半导体器件物理英文课件

2024-8-281§1-1Introduction2024-8-2822024-8-2832024-8-2842024-8-2852024-8-2862024-8-2872024-8-2882024-8-2892024-8-28102024-8-28112024-8-28122024-8-28132024-8-28142024-8-28152024-8-28162024-8-28172024-8-28182024-8-28192024-8-28202024-8-28212024-8-28222024-8-28232024-8-2824§1-2CrystalStructure2024-8-2825Solidstatematerialsincludes:insulator,semiconductorsandconductors.Semiconductormaterials2024-8-2826CompoundsSemiconductorsAbinarycompoundsemiconductorsisacombinationoftwo

elementsfromtheperiodictable.Ex:GaAs,GaP……etc.

Ternaryandquaternarycompounds.Ex:GaxIn1-xAsyP1-y

Element&CompoundSemiconductorsElementSemiconductors2024-8-2827SolidsAsatomsarebroughttogethertoformsolid,theinteractionofthevalanceelectronsholdthecrystaltogetheranddetermineitselectricalbehavior.CovalentBond:

sharingofapairofelectrons,SicrystalIonicBond:

transferofelectron,NaClMetallicBond:

Attractionbetweentheioncoreandtheseaofelectrons,AI(FCC)2024-8-2828Solids

Solidscanbeclassifiedascrystal,polycrystallineandamorphousCrystal–threedimensionallongrangeorderofatoms.Polycrystalline–mediumrangeorder,manysmallregionscalledgrains,eachhavingcrystallinestructure,joinedat“grainboundaries”whicharefullofdefects.Amorphous–nowelldefinedorder.2024-8-2829CrystalStructure2024-8-2830

Crystal

Lattice:Theperiodicarrangementofpointsinacrystal.

Basis:Theconstituentatomsattachedtoeachlattice

point.Everybasisisidenticalincomposition,

arrangement,andorientation.

Crystal=Lattice+BasisR=ma+nb+pca:latticeconstant2024-8-2831CrystalThelatticesisdefinedbythreefundamentaltranslationvectors2024-8-2832Unitcell:LatticecanbeconstructedbyrepeatedlyarrangingunitcellUnitCell2024-8-2833PrimitiveCell:AunitcelliscalledasprimitiveunitcellifthereisnocellofsmallervolumethatcanserveasabuildingblockforcrystalstructurePrimitiveCell2024-8-2834CrystalStructure2024-8-2835CrystalStructure2024-8-2836CrystalStructure2024-8-2837SystemBravaislatticeUnitcellSymmetryTriclinicSimpleNoneMonoclinicSimpleBase-centeredOne2-foldrotationaxisOrthorhombicSimpleBase-centerBody-centeredFace-centeredThreemutualityorthogonal2-foldTetragonalSimpleBody-centeredOne4-foldrotationaxisCubicSimpleBody-centeredFour-3-foldrotationaxis(alongcubediagonal)TrigonalSimpleOne3-foldrotationaxisHexagonalSimpleOne3-foldrotationaxis2024-8-2838MillerIndicesTheMillerindicesareobtainedusingthefollowingstepsFindtheinterceptsoftheplaneonthethreeCartesiancoordinateintermsofthelatticeconstant.Takethereciprocalsofthesenumbersandreducethemtothesmallestthreeintegershavingthesameradio.Enclosetheresultinparentheses(hkl)astheMillerindicesforasingleplane.2024-8-2839MillerIndices12=4x+3yMillerindices[43]2024-8-2840MillerIndices

2024-8-2841MillerIndices(hkl):Foraplanethatinterceptsthex-axisonthenegativesideoftheorigin.(100){hkl}:Forplanesofequivalentsymmetry.

(100)(010)(001)(100)(010)(001)<hkl>:Forafullsetofequivalentdirections.

