HeterojunctionPresentedby:LeiWangHeterojunctionjunctions

p-n

junctionswithinasinglesemiconductor(homojunction)junctionsbetweenametalandasemiconductor?junctionsbetweentwosemiconductorswithdifferentbandgaps.2HeterojunctionHomojunctionAjunctionbetweenametalandasemiconductor3Heterojunction(1)differentmaterials,anddifferentdopedtype5Heterojunction6Heterojunctionx0x2x1BarrierWidthW,

Chargesinthedepletionregion,Possion’sequations:7Heterojunctionx0x2x1Atequilibrium,thepotentialcontactV0,9Heterojunctionx0x2x110Heterojunctionx0x2x111Heterojunction13Heterojunction(2)Differentmaterial,butonedopedtype(Example5-7,P237)

TheseelectronsareconfinedinanarrowpotentialwellintheGaAsconductionband.Ifweconstructadeviceinwhichconductionoccursparalleltotheinterface,theelectronsinthepotentialwellformatwo-dimensionelectrongaswithveryhighmobility.14HeterojunctionSomeexamples:HeterojunctionsapplicationsHEMTHBJHL(D)156.3.2TheHighElectronMobilityTransistor(HEMT)

Toimprovetheperformancesofatransistor,itshouldmaintainhightransconductanceincreasetheoperationspeed(orhighmobility)Tomaintainthehightransconductance,thechannelconductivitymustbeashighaspossible.Thiscanbecompletedbyincreasingthedopinginthechannelandthuscarrierconcentration.However,increaseddopingalsocausesincreasedscatteringbytheionizedimpurities,whichleadstoadegradationofmobility.(seeP105,Fig.3-23)17ItisalsocalledamodulationdopedFET(MODFET)(調(diào)制摻雜場效應(yīng)晶體管

)orhighelectronmobilitytransistor(HEMT)Eg=1.85eV(AlGaAs)Eg=1.43eV(GaAs)UndopedGaAsn-dopedHeterojunction!WhatisneededisawayofcreatingahighelectronconcentrationinthechannelofaMESFETbysomemeansotherthandoping.(page262)187.9HeterojunctionBipolarTransistor(HBT)Forbipolartransistoramplification,wehopeemitterinjectionefficiencycanbeashighaspossible,andtherefore,and.From(7-25),highvalueofcanbemaintainedbyusinglightlydopedmaterialforthebaseregionandheavilydopedmaterialfortheemitter.197.8.4High-FrequencyTransistor2.

CjeCjc:theemitterandcollectorareasmustbesmall3.rb,rc,re:RCchargingtimes,theseriesresistancemustbedesignedtobeassmallaspossible.4.Wb:thebasewidthmustbeasshortaspossible,andD（

t=Wb2/2Dp)

:D/=kT/q,n-p-ntransistorsareusuallypreferred.1.Thephysicalsizeofthedevicemustbekeptsmall217.8.4High-FrequencyTransistorWhatisthesolutiontothisproblem?HeterojunctionBipolarTransistor!227.8.4High-FrequencyTransistorForn-p-ntransistor238.4.