材料科学(10)12章-Electrical-Properties(必威体育精装版整理版).pptVIP

ElectricalPropertiesChapter121

ElectronMobilityElectronsandHoles;Ohm’sLaw;ResistivityEnergyBandGapsFermiEnergyLevelConductivityInsulators,Semiconductors,MetalsDopingIntrinsicandExtrinsicSemiconductorsTemperatureDependenceofConductivityOutline

ElectronMobilityAnelectronmovesrandomlyinametal(withameanspeedu)beingfrequentlyandrandomlyscatteredbythermalvibrationsoftheatoms.Withoutapplyinganelectricfieldthereisnonetdriftinanydirection

AppliedField–netdriftInthepresenceofanappliedfield,thereisanetdriftalongthex-direction.Thisnetdriftalongtheforceofthefieldissuperimposedontherandommotionoftheelectron.Aftermanyscatteringeventstheelectronhasbeendisplacedbyanetdistance,Δx,fromitsinitialpositiontowardthepositiveterminal.Theelectronsscatterbycollisionswithatomsandvacanciesthatdrasticallychangetheirdirectionofmotion.Electronsmoverandomlybutwithanetdriftinthedirectionoppositetotheelectricfield.

Chargecarriers,suchaselectrons,aredeflectedbyatomsordefectsandtakeanirregularpaththroughaconductor.Theaveragerateatwhichthecarriersmoveisthedriftvelocity,v.Valenceelectronsinthemetallicbondmoveeasily.Covalentbondsmustbebrokeninsemiconductorsandinsulatorsforanelectrontobeabletomove.(d)Entireionsmustdiffusetocarrychargeinmanyionicallybondedmaterials.

Movementofanelectronthrough(a)aperfectcrystal,(b)acrystalheatedtoahightemperature,and(c)acrystalcontainingatomicleveldefects.Scatteringoftheelectronsreducesthemobilityandconductivity.Scatteringofelectronsbyimperfections:impurityatoms,vacancies,interstitialatoms,dislocations,andthermalvibrationsofatomsthemselves

Whenanelectricalpotential,V[volts,J/C]isappliedacrossapieceofmaterial,acurrentofmagnitude,I[amperes,C/s]flows.Ohmslaw:I=V/R,