[100][010][001][100][010][001][100][hkl]:Foracrystaldirection.2024-8-2842CrystalStructureTwointerveningFCCcellsoffsetby¼ofthecubicdiagonalfromdiamondstructureandzincblendestructure:2024-8-2843DirectLattice:BravaisCellReciprocalLattice:Wigner-SeitzCell2024-8-2844ImperfectionsinSolid

LatticeThermalVibrationPointDefects

-Vacancy

-interstitial

-Vacancy+interstitial=FrenkelDefectLineDefects-Dislocation:disruptingnotonlygeometricperiodicitybutalsoideaatomicbonds2024-8-2845ImperfectionsinSolid2024-8-2846ImpuritiesinSolidIntentionallyaddingimpuritiesintocrystalcanaltertheelectricalproperty

-Diffusion

-IonImplantation2024-8-2847FromSandtoWaferQuartziteCoalMGSCarbonmonoxidepurificationMGShrydrochlorideTCShydrogenTCShydrogenhrydrochlorideEGS2024-8-2848GrowthTechniquesReductionofquartzitetometallurgicalgradesilicon(MGS)withapurityof~98%.ConversionofMGStotrichlorosilane(SiHCl3).PurificationofSiHCl3bydistillation.Chemicalvapordeposition(CVD)ofSifromthepurifiedSiHCl3,asEGS.2024-8-2849SiCrystalIncrystalstructure,atomsharesitsvalenceelectronswiththeneighbors.Thesesharingofelectronsiscovalentbonding.2024-8-2850Electron&HoleAthighertemperature,thermalvibrationmaybreakthecovalentbonds;thefreeelectronscanparticipateincurrentconduction.Whenelectronsleavethecovalentbond,thevacancieswereconsideredasaparticlesimilartoanelectron.Thisfictitiousparticleiscalledahole.2024-8-2851§1-3EnergyBands2024-8-2852Wave-ParticleDualityWave-particledualityappliesprimarytosmallparticles,suchaselectron,neutron,photonWavesbehavesasiftheyareparticlesandsometimesparticlesbehavesasiftheyarewavesDeBroglierelationship2024-8-2853One-electronApproximation2024-8-2854

HydrogenAtomicModelBohr’sModel2024-8-2855HydrogenAtomicModelForanatom,eachelectronmusthaveaseparatedistinctenergystatedefinedby4quantumnumbers:Principequantumnumber,n=1,2,3Angularmomentumquantumnumber,I=0,1,2,…,n-1Magneticquantumnumber,m=0,±1,…,±IElectronspin,s=±1/2Onlyhydrogenatomcanbesolvedduetoelectron-electroninteraction2024-8-2856BandFormationTheinteractionresultsinthediscretequantizedenergylevelsplittingintotwodiscreteenergylevelsProbabilitydensityfunctionofaisolatedhydrogenatom2024-8-2857EnergyBandDegenerateWhenNisolatedatomsarebroughttogethertoformasolid,theorbitsoftheouterelectronsofdifferentatomsoverlapandinteractwitheachother.2024-8-2858EnergyBandSchematicshowingthesplittingofthreeenergystatesintoallowedbandsofenergies2024-8-2859BandFormationSiAtomInteractionporbital:sixallowedstatessorbital:twoallowedstates2024-8-2860EnergyBandSchematicdiagramoftheformationofasiliconcrystalfromNisolatedsiliconatoms2024-8-2861EnergyBand2024-8-2862EnergyMomentumDiagramForafreeelectron,

energyEcanbegivenbym0:effectivemass

P:momentum2024-8-2863BolchElectronWavefunctionInperiodicpotential,anelectronwillbehaveinthismanner,i.e.,Blochelectronisalsoperiodic2024-8-2864Energy(E)vs.Wavevector(k)FreeElectron:Incrystal,free-electronE-kisnolongervalid,discontinuityatk=nπ/aemerges.

→Creatingenergygap2024-8-2865BrillouinZone

ReducedBrillouinZone2024-8-2866Energy(E)vs.Wavevector(k)2024-8-2867EffectiveMassConceptElectronsinconductionbandandholesinvalencebandaresimilartofreeelectronssincetheycanmoverelativelyfreelyWecantreatelectronsandholesasclassicalparticlesmn:effectivemass

p:crystalmomentumofelectron2024-8-2868EffectiveMassConcept2024-8-2869EnergyBandDiagramofSiliconEg=1.12eVkT/q=0.0259eV@300k2024-8-2870EnergyBand:TemperatureEffect

BandGapvs.TemperatureSiGaAs2024-8-2871EnergyBandConductor,Semiconductor&insulator2024-8-2872BandStructureDirectSemiconductorThetopofthehighest

(occupied)valenceband

andthebottomofthe

lowest(unoccupied)

conductionbandareatthe

samevalueink-space.Examples:

GaAs,InP,GaN,ZnO.2024-8-2873BandStructureIndirectSemiconductor:Theextremeatthetop

ofthevalenceband

andatthebottomof

theconductionband

areatdifferentk-values.

