Chapter7:BipolarJunctionTransistorsPresentedby:LeiWang7.6SwitchingRcU+-27.6SwitchingIfiBissuchthattheoperatingpointliessomewherebetweenthetwoendpointsoftheloadline,thetransistoroperatesinthenormalactivemode.Thatis,theemitterjunctionisforwardbiasedandthecollectorisreversebiased.Ifthebasecurrentiszeroornegative,thepointCisreachedatthebottomendoftheloadline,andthecollectorcurrentisnegligible.Thisisthe“off”stateofthetransistor.IfiBispositiveandsufficientlylarge,thedeviceisdriventothesaturationregime.Thisisthe“on”stateofthetransistor,inwhichalargevalueofiCflowswithaverysmallvoltagedropvCE.37.6.1CutoffIftheemitterjunctionisreversebiasedinthecutoffregime(negativeiB),theexcessholeconcentrationsattheedgesofthereverse-biasedemitterandcollectorjunctionwillbeasand,57.6.1CutoffThebasecurrentiBcanbeapproximatedforasymmetricaltransistoronachargestoragebasisas–qApnWb/τp.Inthiscalculationanegativeexcessholeconcentrationcorrespondstogenerationinthesamewaythatapositivedistributionindicatesrecombination.Physically,asmallsaturationcurrentflowsfromntopineachreverse-biasedjunction,andthiscurrentissuppliedbythebasecurrentiB(which

isnegativewhenflowing

intothebaseofap-n-pdeviceaccordingtoourdefinitions.)67.6.1CutoffIftheshort-circuitsaturationcurrentsIESandICSaresmallandαNandαIarebothnearunity,thesecurrentswillbenegligibleandthecutoffregimewillcloselyapproximatethe“off”conditionofanidealswitch.77.6.2Saturation(1)Whenpc=0(collectorjunctioniszerobiased),thedeviceissaturated.Withtheloadlinefixedbythebatteryandthe5-kΩresistor,saturatedisreachedbyincreasingthebasecurrentiB.-vCEisonlyafractionofavolt,andicisapproximately40V/5k=8mA.97.6.2Saturation(2)Theforwardbiasofthecollectorjunctionleadstopositivepc,drivingthedevicefurtherintosaturation.AnincreaseiniBcallsforanincreaseintheareaunderthep(xn)distribution.Thecollectorcurrentstaysessentiallyconstantwhilethebasecurrentincreases.Thisisapproximatelythe“on”state.107.6.2SaturationWhereasthedegreeof“over-saturation”doesnotaffectthevalueoficsignificantly,itisimportantindeterminingthetimerequiredtoswitchthedevicefromonestatetotheother.117.6.4SpecificationsforswitchingtransistorsSincetheemitterjunctionisreversebiasedincutoff,itisnecessaryfortheemitterspacechargelayertobechargedtotheforwardbiasconditionbeforecollectorcurrentcanflow.Adelaytimetdshouldbeaccounted.back137.8FrequencylimitationsoftransistorsJunctioncapacitances:Chargingtimesrequiredwhenexcesscarrierdistributionarealtered;

TransittimeWhatwillbethepropertiesofbipolartransistorsunderhighfrequencyoperation147.8FrequencylimitationsoftransistorsThefrequencylimitations:JunctioncapacitancesofbothemitterandcollectorjunctionChargingtimeswhenexcesscarrierdistributionsarealteredTransittimeofcarriersacrossthebaseregion157.8.1CapacitanceandchargingtimesBJToperatesinthenormalstate,thetotalcurrentisequaltod-cplusa-c:theamplitudeofa-cismuchlowerthanthatofd-c.d-c:VBEVCEICIBIEa-c:vbevceicibieTotal:vBEvCEiCiBiE177.8.1CapacitanceandchargingtimesWhenasmalla-csignalisappliedtotheemitterp-njunctionalongwithd-clevel,187.8.1CapacitanceandchargingtimesThechargeinthebaseregionisQN(t)Gse:a-cconductanceofdiodeCse:a-ccapacitanceofdiode197.8.1CapacitanceandchargingtimesHybrid-pimodelEquivalenta-ccircuit217.8.1CapacitanceandchargingtimesSeveralchargingtimesareimportantfromFig.7-24b:ThetimetochargetheemitterandthecollectordepletionregionsThedelaytimeinalteringthechargedistributionsinthebaseregionCutofffrequencyfT:thefrequencyatwhichthea-camplification((a-c)=ic/ib)forthedevicedropstounity.???Otherdelaytimes:ChargestoragetimeinthecollectorregionTransittimethroughthecollectordepletionregionAllofthesetimesareincludedind,thentheupperfrequencylimit227.8.1Capacitanceandchargingtimes通常情況下，當(dāng)晶體管的工作頻率變化時(shí)，晶體管的放大特性發(fā)生變化。頻率升高，放大系數(shù)減小。這主要是因?yàn)閯?shì)壘電容和擴(kuò)散電容的充放電效應(yīng)。通常定義當(dāng)電流放大系數(shù)下降到低頻值的1/2時(shí)的頻率稱為截止頻率(cutofffrequency)。當(dāng)放大系數(shù)下降為1時(shí)的頻率稱為特征頻率（characteristicsfrequency)。237.8.3Webstereffectt=wb2/2Dpisvalidforlowinjection,itwillbereducedbyuptoafactorof2underhighlevelinjection.

257.8.3WebstereffectUnderhighlevelinjection,tomatchtheminoritycarrierconcentration,themajoritycarrierconcentrationdecreasesfromthebase-emitterjunctiontobase-collectorjunction.Thistendstocreateadiffusionofmajoritycarrierfromemittertobase.Abuilt-inelectricfielddevelopsinthebasetocreateanopposingmajoritycarrierdriftcurrent.Thedirectionofthisinducedfieldaidstheminoritycarriertransportfromtheemittertothecollector,reducingthetransittimet.267.8.4High-FrequencyTransistor2.

CjeCjc:theemitterandcollectorareasmustbesmall3.rb,rc,re:RCchargingtimes,theseriesresistancemustbedesignedtobeassmallaspossible.4.Wb:thebasewidthmustbeas