Download - Semiconductor devices Neaman

8/11/2019 Semiconductor devices Neaman

1/31

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


2/31

2

9.1|THESCHOTTKYBARRIER

DIODE

9.1.1 QualitativeCharacteristics

Theparameterm isthemetal

workfunction(measuredinvolts),

s isthesemiconductorwork

function,

Xisknownastheelectronaffinity.


3/31

3

InorderfortheFermileveltobecomeaconstantthroughthesystemin

thermalequilibrium,electronsfromthesemiconductorflowintothelower

energystatesinthemetal.

Positivelychargeddonoratomsremaininthesemiconductor,creatingaspacechargeregion.


4/31


5/31

5

9.1.2 IdealJunctionProperties


6/31

6

CommentofEx9.2,

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

height of B0 =0.54V.Theseresultsagreefairlywell.


7/31

7

thebuiltinpotentialbarrierofthegallium

arsenideSchottky diodeislargerthanthatof

thesilicondiode.

9.1.3 Nonideal EffectsontheBarrierHeigh


8/31

8

CommentofEx.9.3,

AlthoughtheSchottky barrierloweringmayseemlikeasmallvalue, itcanthushave

asignificanteffectonthecurrentinaSchottky barrierdiode.


9/31

9

InterfaceStates

Thebarrierheightofthemetalsemiconductorjunctionisdeterminedby

boththemetalworkfunctionandthesemiconductorsurfaceorinterface

states.

assumethatallstatesbelowthe

surfacepotential0 aredonor

states,whichwillbeneutralifthe

statecontainsanelectronand

positivelychargedifthestatedoesnotcontainanelectron.

assumethatallstatesabove0 are

acceptorstates,whichwillbeneutralifthestatedoesnotcontainan

electronand negativelychargedif

thestatecontainsanelectron.


10/31

10

Weassumethatanarrow

interfaciallayerofinsulatorexistsbetween

themetaland

semiconductor.


11/31

11

9.1.4 CurrentVoltageRelationship

Thecurrenttransportinametalsemiconductorjunctionisduemainlytomajoritycarriersasopposedtominoritycarriersinapn junction.

Thebasicprocessintherectifyingcontactwithanntype

semiconductorisbytransportofelectronsoverthepotentialbarrier,

whichcanbedescribedbythethermionicemissiontheory.


12/31


13/31

13

Thereversebiasedcurrent

increaseswithreverse

biasedvoltagebecauseof

thebarrierloweringeffect.

ThenetresultisthatA*andJsT

willvarywidelybetweensilicon

andgalliumarsenide.


14/31

14

9.1.5 ComparisonoftheSchottky BarrierDiodeandthepn JunctionDiode

therearetwoimportantdifferencesbetweenaSchottky diodeandapn

junctiondiode:

Thefirstisinthemagnitudesofthereversesaturationcurrentdensities

Thesecondisintheswitchingcharacteristics.

Thecurrentinapn junctionisdeterminedbythediffusionofminority

carrierswhilethecurrentinaSchottky barrierdiodeisdeterminedbythermionicemissionofmajoritycarriersoverapotentialbarrier.

CommentofEx.9.5,Theidealreversesaturationcurrentdensityofthe

Schottky barrierjunctionisordersofmagnitudelargerthanthatofthe

idealpn junctiondiode.


15/31

15

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.


16/31

16

9.2|METALSEMICONDUCTOROHMICCONTACTSContactsmustbemadebetweenanysemiconductordevice,or

integratedcircuit,andtheoutsideworld.Thesecontactsaremadevia

ohmic contacts.

9.2.1 IdealNonrectifying Barrier

consideredanidealmetalntypesemiconductorcontactwhenm >s


17/31

17

Ifapositivevoltageisappliedto

themetal,thereisnobarrierto

electronsflowingfromthesemiconductorintothemetal.

Ifapositivevoltageisappliedto

thesemiconductor,theeffective

barrierheightforelectronsflowingfromthemetalintothe

semiconductorwillbe

approximatelyBn =n,.

anidealnonrectifying

contactbetweenametal

andaptypesemiconductor

whenm >s.Thisjunctionisalsoan

ohmic contact.


18/31

18

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.


19/31

19


20/31

20

Fordopingconcentrationsgreaterthan

approximately1019 cm3,thetunnelingprocess

dominatesandRc showstheexponential

dependenceonNd. Forlowerdopingconcentrations,theRc valuesare

dependentonthebarrierheightsandbecomealmost

independentofthedoping.


21/31

21

Toformagoodohmic contact,

weneedtocreatealowbarrier

anduseahighlydoped

semiconductoratthesurface.

Lowbarriersarenotpossibleonsomematerials,soaheavily

dopedsemiconductoratthe

surfacemustbeusedtoforma

tunnelingcontact.


22/31

22

9.3|HETEROJUNCTIONS Twodifferentsemiconductormaterialsareusedtoformajunction,the

junctioniscalledasemiconductorheterojunction.

Inordertohaveausefulheterojunction,thelatticeconstantsofthe

twomaterialsmustbewellmatched.

9.3.2 EnergyBandDiagrams

theforbiddenbandgap ofthewidegapmaterialcompletelyoverlapsthe

bandgap ofthenarrowgapmaterial,calledstraddling,appliestomost

heterojunctions.


23/31

23

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.


24/31

24

electronsfromthenarrowgapnregionandholes

fromthewidegapPregion

mustflowacrossthe

junction.


25/31

25

9.3.3 TwoDimensionalElectron

Gas

electronsfromthewidebandgap

AlGaAs flowintotheGaAs,

forminganaccumulationlayerof

electronsinthepotentialwell

adjacenttotheinterface.

theenergyofanelectron

containedinapotentialwellis

quantized,ie, twodimensional

electrongas.


26/31

26

Themovementoftheelectrons

paralleltotheinterfacewillstillbe

influencedbythecoulombattraction

oftheionizedimpuritiesinthe

AlGaAs.Theeffectoftheseforces

canbefurtherreducedbyusinga

gradedAlGaAsGaAs heterojunction.

Thequalitativedistributionof

electronsinthepotentialwell


27/31

27

*9.3.4 EquilibriumElectrostatics


28/31

28


29/31

29


30/31

30


31/31

31

*9.3.5 CurrentVoltageCharacteristics

TheenergybanddiagramsinFigures9.18and9.23demonstratethatthe

barrierheightsforelectronsandholesinaheterojunction canbesignificantlydifferent.

ThebarrierheightforelectronsinFigure9.18islargerthanthatforholes,

sowewouldexpectthecurrentduetoelectronstobeinsignificant

comparedtotheholecurrent.Ifthebarrierheightforelectronsis0.2eV

largerthanthatforholes,theelectroncurrentwillbeapproximatelya

factorof104 smallerthantheholecurrent,assumingallotherparameters

areequal.

TheoppositesituationexistsforthebanddiagramshowninFigure9.23.