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1、CarrierTransportPhenomenaCarrierTransportPhenomena1.CarrierDriftCarrierDrift2.CarrierMobility3.CarrierDiffusion4.EinsteinRelation5.TotalCurrentS.EquationSummaryEquationSummaryReference•D.A.Neamen,“SemiconductorPhysicsandDevices”(McGraw-Hill,2003),Cha
2、pter5.CarrierTransportPhenomena–CarrierDrift1.CarrierDrift•Therearetwomechanismsthatcausechargestomoveinsemiconductors:driftanddiffusion.•Driftisthemotionofchargecarriersduetothepresenceofanelectricfield.•Carriers(ieelectronsandholes)willmoveunderthe
3、influenceofanCarriers(i.e.,electronsandholes)willmoveundertheinfluenceofanelectricfieldbecausethefieldwillexertaforceonthecarriersaccordingto:rrF=Q⋅EwhereQisthechargeofthecarrier(+eforholes,-eforelectrons).CarrierTransportPhenomena–CarrierDrift•Curre
4、ntistherateofflowofcharge(C/s).So,fora(semi)conductorofcross-sectionalareaA,wehave,Idrift=NAQvdwhereI=diftdriftcurrent(At(AorC/C/)s)driftN=carrierconcentration(cm-3)A=conductorarea(cm2)Q=carriercharge(C)=carriercharge(C)v=driftvelocityofcarriers(cm/s
5、)dCarrierTransportPhenomena–CarrierDrift•Withzeroappliedfield,thedriftvelocityofanelectron(hole)iszero,butthisdoesnotmeanthattheelectron(hole)isstationary.•Anelectron(hole)hasthermalenergyandwillbouncerandomlytoandfrobyinteractingwiththeatomsinthelat
6、ticesuchthatitsaveragedisplacementwithtimeiszero.Therandomthermalvelocity,isveryhigh–offththdfeorderof10107cm/t/satroomttemperatture.•Withanelectricfield,theelectron(hole)willstillbescatteredbythelattice,butitwillexperiencenetmotioninthedirectionofth
7、eelectricfield(inthe–vesenseforanelectron).Thecarriermoveswithanadditionaldriftvelocitythatisverymuchlower(e.g.at100V/cm,electronsdriftat~105cm/sinSi).CarrierTransportPhenomena–CarrierDrift•Thedriftvelocityistheaveragevelocityofalltheelectrons(holes)
8、inresponsetoanappliedelectricfield,andisgivenby:(−e)τvd=Em*whereτistheisthemeantimebetweencollisionsmeantimebetweencollisions,andEistheexternal(applied)istheexternal(applied)electricfield.•Thedriftvelocityincreaseslinearlywiththeappliedfield(atsmallf
