半导体毕业论文外文文献翻译.docx
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半导体毕业论文外文文献翻译半导体毕业论文外文文献翻译附附录录附录附录A:
外文资料翻译外文资料翻译原文部分原文部分SemiconductorAsemiconductorisasolidmaterialthathaselectricalconductivitybetweenthoseofaconductorandaninsulator;itcanvaryoverthatwiderangeeitherpermanentlyordynamically.1Semiconductorsareimportantinelectronictechnology.Semiconductordevices,electroniccomponentsmadeofsemiconductormaterials,areessentialinmodernconsumerelectronics,includingcomputers,mobilephones,anddigitalaudioplayers.Siliconisusedtocreatemostsemiconductorscommercially,butdozensofothermaterialsareused.BraggreflectioninadiffuselatticeAsecondwaystartswithfreeelectronswaves.Whenfadinginanelectrostaticpotentialduetothecores,duetoBraggreflectionsomewavesarereflectedandcannotpenetratethebulk,thatisabandgapopens.Inthisdescriptionitisnotclear,whilethenumberofelectronsfillsupexactlyallstatesbelowthegap.EnergylevelsplittingduetospinstatePauliexclusionAthirddescriptionstartswithtwoatoms.Thesplitstatesformacovalentbondwheretwoelectronswithspinupandspindownaremostlyinbetweenthetwoatoms.Addingmoreatomsnowissupposednottoleadtosplitting,buttomorebonds.Thisisthewaysiliconistypicallydrawn.Thebandgapisnowformedbyliftingoneelectronfromthelowerelectronlevelintotheupperlevel.Thislevelisknowntobeanti-bonding,butbulksiliconhasnotbeenseentoloseatomsaseasyaselectronsarewanderingthroughit.Alsothismodelismostunsuitabletoexplainhowingradedhetero-junctionthebandgapcanvarysmoothly.EnergybandsandelectricalconductionLikeinothersolids,theelectronsinsemiconductorscanhaveenergiesonlywithincertainbands(ie.rangesoflevelsofenergy)betweentheenergyofthegroundstate,correspondingtoelectronstightlyboundtotheatomicnucleiofthematerial,andthefreeelectronenergy,whichistheenergyrequiredforanelectrontoescapeentirelyfromthematerial.Theenergybandseachcorrespondtoalargenumberofdiscretequantumstatesoftheelectrons,andmostofthestateswithlowenergy(closertothenucleus)arefull,uptoaparticularbandcalledthevalenceband.Semiconductorsandinsulatorsaredistinguishedfrommetalsbecausethevalencebandinthesemiconductormaterialsisverynearlyfullunderusualoperatingconditions,thuscausingmoreelectronstobeavailableintheconductionband.Theeasewithwhichelectronsinasemiconductorcanbeexcitedfromthevalencebandtotheconductionbanddependsonthebandgapbetweenthebands,anditisthesizeofthisenergybandgapthatservesasanarbitrarydividingline(roughly4eV)betweensemiconductorsandinsulators.Inthepictureofcovalentbonds,anelectronmovesbyhoppingtoaneighboringbond.BecauseofthePauliexclusionprincipleithastobeliftedintothehigheranti-bondingstateofthatbond.Inthepictureofdelocalizedstates,forexampleinonedimensionthatisinawire,foreveryenergythereisastatewithelectronsflowinginonedirectionandonestatefortheelectronsflowingintheother.Foranetcurrenttoflowsomemorestatesforonedirectionthanfortheotherdirectionhavetobeoccupiedandforthisenergyisneeded.Forametalthiscanbeaverysmallenergyinthesemiconductorthenexthigherstateslieabovethebandgap.Oftenthisisstatedas:
fullbandsdonotcontributetotheelectricalconductivity.However,asthetemperatureofasemiconductorrisesaboveabsolutezero,thereismoreenergyinthesemiconductortospendonlatticevibrationandmoreimportantlyforusonliftingsomeelectronsintoanenergystatesoftheconductionband,whichisthebandimmediatelyabovethevalenceband.Thecurrent-carryingelectronsintheconductionbandareknownasfreeelectrons,althoughtheyareoftensimplycalledelectronsifcontextallowsthisusagetobeclear.Electronsexcitedtotheconductionbandalsoleavebehindelectronholes,orunoccupiedstatesinthevalenceband.Boththeconductionbandelectronsandthevalencebandholescontributetoelectricalconductivity.Theholesthemselvesdontactuallymove,butaneighboringelectroncanmovetofillthehole,leavingaholeattheplaceithasjustcomefrom,andinthiswaytheholesappeartomove,andtheholesbehaveasiftheywereactualpositivelychargedparticles.Onecovalentbondbetweenneighboringatomsinthesolidistentimesstrongerthanthebindingofthesingleelectrontotheatom,sofreeingtheelectrondoesnotimplydestructionofthecrystalstructure.Holes:
