Semiconductor devices Neaman

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    CHAPTER9 MetalSemiconductor and

    SemiconductorHeterojunctions

    Determinetheenergybanddiagramofametalsemiconductor

    junction.

    Investigatetheelectrostaticsoftherectifyingmetal

    semiconductorjunction,whichisknownastheSchottky barrier

    diode.

    DerivetheidealcurrentvoltagerelationoftheSchottky barrierdiode.

    Discussdifferencesinthecurrenttransportmechanism

    betweentheSchottky barrierdiodeandpn junctiondiode,and

    discussdifferencesinturnonvoltageandswitchingtimes. Discussohmic contacts,whicharelowresistance,nonrectifying

    metalsemiconductorjunctions.

    Investigatethecharacteristicsofasemiconductor

    heterojunction. 1

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    9.1|THESCHOTTKYBARRIER

    DIODE

    9.1.1 QualitativeCharacteristics

    Theparameterm isthemetal

    workfunction(measuredinvolts),

    s isthesemiconductorwork

    function,

    Xisknownastheelectronaffinity.

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    InorderfortheFermileveltobecomeaconstantthroughthesystemin

    thermalequilibrium,electronsfromthesemiconductorflowintothelower

    energystatesinthemetal.

    Positivelychargeddonoratomsremaininthesemiconductor,creatingaspacechargeregion.

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    9.1.2 IdealJunctionProperties

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    CommentofEx9.2,

    The experimental value of 0.52 V can be compared with the ideal barrier

    height of B0 =0.54V.Theseresultsagreefairlywell.

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    thebuiltinpotentialbarrierofthegallium

    arsenideSchottky diodeislargerthanthatof

    thesilicondiode.

    9.1.3 Nonideal EffectsontheBarrierHeigh

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    CommentofEx.9.3,

    AlthoughtheSchottky barrierloweringmayseemlikeasmallvalue, itcanthushave

    asignificanteffectonthecurrentinaSchottky barrierdiode.

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    InterfaceStates

    Thebarrierheightofthemetalsemiconductorjunctionisdeterminedby

    boththemetalworkfunctionandthesemiconductorsurfaceorinterface

    states.

    assumethatallstatesbelowthe

    surfacepotential0 aredonor

    states,whichwillbeneutralifthe

    statecontainsanelectronand

    positivelychargedifthestatedoesnotcontainanelectron.

    assumethatallstatesabove0 are

    acceptorstates,whichwillbeneutralifthestatedoesnotcontainan

    electronand negativelychargedif

    thestatecontainsanelectron.

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    Weassumethatanarrow

    interfaciallayerofinsulatorexistsbetween

    themetaland

    semiconductor.

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    9.1.4 CurrentVoltageRelationship

    Thecurrenttransportinametalsemiconductorjunctionisduemainlytomajoritycarriersasopposedtominoritycarriersinapn junction.

    Thebasicprocessintherectifyingcontactwithanntype

    semiconductorisbytransportofelectronsoverthepotentialbarrier,

    whichcanbedescribedbythethermionicemissiontheory.

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    Thereversebiasedcurrent

    increaseswithreverse

    biasedvoltagebecauseof

    thebarrierloweringeffect.

    ThenetresultisthatA*andJsT

    willvarywidelybetweensilicon

    andgalliumarsenide.

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    9.1.5 ComparisonoftheSchottky BarrierDiodeandthepn JunctionDiode

    therearetwoimportantdifferencesbetweenaSchottky diodeandapn

    junctiondiode:

    Thefirstisinthemagnitudesofthereversesaturationcurrentdensities

    Thesecondisintheswitchingcharacteristics.

    Thecurrentinapn junctionisdeterminedbythediffusionofminority

    carrierswhilethecurrentinaSchottky barrierdiodeisdeterminedbythermionicemissionofmajoritycarriersoverapotentialbarrier.

    CommentofEx.9.5,Theidealreversesaturationcurrentdensityofthe

    Schottky barrierjunctionisordersofmagnitudelargerthanthatofthe

    idealpn junctiondiode.

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    SinceJsT >Js ,theforwardbias

    characteristicsofthetwotypes

    ofdiodeswillalsobedifferent.

    CommentofEx.9.6,

    theSchottky barrierdiodehasa

    turnonvoltagethatis

    approximately0.37Vsmaller

    thantheturnonvoltageofthe

    pn junctiondiode.

    Thesecondmajordifferenceisinthefrequencyresponse

    thereisnodiffusioncapacitanceassociatedwithaforwardbiased

    Schottky diode.Theeliminationofthediffusioncapacitancemakesthe

    Schottky diodeahigherfrequencydevicethanthepn junctiondiode.

    whenswitchingaSchottky diodefromforwardtoreversebias,thereis

    nominoritycarrierstoredchargetoremove,asisthecaseinthepn

    junctiondiode.

