【毕业论文】外文翻译--半导体.doc
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【毕业论文】外文翻译--半导体.doc
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无锡职业技术学院
毕业设计说明书(英文翻译)
Semiconductors
1SemiconductorsFundamentals
1.1SemiconductorsMaterials
Solid-statematerialscanbegroupedintothreeclasses—insulators,semiconductorsandconductors.Insulatorssuchasfusedquartzandglasshaveverylowconductivities.Semiconductorshaveconductivities,typicallybetweenthoseofinsulatorsandthoseofconductors.Theconductivityofasemiconductorisgenerallysensitivetotemperature,illumination,magneticfield,andminuteamountofimpurityatoms.Thissensitivityinconductivitymakesthesemiconductoroneofmostimportantmaterialsforelectronicapplication.
Thestudyofsemiconductormaterialsbeganintheearlynineteenthcentury.Overtheyearsmanysemiconductorshavebeeninvestigated.Theelementsemiconductors,thosecomposedofsinglespeciesofatoms,suchassilicon(Si)andgermanium(Ge)canbefoundinColumn4.however,numerouscompoundsemiconductorsarecomposedoftwoormoreelements.Forexample,galliumarsenide(GaAs)isan3-5compoundthatisacombinationofgallium(Ga)fromColumn3andarsenic(As)fromColumn5.
Priortotheinventionofthebipolarin1947,semiconductorswereusedonlyastwo-terminaldevices,suchasrectifiersandphotodiodes.Intheearly1950s,germaniumwasthemajorsemiconductormaterial.However,germaniumprovedunsuitableinmanyapplicationsbecausegermaniumdevicesexhibitedhighleakagecurrentsatonlymoderatelyelevatedtemperatures.Inaddition,germaniumoxideiswatersolubleandunsuitedfordevicefabrication.Sincetheearly1960ssiliconhasbecomeapracticalsubstituteandhasnowvirtuallysupplantedgermaniumasamaterialforsemiconductorfabrication.Themainreasonswenowusesiliconarethatsilicondevicesexhibitmuchlowerleakagecurrents,andhigh-qualitysilicondioxidecanbegrownthermally.Thereisalsoaneconomicconsideration.Siliconintheformofsilicaandsilicatescomprises25%oftheEarth’scrust,andsiliconissecondonlytooxygeninabundance.Atpresent,siliconisoneofthemoststudiedelementsintheperiodictable;andsilicontechnologyisbyfarthemostadvancedamongallsemiconductortechnologies.
Manyofthecompoundsemiconductorshaveelectricalandopticalpropertiesthatareabsentinsilicon.Theresemiconductorsespeciallygalliumarsenide(GaAs)areusedmainlyformicrowaveandphotonicapplications.Althoughwedonotknowasmuchaboutthetechnologyofcompoundsemiconductorsaswedoaboutthatofsilicon,compoundsemiconductortechnologyhaspartlybecauseoftheadvancesinsilicontechnology.
1.2CrystalStructure
Solidsmaybeclassifiedbystructuralorganizationintocrystalline,polycrystalline,and,amorphoustypes.Anamorphoussoliddoesnothaveawell-definestructure;infact,itisdistinguishedbyitsformlessness.Inthepastdecade,amorphoussiliconhasreceivedagreatdealofattention,primarilyduetoitsapplicationinlowcostsolarcells.Recently,amorphoussiliconsolarcellswithanefficiencyofgreaterthan10percenthavebeenrealized,andlower-efficiencydevicesarebeingusedinconsumerelectronics,e.g.,hand-heldcalculatorsandcameras.Exploratoryworkisbeingperformedtobuildamorphousfield-effecttransistors(FETs)forlarge-areadisplaysandimagesensors.Nevertheless,amorphoussemiconductorsarenotexpectedtoplayanimportantroleinmicroelectronicsintheforeseeablefuture.Itshoudbepointedoutthatsilicondioxide,anextremelyimportantmaterialinsemiconductortechnologyisalsoanamorphoussolid.However,itisusedasaninsulator,soitselectricalconductionpropertyisnotofimportancetous.
Inapolycrystallinesolid,therearemanysmallregions,eachhavingawell-organizedstructurebutdifferingfromitsneighboringregions.Suchamaterialcanbeproducedinexpensivelyandisusedextensivelyinmicroelectronics,e.g.,polycrystallinesilicon(Poly-Si),whichisusedasaconductor,contact,orgateintransistors.Bothamorphousandpolycrystallinematerialsarestructurallymorecomplex,resultinginlesswell-defineddevicephysics.
1.3ValenceBondsModelofSolid
Inacrystallattice,apositivelychargednucleusissurroundedbynegativelychargedorbitingelectronsineachconstituentatom.Iftheatomsarecloselypacked,theorbitsoftheouter-shellelectronswilloverlaptoproducestronginteratomicforces.Theouterelectrons,i.e.,valenceelectrons,areofprimaryimportanceindeterminingtheelectricalpropertiesofthesolid.Inametallicconductorsuchasaluminumorgold,thevalenceelectronsaresharedbyalltheatomsinthesolid.Theseelectronsarenotboundtoindividualatomsandarefreetocontributetotheconductionofcurrentupontheapplicationofanelectricfield.Thefree-electrondensityofametallicconductorisontheorderof1023cm-3,andtheresultingresistivityissmallerthan10-5cm.Inaninsulatorsuchasquartz(SiO2),almostallthevalenceelectronsremaintightlyboundtotheconstituentatomsandarenotavailableforcurrentconduction.Asaresult,theresistivityofsiliconisgreaterthan1016cm.