Examples:Ge,Si.2024-8-2874Donor&AcceptorDeepimpurityLevel&ShallowImpurityLevel2024-8-2875§1-4CarrierConcentrationatThermalEquilibrium2024-8-2876DistributionFunctionsandDensitiesofStatesLetusconsiderthesituationwhenthenumberofstatesismuchgreaterthanthenumberofparticlesandtheprobabilityoffindingaparticleinagivenstatesismuchsmallerthanunity.Inthiscase,thePauliexclusionprincipleisnotimportant(sinceitisveryunlikelythattwoparticleswilloccupythesameenergylevel),andtheprobabilityoffindingaparticleinthestatewithenergyE,isgivenbywhereNiisthetotalnumberofparticlesinthisstate.Theaverageparticleenergycanbefoundas2024-8-2877Fermi-DiracDistributionFunctionForelectrons,thePauliexclusionprinciplestatesthatnomorethantwoelectrons(withoppositespins)canoccupyagivenenergylevel.Electronstendtooccupystateswithlowenergiesfirst.Hence,allthestateswithlowenergiesarefilledinexactlythesameway–oneelectronineachenergystate(countingthetwostateswiththesameenergyavailableforelectronswithoppositespinsastwoseparatestates).Atsuchlowenergies,theelectronprobabilityfunction,f,mustbeequaltounitysinceallthesestatesareoccupied.Howeverathighvaluesofenergy,whentheprobabilityofoccupyinganenergystateismuchsmallerthanunity,thePauliprinciplepresentsnolimitation,andthedistributionfunctionshouldreducetotheBoltzmanndistributionfunction.AmoredetailedanalysisshowsthattheelectrondistributionfunctionisgivenbytheFermi-Diracdistributionfunction2024-8-2878

BoltzmannDistributionFunctionInequilibrium,theprobabilitiesofhavingparticlesintwoenergystates,EkandEi,arerelatedviatheBoltzmannfactors:Thisequationmeansthattheprobabilityoffindingaparticlesinagivesenergystates,Ei,decreasesexponentiallywithEi.Foracontinuousenergyspectrum,theprobabilityoffindingaparticlewiththeenergybetweenEandE+dEisgivenbyThefunctionfiscalledtheBoltzmanndistributionfunction.2024-8-2879IntrinsicCarrierConcentrationIntrinsicSemiconductorisonethatcontainsrelativelysmallamountofimpuritiescomparedwiththethermallygeneratedelectronsandholesFermi-DiracDistributionFunction2024-8-2880Fermi-DiracDistributionprobabilitythataquantumstateattheenergyEwillbeoccupiedbyanelectron2024-8-2881f(T)2024-8-2882

FermiLevelatT=0oKTheFermiprobabilityfunctionversusforenergyforT=0oKDiscreteenergystatesandquantumstatesforaparticularsystematT=0oK2024-8-2883ThermalExcitation(T>0°K)DiscreteenergystatesandquantumstatesfortheparticularsystematT>0oK2024-8-2884

IntrinsicCarrierConcentration2024-8-2885

DensityofStatevs.Energy2024-8-2886

IntrinsicCarrierConcentrationIntrinsicsemiconductor(a)schematicbanddiagram(b)densityofstates(c)Fermidistributionfunction(d)carrierconcentration2024-8-2887

