acceptor受主donorrecombination復(fù)合majority多子minority少子transitionregion過渡區(qū)depletionregion耗盡區(qū)contactbarrier接觸勢(shì)壘p-njunctionpn結(jié)heterojunction/異質(zhì)結(jié)EHP電子空穴對(duì)homojunction/同質(zhì)結(jié)

Schottkybarrier/肖特基勢(shì)壘barrierheight/勢(shì)壘高度ideal/理想的workfunction/功函數(shù)practical/實(shí)際的electronaffinity/電子親和能Fermilevel/費(fèi)米能級(jí)rectifier/整流器electrostaticpotential/靜電勢(shì)breakdown/擊穿rectifyingcontacts/整流接觸Ohmiccontacts/歐姆接觸surfacestate/表面態(tài)lattice-matched/晶格匹配的tunnelingeffect/隧道效應(yīng)半導(dǎo)體器件工作的基本方程泊松方程電流密度方程電流連續(xù)性方程5.7.1SchottkyBarriersPossibly,inretrospect,Schottky'smostimportantscientificachievementwastodevelop(in1914)thewell-knownclassicalformula,nowwritten-q2/4πε0x,fortheinteractionenergybetweenapointcharge

qandaflat

metalsurface,whenthechargeisatadistancexfromthesurface.Owingtothemethodofitsderivation,thisinteractioniscalledthe"imagepotentialenergy"(imagePE).Schottkybasedhisworkonearlierworkby(Lord)KelvinrelatingtotheimagePEforasphere.Schottky'simagePEhasbecomeastandardcomponentinsimplemodelsofthebarriertomotion,M(x),experiencedbyanelectrononapproachingametalsurfaceorametal-semiconductorinterfacefromtheinside.1.Schottkyeffect(TheimagePEisusuallycombinedwithtermsrelatingtoanappliedelectricfield

Fandtotheheighth(intheabsenceofanyfield)ofthebarrier.Thisleadstothefollowingexpressionforthedependenceofthebarrierenergyondistancex,measuredfromthe"electricalsurface"ofthemetal,intothevacuumorintothesemiconductor:

Here,eistheelementarypositivecharge,ε0istheelectricconstantandεristherelativepermittivityofthesecondmedium(=1forvacuum).Inthecaseofametal-semiconductorjunction,thisiscalledaSchottkybarrier;inthecaseofthemetal-vacuuminterface,thisissometimescalledaSchottky-Nordheimbarrier.Inmanycontexts,hhastobetakenequaltothelocalworkfunction

φ.ThisSchottky-Nordheimbarrier(SNbarrier)hasplayedinimportantroleinthetheoriesofthermionicemissionandoffieldelectronemission.Applyingthefieldcausesloweringofthebarrier,andthusenhancestheemissioncurrentinthermionicemission.Thisiscalledthe"Schottkyeffect",andtheresultingemissionregimeiscalled"Schottkyemission".Theworkfunction(功函數(shù)):TheenergywiththeworkfunctionisrequiredtoremoveanelectronattheFermileveltothevacuumoutsidethemetal.(Al=4.3eV)theelectronaffinity(電子親和能):Theenergywiththeelectronaffinityisrequiredtoremoveanelectronatthebottomoftheconductionbandtothevacuumoutsidethesemiconductors.(Si=4.1eV)TwokindsofMScontactswilloccur:rectifyingandohmicTherectifyingcontactissimilartothep-njunctioninfunction.(diode)Theohmiccontactisusuallyusedtoconnectsemiconductorstoelectrodesorwires.(resistor)（1）qm>qsMetal/n-typesemiconductor2.Schottkybarriers（肖特基勢(shì)壘）chargetransferoccursuntiltheFermilevelsalignatequilibriumtheelectrostaticpotentialofthesemiconductormustberaisedV0theelectronenergymustbelowered.–qV0thedepletionregionisformednearthejunction.Contactpotentialbarrier:qV0=qm-qspotentialbarrierheightqB=qm-qΦBiscalledSchottkybarrier5.7.2Rectifyingcontacts(diode)YoucantreattheSchottkybarrierforM/n-typeS(orM/p-typeS)astheP+N(orN+P)junctionexceptthatnoholeinjectionoccursfromthemetalintothesemiconductor.(a)ForwardbiasElectronsflowfromsemiconductortoMetalunderforwardbias.Case1:Metal-ntypecontact(b)ReversebiasOneimportantfeatureisthatthesaturationcurrentI0

dependsuponBandisunaffectedbythebiasvoltage.I0∝Exp(-qB/kT)Howaboutp-njunction?Forpnjunction:ForM-Sjunction

~10-11A/cm25.7.3Ohmiccontacts(ideal,resistor)Inmanycaseswewishtohaveanohmicmetal-semiconductorcontact,havingalinearI-Vcharacteristicinbothbiasingdirections.Forexample,theinterconnectedlines(互連線)arerequiredinIC.So,itisimportantthatsuchcontactsbeohmic,withminimalresistanceandnotendencytorectifysignals.RThebarriertoelectronflowbetweenthemetalandsemiconductorissmallandeasilyovercomebyasmallvoltage.Case1M/ntype,qΦm<qΦsCase2:metal-ptypesemiconductor,qm>qsUnliketherectifyingcontacts,nodepletionregionoccursinthesemiconductorinthesetwocasessincetheelectrostaticpotentialdifferencerequiredtoaligntheFermilevelatequilibriumcallsforaccumulationofmajoritycarriersinthesemiconductor.5.7.3Ohmiccontacts(practical)Inpractice,itisalwaysimpossibleforustoformtheohmiccontactsofmetal-semiconductorbychoosingtheappropriateworkfunctionofthemetal.Thereasonisthatthesemiconductorsurfacecontainssurfacestatesduetoincompletecovalentbonds.Themetal/semiconductorcontactsformalwaysabarrierwhatevermetalsareused.So,toobtaintheohmiccontactsformetal/semiconductor,apracticalmethodisbydopingthesemiconductorheavilyinthecontactregions.Therefore,ifabarrierexistsattheinterface,