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    9.2|METALSEMICONDUCTOROHMICCONTACTSContactsmustbemadebetweenanysemiconductordevice,or

    integratedcircuit,andtheoutsideworld.Thesecontactsaremadevia

    ohmic contacts.

    9.2.1 IdealNonrectifying Barrier

    consideredanidealmetalntypesemiconductorcontactwhenm >s

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    Ifapositivevoltageisappliedto

    themetal,thereisnobarrierto

    electronsflowingfromthesemiconductorintothemetal.

    Ifapositivevoltageisappliedto

    thesemiconductor,theeffective

    barrierheightforelectronsflowingfromthemetalintothe

    semiconductorwillbe

    approximatelyBn =n,.

    anidealnonrectifying

    contactbetweenametal

    andaptypesemiconductor

    whenm >s.Thisjunctionisalsoan

    ohmic contact.

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    ifm s forthemetalptypesemiconductorcontact,wemaynotnecessarilyformagoodohmic contact.

    9.2.2 TunnelingBarrier

    Thespacechargewidthinarectifyingmetalsemiconductor

    contactisinverselyproportionaltothesquarerootofthe

    semiconductordoping.

    CommentofEx9.7,

    Inaheavilydoped

    semiconductor,the

    depletionwidthisonthe

    orderofangstroms,sothat

    tunnelingisnowadistinctpossibility.Forthesetypes

    ofbarrierwidths,tunneling

    maybecomethedominant

    currentmechanism.

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    Fordopingconcentrationsgreaterthan

    approximately1019 cm3,thetunnelingprocess

    dominatesandRc showstheexponential

    dependenceonNd. Forlowerdopingconcentrations,theRc valuesare

    dependentonthebarrierheightsandbecomealmost

    independentofthedoping.

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    Toformagoodohmic contact,

    weneedtocreatealowbarrier

    anduseahighlydoped

    semiconductoratthesurface.

    Lowbarriersarenotpossibleonsomematerials,soaheavily

    dopedsemiconductoratthe

    surfacemustbeusedtoforma

    tunnelingcontact.

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    9.3|HETEROJUNCTIONS Twodifferentsemiconductormaterialsareusedtoformajunction,the

    junctioniscalledasemiconductorheterojunction.

    Inordertohaveausefulheterojunction,thelatticeconstantsofthe

    twomaterialsmustbewellmatched.

    9.3.2 EnergyBandDiagrams

    theforbiddenbandgap ofthewidegapmaterialcompletelyoverlapsthe

    bandgap ofthenarrowgapmaterial,calledstraddling,appliestomost

    heterojunctions.

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    Therearefourbasictypesofheterojunction.Thoseinwhichthe

    dopanttypechangesatthejunctionarecalledanisotype.Wecan

    formnP orNpjunctions,wherethecapitalletterindicatesthelargerbandgap material.Heterojunctions withthesamedopanttypeon

    eithersideofthejunctionarecalledisotype.WecanformnN andpP

    isotype heterojunctions.

    Ifthevacuumleveliscontinuous,

    thenthesame Ec and Ev

    discontinuitieswillexistattheheterojunction interface.This

    idealsituationisknownasthe

    electronaffinityrule.

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    electronsfromthenarrowgapnregionandholes

    fromthewidegapPregion

    mustflowacrossthe

    junction.

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    9.3.3 TwoDimensionalElectron

    Gas

    electronsfromthewidebandgap

    AlGaAs flowintotheGaAs,

    forminganaccumulationlayerof

    electronsinthepotentialwell

    adjacenttotheinterface.

    theenergyofanelectron

    containedinapotentialwellis

    quantized,ie, twodimensional

    electrongas.

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    Themovementoftheelectrons

    paralleltotheinterfacewillstillbe

    influencedbythecoulombattraction

    oftheionizedimpuritiesinthe

    AlGaAs.Theeffectoftheseforces

    canbefurtherreducedbyusinga

    gradedAlGaAsGaAs heterojunction.

    Thequalitativedistributionof

    electronsinthepotentialwell

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    *9.3.4 EquilibriumElectrostatics

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    *9.3.5 CurrentVoltageCharacteristics

    TheenergybanddiagramsinFigures9.18and9.23demonstratethatthe

    barrierheightsforelectronsandholesinaheterojunction canbesignificantlydifferent.

    ThebarrierheightforelectronsinFigure9.18islargerthanthatforholes,

    sowewouldexpectthecurrentduetoelectronstobeinsignificant

    comparedtotheholecurrent.Ifthebarrierheightforelectronsis0.2eV

    largerthanthatforholes,theelectroncurrentwillbeapproximatelya

    factorof104 smallerthantheholecurrent,assumingallotherparameters

    areequal.

    TheoppositesituationexistsforthebanddiagramshowninFigure9.23.