Maxwell-BoltzmanApproximationFermi-Diracdistribution:ForE-Ef>>kTMaxwell-Boltzmanapproximation2024-8-2888DensityofStatesandEffectiveDensityofStateswhereiscalledtheeffectivedensityofstatesfortheconductionband2024-8-2889DensityofStatesandEffectiveDensityofStatesIstheFermiintegralforconductionband,wherewhenηn>3,2024-8-2890whenηn<-3,ExampleExpressthevalueofenergycorrespondingtothepeakofdn/dEdistributionintermstemperature,T,assumingthatEc-EF>>kBT2024-8-2891

IntrinsicCarrierConcentration2024-8-2892IntrinsicCarrierConcentrationEffectivedensityofstatesinconductionbandEffectivedensityofstates

invalanceband2024-8-2893CarrierConcentrationinIntrinsicSilicon2024-8-2894FermiLevelinIntrinsicSiliconMassactionlawFermilevel:chargeneutralityn=p=niInintrinsicsiliconEcEvEiFermilevellocatesnearmiddle

bandgap2024-8-2895IntrinsicCarrierDensitiesIntrinsiccarrierdensitiesinSiandGaAsasafunctionofthereciprocaloftemperature2024-8-2896Donor&AcceptorExtrinsic:Asemiconductordopedwithoracceptorimpurities2024-8-2897Donor&AcceptorExcitedbythermalenergy

Mostofthedopantslocateatshallowenergylevel.Activationcanbemodeledbyhydrogenionizationwithreplacingdielectricconstant&effectivemass2024-8-2898CompensatedSemiconductor2024-8-2899NondegenerateSemiconductorElectronandholeconcentrationaremuchlowerthantheeffectivedensityofstatesInotherwords,EFisatleast3kTaboveEVor3kTbelowECForshallowdonorsoracceptors,thereusuallyisenergythermalenergytosupplytheenergyEDtoionizeallimpurities,i.e.,completeionizationTheconcentrationofelectron(hole)equalstothatofdonor(acceptor)ion2024-8-28100Donor&Acceptor

WhenCompleteIonization

ND:donorconcentration

NA:acceptorconcentration

2024-8-28101Inp-typesemiconductor

Donor&AcceptorWhendonorandacceptorexistinthesametimeInn-typesemiconductor2024-8-28102

CarrierConcentrationinn-DopedSilicon

2024-8-28103ChargeNeutrality2024-8-28104FermiLevelinExtrinsicSiliconThepositionofFermileveldeterminetheconcentrationofelectronsandholes2024-8-28105Donor&Acceptor2024-8-28106TemperatureEffect2024-8-28107Donor&Acceptor2024-8-28108DopantSolubilityinSilicon2024-8-28109DegenerateSemiconductorWhenthedopingconcentrationbecomesequalorlargerthattheeffectivedensityofstates,wecannolongerusetheapproximationofM-Bstatistics.Concentrationshouldbecalculatednumerically.EFwillbeaboveECorbelowEVBroadenshapeofimpurityenergydistributionresultinenergybandgapnarrowing2024-8-28110DegenerateSemiconductor2024-8-28111DegenerateSemiconductorLocalvariationofpotentialbyimpurity2024-8-28112§1-5CarrierTransport

Phenomena2024-8-28113TransportinDevicesThermalMotioninEquilibriumDrift:underinfluenceofelectricfieldDiffusion:concentrationgradientGenerationandRecombinationThermionicEmission

TunnelingImpactIonization2024-8-28114TopicsinTransportCurrentdensityequation:Drift&DiffusionContinuityequation:Generation-RecombinationOthertransportmechanisms:thermionicemission,tunneling,impactionizationIntroductionofresistivity,mobility……2024-8-28115ThermalMotionInthermalequilibrium,mobileelectronsintheconductionbandwillbeinrandomthermalmotion.Fromstatisticalfreedom.ThusKineticEnergyofanelectronwherek=Boltzmann’sconstantmn=Conductivityeffectivemassofelectron(nottoconfusedwiththedensityofstateeffectivemassVth=thermalvelocity~107cm/sec@300°K2024-8-28116ThermalMotionElectronsmovingrapidlyinalldirectionsThethermalmotionofanindividualelectroncanbevisualizedasasuccessiverandomscatteringwithlattervibration,impurity,andsoon.

