黄汉忠—130—131—b13

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1、Theequationofapositivelychargedholeinthepresenceofanelectricfieldis(4.3)whereisthemagnitudeoftheelectroniccharge,istheacceleration,istheelectricfield,andistheeffectivemassofthrhole.Iftheelectricfieldisconstant,thenweexpectthevelocitytoincreaselinearlywithtime.However,chargedpart

2、iclesinasemiconductorareinvolvedincollisionswithionizedimpurityatomsandwithwavesofthermallyvibratinglatticeatoms.Thesecollisions,orscatteringevents,alterthevelocitycharacteristicsoftheparticle.astheholeacceleratesinacrystalduetotheelectricfield,thevelocityincreases.Whenthecharge

3、dparticlelosesmost,orall.Ofitsenergy.theparticlewillagainbegintoaccelerateandgainenergyuntilitisagaininvolvedinascatteringprocess.thiscontinuesoverandoveragain.throughoutthisprocess,theparticlewillgainanaveragedriftvelocitywhich,forlowelectricfields,isdirectlyproportionaltotheel

4、ectricfield.wecanthenwrite(4.4)HolemobilityWhereistheproportionalityfactorandiscalledtheholemobility.Themobilityisanimportantparameterofthesemiconductorsinceitdescribeshowwellaparticlewillmoveduetoanelectricfield.Theunitofmobilityisusuallyexpressedintermsofcm2/V-s.Holedriftcurre

5、ntdensityBycombiningEquations(4.2)and(4.3),wecanwritethedriftcurrentdensityduetoholesas(4.5)Thedriftcurrentduetoholesisinthesamedirectionastheappliedelectricfield.Thesamediscussionofdriftappliestoelectrons.(4.6)whereisthedriftcurrentdensityduetoelectronsandistheaveragedriftveloc

6、ityofelectrons.Thenetchargedensityofelectronsisnegative(-e).Theaveragedriftvelocityofanelectronisalsoproportionaltotheelectricfieldforsmallfields.However,sincetheelectronisnegativelycharged,thenetmotionoftheelectronisoppositetotheelectricfielddirection.Wecanthenwrite(4.7)Electro

7、nmobilityWhereistheelectronmobilityandisapositivequantity.Equation(4.6)cannowbewrittenas(4.8)ElectrondriftcurrentdensityTheconventionaldriftcurrentduetoelectronsisalsointhesamedirectionastheappliedelectricfieldeventhoughtheelectronmovementisintheoppositedirection.Table4.1/Typica

8、lmobilityvaluesatT=300Kandlowdopingconcentratio