文献翻译——LED显示屏趋势素描

第1页外文文献资料LEDdisplaytrendsketchAsemiconductorisasolidmaterialthathaselectricalconductivitybetweenthoseofaconductorandaninsulator;itcanvaryoverthatwiderangeeitherpermanentlyordynamically.Semiconductorsareimportantinelectronictechnology.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)betweentheenergy第2页ofthegroundstate,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.Theholes第3页themselvesdontactuallymove,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.UnderabsolutezeroconditionstheFermienergycanbethoughtofastheenergyuptowhichavailableelectronstates.areoccupied.Athighertemperatures,theFermienergyistheenergyatwhichtheprobabilityofastatebeingoccupiedhasfallento0.5.Thedependenceoftheelectronenergydistributionontemperaturealso第4页explainswhytheconductivityofasemiconductorhasastrongtemperaturedependency,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,inwhichanelectronlosesanamount第5页ofenergylargerthanthebandgap,beaccompaniedbytheemissionofthermalenergy(intheformofphonons)orradiation(intheformofphotons).Insomestates,thegenerationandrecombinationofelectronholepairsareinequipoise.Thenumberofelectron-holepairsinthesteadystateatagiventemperatureisdeterminedbyquantumstatisticalmechanics.Theprecisequantummechanicalmechanismsofgenerationandrecombinationaregovernedbyconservationofenergyandconservationofmomentum.Astheprobabilitythatelectronsandholesmeettogetherisproportionaltotheproductoftheiramounts,theproductisinsteadystatenearlyconstantatagiventemperature,providingthatthereisnosignificantelectricfield(whichmightflushcarriersofbothtypes,ormovethemfromneighbourregionscontainingmoreofthemtomeettogether)orexternallydrivenpairgeneration.Theproductisafunctionofthetemperature,astheprobabilityofgettingenoughthermalenergytoproduceapairincreaseswithtemperature,beingapproximately1exp(EG/kT),wherekisBoltzmannsconstant,TisabsolutetemperatureandEGisbandgap.Theprobabilityofmeetingisincreasedbycarriertrapsimpuritiesordislocationswhichcantrapanelectronorholeandholdituntilapairiscompleted.Suchcarriertrapsaresometimespurposelyaddedtoreducethetimeneededtoreachthesteadystate.DopingofsemiconductorsThepropertyofsemiconductorsthatmakesthemmostusefulforconstructingelectronicdevicesisthattheirconductivitymayeasilybemodifiedbyintroducingimpuritiesintotheircrystallattice.Theprocessofaddingcontrolledimpuritiestoasemiconductorisknownasdoping.Theamountofimpurity,ordopant,addedtoanintrinsic(pure)semiconductorvariesitslevelofconductivity.Dopedsemiconductorsareoftenreferredtoasextrinsic.DopantsThematerialschosenassuitabledopantsdependontheatomicpropertiesofboththedopantandthematerialtobedoped.Ingeneral,dopantsthatproducethedesiredcontrolledchangesareclassifiedaseitherelectronacceptorsordonors.Adonoratomthatactivates(thatis,becomes第6页incorporatedintothecrystallattice)donatesweakly-boundvalenceelectronstothematerial,creatingexcessnegativechargecarriers.Theseweakly-boundelectronscanmoveaboutinthecrystallatticerelativelyfreelyandcanfacilitateconductioninthepresenceofanelectricfield.(Thedonoratomsintroducesomestatesunder,butveryclosetotheconductionbandedge.Electronsatthesestatescanbeeasilyexcitedtoconductionband,becomingfreeelectrons,atroomtemperature.)Conversely,anactivatedacceptorproducesahole.Semiconductorsdopedwithdonorimpuritiesarecalledn-type,whilethosedopedwithacceptorimpuritiesareknownasp-type.Thenandptypedesignationsindicatewhichchargecarrieractsasthematerialsmajoritycarrier.Theoppositecarrieriscalledtheminoritycarrier,whichexistsduetothermalexcitationatamuchlowerconcentrationcomparedtothemajoritycarrier.Forexample,thepuresemiconductorsiliconhasfourvalenceelectrons.Insilicon,themostcommondopantsareIUPACgroup13(commonlyknownasgroupIII)andgroup15(commonlyknownasgroupV)elements.Group13elementsallcontainthreevalenceelectrons,causingthemtofunctionasacceptorswhenusedtodopesilicon.Group15elementshavefivevalenceelectrons,whichallowsthemtoactasadonor.Therefore,asiliconcrystaldopedwithboroncreatesap-typesemiconductorwhereasonedopedwithphosphorusresultsinann-typematerial.CarrierconcentrationTheconcentrationofdopantintroducedtoanintrinsicsemiconductordeterminesitsconcentrationandindirectlyaffectsmanyofitselectricalproperties.Themostimportantfactorthatdopingdirectlyaffectsisthematerialscarrierconcentration.Inanintrinsicsemiconductorunderthermalequilibrium,theconcentrationofelectronsandholesisequivalent.Wherenistheconcentrationofconductingelectrons,pistheelectronholeconcentration,andniisthematerialsintrinsiccarrierconcentration.Intrinsiccarrierconcentrationvariesbetweenmaterialsandisdependentontemperature.Siliconsni,forexample,isroughly1.61010cm-3at300kelvin(roomtemperature).Ingeneral,anincreaseindopingconcentrationaffordsanincreasein第7页conductivityduetothehigherconcentrationofcarriersavailableforconduction.Degenerately(veryhighly)dopedsemiconductorshaveconductivitylevelscomparabletometalsandareoftenusedinmodernintegratedcircuitsasareplacementformetal.Oftensuperscriptplusandminussymbolsareusedtodenoterelativedopingconcentrationinsemiconductors.Forexample,n+denotesann-typesemiconductorwithahigh,oftendegenerate,dopingconcentration.Similarly,pwouldindicateaverylightlydopedp-typematerial.Itisusefultonotethatevendegeneratelevelsofdopingimplylowconcentrationsofimpuritieswithrespecttothebasesemiconductor.Incrystallineintrinsicsilicon,thereareapproximately51022atoms/cm.Dopingconcentrationforsiliconsemiconductorsmayrangeanywherefrom1013cm-3to1018cm-3.Dopingconcentrationaboveabout1018cm-3isconsidereddegenerateatroomtemperature.Degeneratelydopedsiliconcontainsaproportionofimpuritytosiliconintheorderofpartsperthousand.Thisproportionmaybereducedtopartsperbillioninverylightlydopedsilicon.Typicalconcentrationvaluesfallsomewhereinthisrangeandaretailoredtoproducethedesiredpropertiesinthedevicethatthesemiconductorisintendedfor.EffectonbandstructureDopingasemiconductorcrystalintroducesallowedenergystateswithinthebandgapbutveryclosetotheenergybandthatcorrespondswiththedopanttype.Inotherwords,donorimpuritiescreatestatesneartheconductionbandwhileacceptorscreatestatesnearthevalenceband.Thegapbetweentheseenergystatesandthenearestenergybandisusuallyreferredtoasdopant-sitebondingenergyorEBandisrelativelysmall.Forexample,theEBforboroninsiliconbulkis0.045eV,comparedwithsiliconsbandgapofabout1.12eV.BecauseEBissosmall,ittakeslittleenergytoionizethedopantatomsandcreatefreecarriersintheconductionorvalencebands.Usuallythethermalenergyavailableatroomtemperatureissufficienttoionizemostofthedopant.DopantsalsohavetheimportanteffectofshiftingthematerialsFermileveltowardstheenergybandthatcorrespondswiththedopantwiththegreatestconcentration.SincetheFermilevelmustremainconstantina第8页systeminthermodynamicequilibrium,stackinglayersofmaterialswithdifferentpropertiesleadstomanyusefulelectricalproperties.Forexample,thep-njunctionspropertiesareduetotheenergybandbendingthathappensasaresultoflininguptheFermilevelsincontactingregionsofp-typeandn-typematerial.Thiseffectisshowninabanddiagram.Thebanddiagramtypicallyindicatesthevariationinthevalencebandandconductionbandedgesversussomespatialdimension,oftendenotedx.TheFermienergyisalsousuallyindicatedinthediagram.SometimestheintrinsicFermienergy,Ei,whichistheFermilevelintheabsenceofdoping,isshown.Thesediagramsareusefulinexplainingtheoperationofmanykindsofsemiconductordevices.PreparationofsemiconductormaterialsSemiconductorswithpredictable,reliableelectronicpropertiesarenecessaryformassproduction.Thelevelofchemicalpurityneededisextremelyhighbecausethepresenceofimpuritieseveninverysmallproportionscanhavelargeeffectsonthepropertiesofthematerial.Ahighdegreeofcrystallineperfectionisalsorequired,sincefaultsincrystalstructure(suchasdislocations,twins,andstackingfaults)interferewiththesemiconductingpropertiesofthematerial.Crystallinefaultsareamajorcauseofdefectivesemiconductordevices.Thelargerthecrystal,themoredifficultitistoachievethenecessaryperfection.Currentmassproductionprocessesusecrystalingotsbetweenfourandtwelveinches(300mm)indiameterwhicharegrownascylindersandslicedintowafers.Becauseoftherequiredlevelofchemicalpurityandtheperfectionofthecrystalstructurewhichareneededtomakesemiconductordevices,specialmethodshavebeendevelopedtoproducetheinitialsemiconductormaterial.AtechniqueforachievinghighpurityincludesgrowingthecrystalusingtheCzochralskiprocess.Anadditionalstepthatcanbeusedtofurtherincreasepurityisknownaszonerefining.Inzonerefining,partofasolidcrystalismelted.Theimpuritiestendtoconcentrateinthemeltedregion,whilethedesiredmaterialrecrystalizesleavingthesolidmaterialmorepureandwithfewercrystallinefaults.Inmanufacturingsemiconductordevicesinvolvingheterojunctionsbetween第9页differentsemiconductormaterials,thelatticeconstant,whichisthelengthoftherepeatingelementofthecrystalstructure,isimportantfordeterminingthecompatibilityofmaterials.Withtheinformationdisplaytechnology,ithasformedCRT,LCD,PDP,LED,EL,DLPseriesofinformationdisplayproducts,lookatthedevelopmentofvarioustypesofdisplayproducts,14inchesisthedominantLCD,1432inchesCRTledby4070inchesor100-inchPDPmarketwillbe100inchesormore,whetherindoorsoroutdoors,LEDdisplaytoabsoluteadvantagetobecomemainstream.Inthispaper,LEDdisplayproductapplications,industryandmarkettrendsareintroduced.LEDdisplayproductlineswerealotofgeneralcommonformmainlythefollowingcategories:Accordingtotheuseoftheenvironment:indoordisplayandoutdoordisplays;light-emittingpixelsaccordingtothesizeof:3、3.75、5、8、15、19、26,etc.;light-emittingpixelsaccordingtocolor:color,redandgreendual-color,red,greenandbluecolor(fullcolor);underthedisplaycontrolmode:Communicationscreen,videodisplay;accordingtousagerequirements:dotmatrixscreen,dotmatrixdigitalhybridscreen.Afterseveralyearsofdevelopment,ChinaLEDdisplayindustryhasbeguntotakeshape,somewithstrengthandinfluenceofthecorporatebusinessstrategytothedevelopmentofLEDdisplayasanimportantcontentofChinasaccessiontoWTO,suchasBeijingssuccessfulOlympicbid,aLEDdisplayindustrialdevelopmentopportunities,isexpectedtolasttwoyears,ChinasLEDdisplayindustrywillhavegreateradjustmentanddevelopmentWhilein-scaledevelopment,LEDdisplayhasmaintainedthismoreadvancedlevel.Theearly90sthathaveaproven16-levelgrayscaleand256colorvideowirelessremotecontroltechnologyandotherinternationaladvancedleveloftechnology,inrecentyears,full-colorLEDdisplay,256levelgray-scalevideocontroltechnology,theclusterofwirelesscontrol,multi-groupcontroladvancedtechnology,andhavereachedinternationalstandardsoftechnologyandproductsthere;LEDdisplaycontrolLSIhasbeendedicateddevelopmentandproductionofdomesticenterprisesandapplied.CurrentlythemainLEDdisplaymanufacturersintheEastChina,NorthChina,SouthChinaarea,largemanufacturersmarketrangecoveringalmostthe第10页entiredomesticmarket.ThenumberofdomesticLEDmanufacturersarealsoincreasingyearbyyear.Comparingthecurrentscale(annualproductionof300millionyuan)estimatesthatmorethan150,ofwhichtensofmillionsofannualoutputvalueofmorethan20.Andbythedry-madeLEDdisplayrelativelyhighcost,themarketshareofnearly100%,basicallynomarketofsimilarproductsabroad.Butdomesticmanufacturersofmany,noonecompanyachievedmorethan10%marketshare,intheerascrowded.WithUDdisplayproductsindustry,thegradualandorderlycompetition,themarketwillsoonturnintolarge-scale,brandcompetition,whenthestrongestthreetofivecompaniestogetheraccountformorethan50%marketshare,thedisplaymarketwillmature.LEDdisplaystheinternationalscopeoftheapplicationfocusedonoutdooradvertisingandsports,trafficandperformance,otherexhibitions,leasing,assemblyandotheroccasions,withanestimated30%marketcapacityinaccordancewiththeannualgrowthrateoftheincrease.Atpresent,themajormanufacturersfocusedonJapan,NorthAmericaChinaTaiwanandothercountriesandregions,ChinasLEDmanufacturersinwhichtheinsignificantshareofexports.