Theaveragedistancebetweencollisionsisreferredtomeanfreepath(l).Typicalvalueof10-5cm.Thetimecalledmeanfreetime~1ps(τc=1/Vth)

2024-8-28117CarrierTransport:Drift(ElectricField)2024-8-28118CarrierTransport:MobilityInsteadystate,allmomentumgainedbetweencollisionswillbelosttolatticeinthecollision.Therefore,themomentumgainedbyaccelerationofelectricfieldduringfreemotioncanbeobtainedbyitmeansthatthereexistsarelationbetweendriftvelocityandappliedelectricfieldMobility:μn2024-8-28119ThermalVelocityTheaveragekineticenergyofthermalmotionperoneelectronis3kBT/2whereTisthetemperatureindegreesKelvinandkBistheBoltzmannconstant.Theelectronthermalvelocity,vthn,isfoundbyequatingtheelectronkineticenergyto3kBT/2:2024-8-28120DriftVelocityTheelectrondriftvelocity,vn,causedbyanappliedelectricfield,issuperimposedonthischaoticthermalmotion.Atroomtemperature,theelectronvelocityduetothethermalmotionisusuallygreaterthanoratleastcomparabletothedriftvelocity.Therefore,anexactdescriptionoftheelectronicmotioninasemiconductorhastorandomnessoftheelectronvelocity.2024-8-28121DriftVelocity(cont.)However,anapproximatedescriptionofthedriftvelocitycanbeobtainedfromNewton’ssecondlawofmotionforanelectronmovinginanelectricfieldF.Afreeelectroninspaceisacceleratedbyelectricfieldasfollows:2024-8-28122SecondLawofMotionEffectiveMassInasemiconductor,thefreeelectronmasshastobereplacedbytheeffectivemass,mn:2024-8-28123RelaxationTimeandMeanFreePathTheaveragedistancewhichanelectrontravelsbetweentwocollisionsiscalledthemeanfreepath.Inrelativelyweakelectricfieldswhentheelectrondriftvelocityismuchsmallerthanthenormalvelocity,themeanfreepathisgivenby:2024-8-28124MobilityAtlowfrequencies,ω<<1/τnp,mdv/dt<<qFiscalledtheelectronlowfieldmobilityorjustmobility.2024-8-28125ScatteringMechanismsLow-fieldmobilities:Mathiessens’srule:Inlowelectricfields

·ionizedimpurities

·acousticphononsInhighelectricfields

·opticalphonons

·intervalley

scatteringAthighconcentrations

·carrier-carrier

scattering2024-8-28126ExampleTheelectronmobilityinGaAsattemperaturesT=77KandT=300Kisequalto300,000cm2/Vsand9,000cm2/Vs,respectively.Theelectroneffectivemassisequalto0.067mewheremeisthefreeelectronmass.Findtheelectronmeanfreepathatthesetemperatures.2024-8-28127SolutionThemomentumrelaxationtimeisgivenbyHence,themeanfreepathSubstitutingtheparametervalues,wefind:

λn(77K)=2.67μm,λp(300K)=1570A2024-8-281281.Phononscattering-Latticeatomsvibratewithdiscreteallowablestates(quantummechanics).

2.Ionizadimpurityatomscattering(importantathighdopantconcentrations)

3.Neutalimpurityatomscattering(usuallynegligible)

4.Electron-electronandElectron-holescattering(importantathighcarrierconcentrations)

5.Crystaldefects(importantinpolycrystallinematerial)

6.Surfacescatteringeffects

(importantinMOSdevices)MobilityMobilityisdirectlyrelatedtothemeanfreetimebetweencollisions,whichisinturndeterminedbyscattering2024-8-28129ImpurityScattering2024-8-28130LatticeVibration2024-8-28131MobilityElectronmobilityinsiliconversustemperatureforvariousdonorconcentrations.Forlightlydopedsamples,thelatticescatteringdominates,andthemobilitydecreasesasthetemperatureincrease.Forheavilydopedsamples,theeffectofscatteringisthemostpronouncedatlowtemperature.Foragiventemperature,themobilitydecreaseswithincreasingimpurityconcentrationbecauseofenhancedimpurityscattering.2024-8-28132Mobility2024-8-28133MobilityAtlowfield

