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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.
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