53半导体外文翻译外文文献英文文献文档格式.docx
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53半导体外文翻译外文文献英文文献文档格式.docx
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Semiconductor
Asemiconductorisasolidmaterialthathaselectricalconductivitybetweenthoseofaconductorandaninsulator;
itcanvaryoverthatwiderangeeitherpermanentlyordynamically.[1]
Semiconductorsareimportantinelectronictechnology.Semiconductordevices,electroniccomponentsmadeofsemiconductormaterials,areessentialinmodernconsumerelectronics,includingcomputers,mobilephones,anddigitalaudioplayers.Siliconisusedtocreatemostsemiconductorscommercially,butdozensofothermaterialsareused.
Braggreflectioninadiffuselattice
Asecondwaystartswithfreeelectronswaves.Whenfadinginanelectrostaticpotentialduetothecores,duetoBraggreflectionsomewavesarereflectedandcannotpenetratethebulk,thatisabandgapopens.Inthisdescriptionitisnotclear,whilethenumberofelectronsfillsupexactlyallstatesbelowthegap.
EnergylevelsplittingduetospinstatePauliexclusion
Athirddescriptionstartswithtwoatoms.Thesplitstatesformacovalentbondwheretwoelectronswithspinupandspindownaremostlyinbetweenthetwoatoms.Addingmoreatomsnowissupposednottoleadtosplitting,buttomorebonds.Thisisthewaysiliconistypicallydrawn.Thebandgapisnowformedbyliftingoneelectronfromthelowerelectronlevelintotheupperlevel.Thislevelisknowntobeanti-bonding,butbulksiliconhasnotbeenseentoloseatomsaseasyaselectronsarewanderingthroughit.Alsothismodelismostunsuitabletoexplainhowingradedhetero-junctionthebandgapcanvarysmoothly.
Energybandsandelectricalconduction
Likeinothersolids,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,thereismoreenergyinthesemiconductortospendonlatticevibrationand—moreimportantlyforus—onliftingsomeelectronsintoanenergystatesoftheconductionband,whichisthebandimmediatelyabovethevalenceband.Thecurrent-carryingelectronsintheconductionbandareknownas"
freeelectrons"
althoughtheyareoftensimplycalled"
electrons"
ifcontextallowsthisusagetobeclear.
Electronsexcitedtotheconductionbandalsoleavebehindelectronholes,orunoccupiedstatesinthevalenceband.Boththeconductionbandelectronsandthevalencebandholescontributetoelectricalconductivity.Theholesthemselvesdon'
tactuallymove,butaneighboringelectroncanmovetofillthehole,leavingaholeattheplaceithasjustcomefrom,andinthiswaytheholesappeartomove,andtheholesbehaveasiftheywereactualpositivelychargedparticles.
Onecovalentbondbetweenneighboringatomsinthesolidistentimesstrongerthanthebindingofthesingleelectrontotheatom,sofreeingtheelectrondoesnotimplydestructionofthecrystalstructure.
Holes:
electronabsenceasachargecarrier
Thenotionofholes,whichwasintroducedforsemiconductors,canalsobeappliedtometals,wheretheFermilevellieswithintheconductionband.WithmostmetalstheHalleffectrevealselectronstobethechargecarriers,butsomemetalshaveamostlyfilledconductionband,andtheHalleffectrevealspositivechargecarriers,whicharenottheion-cores,butholes.Contrastthistosomeconductorslikesolutionsofsalts,orplasma.Inthecaseofametal,onlyasmallamountofenergyisneededfortheelectronstofindotherunoccupiedstatestomoveinto,andhenceforcurrenttoflow.Sometimeseveninthiscaseitmaybesaidthataholewasleftbehind,toexplainwhytheelectrondoesnotfallbacktolowerenergies:
Itcannotfindahole.Intheendinbothmaterialselectron-phononscatteringanddefectsarethedominantcausesforresistance.
Fermi-Diracdistribution.StateswithenergyεbelowtheFermienergy,hereμ,havehigherprobabilityntobeoccupied,andthoseabovearelesslikelytobeoccupied.Smearingofthedistributionincreaseswithtemperature.
Theenergydistributionoftheelectronsdetermineswhichofthestatesarefilledandwhichareempty.ThisdistributionisdescribedbyFermi-Diracstatistics.Thedistributionischaracterizedbythetemperatureoftheelectrons,andtheFermienergyorFermilevel.UnderabsolutezeroconditionstheFermienergycanbethoughtofastheenergyuptowhichavailableelectronstatesareoccupied.Athighertemperatures,theFermienergyistheenergyatwhichtheprobabilityofastatebeingoccupiedhasfallento0.5.
Thedependenceoftheelectronenergydistributionontemperaturealsoexplainswhytheconductivityofasemiconductorhasastrongtemperaturedependency,asasemiconductoroperatingatlowertemperatureswillhavefeweravailablefreeelectronsandholesabletodothework.
Energy–momentumdispersion
Intheprecedingdescriptionanimportantfactisignoredforthesakeofsimplicity:
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.
Carriergenerationandrecombination
Whenionizingradiationstrikesasemiconductor,itmayexciteanelectronoutofitsenergylevelandconsequentlyleaveahole.Thisprocessisknownaselectron–holepairgeneration.Electron-holepairsareconstantlygeneratedfromthermalenergyaswell,intheabsenceofanyexternalenergysource.
Electron-holepairsarealsoapttorecombine.Conservationofenergydemandsthattheserecombinationevents,inwhichanelectronlosesanamountofenergylargerthanthebandgap,beaccompaniedbytheemissionofthermalenergy(intheformofphonons)orradiation(intheformofphotons).
Insomestates,thegenerationandrecombinationofelectron–holepairsareinequipoise.Thenumberofelectron-holepairsinthesteadystateatagiventemperatureisdeterminedbyquantumstatisticalmechanics.Theprecisequantummechanicalmechanismsofgenerationandrecombinationaregovernedbyconservationofenergyandconservationofmomentum.
Astheprobabilitythatelectronsandholesmeettogetherisproportionaltotheproductoftheiramounts,theproductisinsteadystatenearlyconstantatagiventemperature,providingthatthereisnosignificantelectricfield(whichmight"
flush"
carriersofbothtypes,ormovethemfromneighbourregionscontainingmoreofthemtomeettogether)orexternallydrivenpairgeneration.Theproductisafunctionofthetemperature,astheprobabilityofgettingenoughthermalenergytoproduceapairincreaseswithtemperature,beingapproximately1×
exp(−EG/kT),wherekisBoltzmann'
sconstant,TisabsolutetemperatureandEGisbandgap.
Theprobabilityofmeetingisincreasedbycarriertraps–impuritiesordislocationswhichcantrapanelectronorholeandholdituntilapairiscompleted.Suchcarriertrapsaresometimespurposelyadded
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