Vd=μζ

Mathiessen’srule2024-8-28134CarrierMobilitiesinSilicon2024-8-28135MaterialPropertiesofImportantSemiconductors2024-8-28136SurfaceScatteringInaMOSFET,carriersareattractedtowardsurface,whichmakesthemsufferingfrommoreseverescattering2024-8-28137Poisson’sEquationPotential:Electricfield:Poisson’sEq.:Guass’slaw2024-8-28138DiffusioncurrentLetusconsiderann-typesamplewithnon-uniformcarrierconcentrationindirectionx(whichmayberelatedtoanon-uniformdoping)andnoelectriccurrent(i=jdrift+jdiff=0)sothat(*)2024-8-28139SinceweobtainandandfromEq.(*)EinsteinRelationship2024-8-28140ExampleTheelectronandholemobilitiesinSiatroomtemperature(T=300K)are1,000cm2/Vsand300cm2/Vs,respectively.Calculatetheelectronandholediffusioncoefficients.2024-8-28141SolutionSinceforT=300K,kBT/q=0.02584eV,

Dn=μn

kBT/q=0.02584×1000=25.8cm2/sand

Dp=μp

kBT/q=0.02584×300=7.75cm2/s2024-8-28142ResistivityofSilicon2024-8-28143SheetResistivity2024-8-28144HallEffectForp-typesemiconductorLorentzforceHallFieldHallvoltageHallcoefficientForn-typesemiconductor2024-8-28145HallEffectWecandomeasurementoftheHallvoltageforaknowncurrentandmagneticfieldyields.Thus,thecarrierconcentrationandcarriertypecanbeobtaineddirectly.2024-8-28146ElectronDriftCurrentDensityjFOhm’sLaw2024-8-28147EnergyBalanceandEnergyRelaxationTimeElectrontemperature2024-8-28148DriftVelocityinSemiconductors2024-8-28149MobilityandSaturationVelocityinSilicon2024-8-28150ElectionandHoleVelocityinSi2024-8-28151Generation&RecombinationInthermalequilibrium:Ifexcesscarriersareintroducedtoasemiconductor.Wehaveanon-equilibriumsituation.Theprocessofintroducingexcesscarriersiscalledinjection.Whentheequilibriumconditionisdisturbed,processexisttorestorethesystemtoequilibrium.Whenweinjectiontheminoritycarrierstothesemiconductor.Theinjectionminoritycarrierswillrecombinewithmajoritycarriers.Thereleasedenergythatresultsfromtherecombinationprocesscanbeemittedasaphoton(radiativerecombination)ordissipatedasheattolattice(nonradiativerecombination).2024-8-28152DirectRecombinationIndirect-bandgapsemiconductor,whenthethermalvibrationcausessomebondsbetweenneighboringatomstobebroken,anelectron-holepairisgenerated.Thethermalenergyenablesavalanceelectrontomakeanupwardtransitiontotheconductionband,leavingaholeinvalenceband.Thisprocessiscalledcarriergeneration.2024-8-28153DirectRecombinationWhenanelectronmakesatransitiondownwardfromtheconductionbandtovalanceband,anelectron-holepairisannihilated.Thisprocessiscalledrecombination.2024-8-28154AugerRecombinationAuger

recombinationoccursbythetransferoftheenergyandmomentumreleasedbytherecombinationofanelectron-holepairtoathirdparticlethatcanbeeitheranelectronorahole2024-8-28155Underthermalequilibrium,thegenerationratemustequaltorecombinationrate.Whenexcesscarriesareintroducedtoadirect-bandgapsemiconductorRecombinationrateThermalequilibriumThesubject0indicatesanequilibriumquantity△meansexcessconcentrationThenetrateofchangeofholeconcentrationisβisthepropotionalityconstant2024-8-28156Insteadystate,UistherecombinationrateByaboveequationsForlowlevelinjection△p,andpno<<nn0τpislifetimeThephysicalmeaningoflifetimecanbeillustratedbythetransientresponseofadeviceafterthesuddenremovalofthelightsource.2024-8-28157

