothecollector ThisusesupmuchofIB Asaresult ICdrops VCE V 8 6E
15、bers MollModel ModernSemiconductorDevicesforIntegratedCircuits C Hu Slide8 22 8 7TransitTimeandChargeStorage WhentheBEjunctionisforward biased excessholesarestoredintheemitter thebase andeveninthedepletionlayers QFisallthestoredexcessholecharge tFdeterminesthehigh frequencylimitofBJToperation tFisdi
16、fficulttobepredictedaccuratelybutcanbemeasured ModernSemiconductorDevicesforIntegratedCircuits C Hu Slide8 23 8 7 1BaseChargeStorageandBaseTransitTime Let sanalyzetheexcessholechargeandtransittimeinthebaseonly ModernSemiconductorDevicesforIntegratedCircuits C Hu Slide8 24 WhatistFBifWB 70nmandDB 10cm2 s Answer 2 5psisaveryshorttime Sincelightspeedis3 108m s lighttravelsonly1 5mmin5ps EXAMPLE BaseTransitTime ModernSemiconductorDevicesforIntegratedCircuits C Hu Slide8 25 Thebasetransittimecanbered