electronabsenceasachargecarrierThenotionofholes,whichwasintroducedforsemiconductors,canalsobeappliedtometals,wheretheFermilevellieswithintheconductionband.WithmostmetalstheHalleffectrevealselectronstobethechargecarriers,butsomemetalshaveamostlyfilledconductionband,andtheHalleffectrevealspositivechargecarriers,whicharenottheion-cores,butholes.Contrastthistosomeconductorslikesolutionsofsalts,orplasma.Inthecaseofametal,onlyasmallamountofenergyisneededfortheelectronstofindotherunoccupiedstatestomoveinto,andhenceforcurrenttoflow.Sometimeseveninthiscaseitmaybesaidthataholewasleftbehind,toexplainwhytheelectrondoesnotfallbacktolowerenergies:
Itcannotfindahole.Intheendinbothmaterialselectron-phononscatteringanddefectsarethedominantcausesforresistance.Fermi-Diracdistribution.StateswithenergybelowtheFermienergy,here,havehigherprobabilityntobeoccupied,andthoseabovearelesslikelytobeoccupied.Smearingofthedistributionincreaseswithtemperature.Theenergydistributionoftheelectronsdetermineswhichofthestatesarefilledandwhichareempty.ThisdistributionisdescribedbyFermi-Diracstatistics.Thedistributionischaracterizedbythetemperatureoftheelectrons,andtheFermienergyorFermilevel.UnderabsolutezeroconditionstheFermienergycanbethoughtofastheenergyuptowhichavailableelectronstatesareoccupied.Athighertemperatures,theFermienergyistheenergyatwhichtheprobabilityofastatebeingoccupiedhasfallento0.5.Thedependenceoftheelectronenergydistributionontemperaturealsoexplainswhytheconductivityofasemiconductorhasastrongtemperaturedependency,asasemiconductoroperatingatlowertemperatureswillhavefeweravailablefreeelectronsandholesabletodothework.EnergymomentumdispersionIntheprecedingdescriptionanimportantfactisignoredforthesakeofsimplicity:
thedispersionoftheenergy.Thereasonthattheenergiesofthestatesarebroadenedintoabandisthattheenergydependsonthevalueofthewavevector,ork-vector,oftheelectron.Thek-vector,inquantummechanics,istherepresentationofthemomentumofaparticle.Thedispersionrelationshipdeterminestheeffectivemass,m*,ofelectronsorholesinthesemiconductor,accordingtotheformula:
Theeffectivemassisimportantasitaffectsmanyoftheelectricalpropertiesofthesemiconductor,suchastheelectronorholemobility,whichinturninfluencesthediffusivityofthechargecarriersandtheelectricalconductivityofthesemiconductor.Typicallytheeffectivemassofelectronsandholesaredifferent.Thisaffectstherelativeperformanceofp-channelandn-channelIGFETs,forexample(Muller&Kamins1986:
427).Thetopofthevalencebandandthebottomoftheconductionbandmightnotoccuratthatsamevalueofk.Materialswiththissituation,suchassiliconandgermanium,areknownasindirectbandgapmaterials.Materialsinwhichthebandextremaarealignedink,forexamplegalliumarsenide,arecalleddirectbandgapsemiconductors.Directgapsemiconductorsareparticularlyimportantinoptoelectronicsbecausetheyaremuchmoreefficientaslightemittersthanindirectgapmaterials.CarriergenerationandrecombinationWhenionizingradiationstrikesasemiconductor,itmayexciteanelectronoutofitsenergylevelandconsequentlyleaveahole.Thisprocessisknownaselectronholepairgeneration.Electron-holepairsareconstantlygeneratedfromthermalenergyaswell,intheabsenceofanyexternalenergysource.Electron-holepairsarealsoapttorecombine.Conservationofenergydemandsthattheserecombinationevents,inwhichanelectronlosesanamountofenergylargerthanthebandgap,beaccompaniedbytheemissionofthermalenergy(intheformofphonons)orradiation(intheformofphotons).Insomestates,thegenerationandrecombinationofelectronholepairsareinequipoise.Thenumberofelectron-holepairsinthesteadystateatagiventemperatureisdeterminedbyquantumstatisticalmechanics.Theprecisequantummechanicalmechanismsofgenerationandrecombinationaregovernedbyconservationofenergyandconservationofmomentum.Astheprobabilitythatelectronsandholesmeettogetherisproportionaltotheproductoftheiramounts,theproductisinsteadystatenearlyconstantatagiventemperature,providingthatthereisnosignificantelectricfield(whichmightflushcarriersofbothtypes,ormovethemfromneighbourregionscontainingmoreofthemtomeettogether)orexternallydrivenpairgeneration.Theproductisafunctionofthetemperature,astheprobabilityofgettingenoughthermalenergytoproduceapairincreaseswithtemperature,beingapproximately1exp(EG/kT),wherekisBoltzmannsconstant,TisabsolutetemperatureandEGisbandgap.Theprobabilityofmeetingisincreasedbycarriertrapsimpuritiesordislocationswhichcantrapanelectronorholeandholdituntilapairiscompleted.Suchcarriertrapsaresometimespurposelyaddedtoreducethetimeneededtoreachthesteadystate.DopingThepropertyofsemiconductorsthatmakesthemmostusefulforconstructingelectronicdevicesisthattheirconductivitymayeasilybemodif