IndirectionRecombinationThedominaterecombination

processinindirection

semiconductor

(EX:Si)isindirecttransitionvialocalized

energystatesin

theforbidden

energygap.2024-8-28158IndirectionRecombinationTheproportionalityconstantcanbevisualizedasthevolumesweptoutperunittimebyanelectron.Ifthecentrelieswithinthisvolume,theelectronwillbecapturedbyit.2024-8-28159Atequilibrium,Ra=RbIndirectionRecombination2024-8-28160IndirectionRecombinationatsteadystateatequilibrium,GL=0Ra=RbandRc=Rd

understeadystatenonequilibrium

GL=Ra-Rb=Rc-Rd≡U2024-8-28161IndirectionRecombination2024-8-28162TheMinority-carrierLifetime

(Single-levelRecombination)TheAsymptoticLifetime

(Multiple-levelRecombination)2024-8-28163Experiments:Solid-stateDiffusionEX.(Goldinsilicon)High-energyRadiationEX.(Electronirradiation,Neutronirradiation,Deuteronirradiation)2024-8-28164Theminority-carrierlifetimemeasurementPC:PhotoconductionEffectStevenson-KeyesMethodPEM:Photo-electromagneticEffect2024-8-28165SurfaceRecombinationTheabruptdiscontinuityofthelatticestructureatthesurface,alargenumberiflocalizedenergystatesorgenerationrecombinationcentresmaybeintroducedatthesurfaceregion.Theseenergystatescalledsurfacestates,maygreatlyenhancetherecombinationrateatthesurfaceregion.

Low-injectionsurface

recombinationvelocity2024-8-28166§1-6PhononSpectraandOptical,Thermal,andHigh-FieldPropertiesofSemiconductors2024-8-28167PHONONSPECTRALA—longitudinalacousticmodesLO—longitudinalopticalmodesTA—transverseacousticmodesTO—transverseopticalmodes2024-8-28168OpticalPropertyT----TransmissioncoefficientR----Reflectioncoefficienta----absorptioncoefficienta~(hν-Eg)γ2024-8-28169ThermalPropertyThermoelectricPowerTodeterminetheconductiontypeofasemiconductorP-type----------------positiveN-type----------------negative2024-8-28170ThermalPropertyThermalconductivityκ~T2024-8-28171High-FieldPropertyNonlinearmobility(μ)Effectivetemperature(Te)Saturationofdriftvelocity2024-8-28172Vd,sat≈107cm/sVelocitySaturationVelocityincreaseslinearlywithincreasingelectricfield,butsaturatesathighfield2024-8-28173DiffusionVelocityoftheGaAsForn-typeGaAs,thedriftvelocityreachesamaximum,thendecreasesasthefieldfurtherincreases.ThisisduetotheenergybandstructureofGaAsallowsthetransferofconductionelectronfromahigh-mobilityenergyminimumtolow-mobility,higherenergysatellitevalleys.2024-8-28174DriftVelocityoftheGaAs2024-8-28175IntervalleyTransfer2024-8-28176DriftVelocityoftheGaAsThesteady-stateconductivityofthen-typeGaAs2024-8-28177Velocity-FieldCurvesforGaAs2024-8-28178AnalyticalApproximationVelocityin105m/s

Mobility,μ，inm2/V-s2024-8-28179ImpactIonizationWhentheelectricfieldishighenough,electronintheconductionbandcangainkineticenergybeforeitcollideswiththelattice.Onimpactwiththelattice,theelectronimpartsmostofitskineticenergytobreakabond,thatis,toionizeavalenceelectronfromthevalencebandtotheconductionbandandtherebygenerateanelectron-holepair.Thenthegenerationpairwillrepeatthisprocessagain.Thisprocessiscalledtheimpactionizationprocess.2024-8-28180Considerafast-movingelectronhasakineticenergyandamomentum.Aftercollision,therearethreecarries:theoriginalelectronplusanelectron-holepair.ImpactIonizationItisobviousthatE0mustbelargerthanthebandgapfortheionizationprocesstooccur.2024-8-28181IonizationRateThenumberofelectronholepairsgeneratedbyanelectronperunitdistancetravellediscalledtheionizationratefortheelectron

αn2024-8-28182