AsdiscussedinSection1.1,eachatominadiamondlatticeissurroundedbyfournearestneighbors.Eachatomhasfourelectronsintheouterorbit,andeachatomsharestherevalanceelectronswithitsfourneighbors.Thissharingofelectronsisknownascovalentbonding;eachelectronpairconstitutesacovalentbond.Covalentbondingoccursbetweenatomsofthehavesimilarouter-shellelectronconfigurations.Eachelectronspendsanequalamountoftimewitheachnucleus.However,bothelectronsspendmostoftheirtimebetweenthetwonuclei.Theforceofattractionfortheelectronsbybothnucleiholdsthetwoatomstogether.Forazincblendelatticesuchasgalliumarsenide,themajorbondingforceisfromthecovalentbonds.However,galliumarsenidehasaslightionicbondingforcethatisanelectrostaticattractiveforcebetweeneachGa-ionanditsfourneighboringAs+ionanditsfourneighboringGa-ions.
Atlowtemperature,theelectronsareboundintheirrespectivetetrahedronlattice;consequently,theyarenotavailableforconduction.Athightemperatures,thethermalenergyenablessomeelectronstobreakthebond,andtheliberatedelectronsarethenfreetocontributetocurrentconduction.Thus,asemiconductorbehaveslikeaninsulatoratlowtemperaturesandaconductorathightemperatures.Atroomtemperature,theresistivityofpuresiliconis2×105cm,whichisconsiderablyhigherthanthatofagoodconductor.
Wheneveravalenceelectronisliberatedinasemiconductor,avacancyisleftbehindinthecovalentbond.Thisdeficiencymaybefilledbyoneoftheneighboringelectrons,whichresultsinashiftofthedeficiencylocation,asfromlocationAtolocationB.Wemaythereforeconsiderthisdeficiencyasaparticlesimilartoanelectron.Thisfictitiousparticleiscalledahole.Itcarriesapositivechargeandmoves,undertheinfluenceofanappliedelectricfield,inthedirectionoppositetothatofanelectron.Theconceptofaholeisanalogoustothatofabubbleinaliquid.Althoughitisactuallytheliquidthatmoves,itismucheasiertotalkaboutthemotionofthebubbleintheoppositedirection.
1.4DonorsandAcceptors
Whenasemiconductorisdopedwithimpurities,thesemiconductorbecomesextrinsicandimpurityenergylevelsareintroduced.Asiliconatomisreplaced(orsubstituted)byanarsenicatomwithfivevalenceelectrons.Thearsenicatomformscovalentbondwithitsfourneighboringsiliconatoms.Thefifthelectronbecomesaconductionelectronthatis“donated”totheconductionband.Thesiliconbecomesn-typebecauseoftheadditionofthenegativechargecarrier,andthearsenicatomiscalledadonor.Similar,whenaboronatomwiththreevalenceelectronssubstituteforasiliconatom,additionalelectronisacceptedtoformfourcovalentbondsaroundtheboron,andapositivelycharged“hole”iscreatedinthevalenceband.Thisisap-typesemiconductor,andtheboronisanacceptor.
Forshallowdonorsinsiliconandgalliumarsenide,thereusuallyisenoughthermalenergytosupplytheenergyEDtoionizealldonorimpuritiesatroomtemperatureandthusprovidesanequalnumberofelectronsintheconductionband.Thisconditioniscalledcompleteionization.Underacompleteionizationcondition,wecanwritetheelectrondensityas
N=ND(1.1)
WeobtaintheFermilevelintermoftheeffectivedensityofstatesNcandthedonorconcentrationND
EC-EF=ktln(NC/ND)(1.2)
Itisclearthatthehigherthedonorconcentration,thesmallertheenergydifference(EC-EF),thatis,theFermilevelwillmoveclosertothebottomoftheconductionband.Similarly,forhigheracceptorconcentration,theFermilevelwillmoveclosertothetopofthevalenceband.
2CharacteristicsofDiodes
2.1Introduction
Mostelectronicdevicesdependontheelectricalcharacteristicsofjunctionsbetweendifferentmaterials.Suchjunctionsaretwo-terminaldevicesandarereferredtoasdiodes.Whenn-typeandp-typesiliconcrystalsarejoinedtogether,apnjunctionisformed.Inpractice,apnjunctionisformedbyaddingaccepterimpuritiestoann-typewaferordonorstoap-typewafer.Thereareavarietyofmethodsforjunctionformation.
Thetechniquesofalloying,epitaxy,diffusion,andionimplantationhavereportedinsomereferencestoproducethepnjunction.Inalloyingtechnique,athinaluminumfilmisevaporatedontoacleann-typesiliconwafer,calledasubstrate,insideavacuumchamber.Thesiliconisthenplacedinafurnacesetatabout600for30min,
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