《电子陶瓷》PPT课件

p-njunctionsElectronsandHolesInanintrinsicS.Mlikesiliconattemperaturesaboveabsolutezero,therewillbesomeelectronswhichareexcitedacrossthebandgapintoC.VandwhichcanproducecurrentWhentheelectroninpuresiliconcrossesthegap,itleavesbehindanelectronvacancyor"hole"intheregularsiliconlatticeUndertheinfluenceofanexternalvoltage,boththeelectronandtheholecanmoveacrossthematerialInann-typesemiconductor,thedopantcontributesextraelectrons,dramaticallyincreasingtheconductivityInap-typesemiconductor,thedopantproducesextravacanciesorholes,whichlikewiseincreasetheconductivityP-andN-typeS.MBandsforDopedSemiconductorsTheapplicationofbandtheoryton-typeandp-typesemiconductorsshowsthatextralevelshavebeenaddedbytheimpurities.Inn-typematerialthereareelectronenergylevelsnearthetopofthebandgapsothattheycanbeeasilyexcitedintotheconductionband.Inp-typematerial,extraholesinthebandgapallowexcitationofvalencebandelectrons,leavingmobileholesinthevalenceband.P-njunctionTheopencirclesontheleftsideofthejunctionaboverepresent"holes"ordeficienciesofelectronsinthelatticewhichcanactlikepositivechargecarriers.Thesolidcirclesontherightofthejunctionrepresenttheavailableelectronsfromthen-typedopant.Nearthejunction,electronsdiffuseacrosstocombinewithholes,creatinga"depletionregion".TheenergylevelsketchaboverightisawaytovisualizetheequilibriumconditionoftheP-Njunction.Theupwarddirectioninthediagramrepresentsincreasingelectronenergy.Completep-njunctionAp-njunctioninthermalequilibriumwithzerobiasvoltageapplied.Electronsandholesconcentrationaredrawnrespectivelywithblueandredlines.Grayregionsarechargeneutral.Lightredzoneispositivelycharged.Lightbluezoneisnegativelycharged.Theelectricfieldisshownonthebottom,theelectrostaticforcebuildingbetweenelectronsandholesandthedirectioninwhichthediffusiontendstomoveelectronsandholes.DepletionRegionWhenap-njunctionisformed,someofthefreeelectronsinthen-regiondiffuseacrossthejunctionandcombinewithholestoformnegativeions.Insodoingtheyleavebehindpositiveionsatthedonorimpuritysites.Depletionregionisanon-conductinglayerMoreDetailInthep-typeregionthereareholesfromtheacceptorimpuritiesandinthen-typeregionthereareextraelectronsWhenap-njunctionisformed,someoftheelectronsfromthen-regionwhichhavereachedtheconductionbandarefreetodiffuseacrossthejunctionandcombinewithholesFillingaholemakesanegativeionandleavesbehindapositiveiononthen-side.Aspacechargebuildsup,creatingadepletionregionwhichinhibitsanyfurtherelectrontransferunlessitishelpedbyputtingaforwardbiasonthejunction.RecombinationCurrentinS.MBothelectronsandholescontributetocurrentflowinanintrinsicsemiconductorThecurrentwhichwillflowinanintrinsicS.Mconsistsofbothelectronandholecurrent.Thatis,theelectronswhichhavebeenfreedfromtheirlatticepositionsintotheconductionbandcanmovethroughthematerialInaddition,otherelectronscanhopbetweenlatticepositionstofillthevacanciesleftbythefreedelectrons.Thisadditionalmechanismiscalledholeconductionbecauseitisasiftheholesaremigratingacrossthematerialinthedirectionoppositetothefreeelectronmovement.BiaseffectonelectronsinD.Z:EquilibriumofjunctionCoulombforcefromionspreventsfurthermigrationacrossthep-njunction.electronswhichhadmigratedacrossfromtheNtoPregioninformingofdepletionlayerhavenowreachedequilibrium.OtherelectronsfromtheNregioncannotmigratebecausetheyarerepelledbythenegativeionsinthePregionandattractedbythepositiveionsintheNregionBiaseffectonelectronsinD.Z:ReverseBiasConnectingtheP-typeregiontothenegativeterminalofthebatteryandtheN-typeregiontothepositiveterminal,producesthereverse-biaseffectAnappliedvoltagewiththeindicatedpolarityfurtherimpedestheflowofelectronsacrossthejunction.Forconductioninthedevice,electronsfromtheNregionmustmovetothejunctionandcombinewithholesinthePregion.Areversevoltagedrivestheelectronsawayfromthejunction,preventingconduction.BiaseffectonelectronsinD.Z:ForwardBiasForward-biasoccurswhentheP-typesemiconductormaterialisconnectedtothepositiveterminalofabatteryandtheN-typeS.Mmaterialisconnectedtothenegativeterminal,asshownbelowAnappliedvoltageintheforwarddirectionasindicatedassistselectronsinovercomingthecoulombbarrierofthespacechargeindepletionregion.Electronswillflowwithverysmallresistanceintheforwarddirection.Or,youcansaythedepletionregionbecomesnarrowenoughBandstructureofp-njunctionI–VCharacteristicsApnjunctionisanon-lineardevice,a“valve”withan“easy”anda“difficult”currentdirectionIthasanonvoltageandabreakdownvoltageIVpnVIBreakdownprocessesarenon-destructiveandarereversibleIntroductionpnandnpdiodecurrent-voltagecharacteristicsVIVIpnVIVInpThesecurrentsdonotobeytheOhm’slawSchottkybarrierstoconductionsSchottkyBarrierVariousSchottkybarrierdiodes:Smallsignalrfdevices(left),mediumandhighpowerSchottkyrectifyingdiodes(middleandright)ASchottkydiodeisaspecialtypeofdiodewithaverylowforward-voltagedropSchottkyDiodeASchottkydiodeconsistofametallayeronasemiconductorsubstrateAccordingtothesimpleSchottkytheory,thebarrierheightFBthatexistsatthemetal/semiconductorinterfaceisgivenbyTheelectronsmustflowoverthebarrierwhenthedeviceisbiased-knownasthermionicemissionFBineVfBinV;FB=-qfBFBorfB?!what’sgoingonhere?ECEVV=0fBVbiVSchottlyDiodesEnergybanddiagramofthemetalandthesemiconductorbefore(a)andafter(b)contactismadeSchottlyDiodesSchottkyDiodeThecurrent-voltagebehaviorofaSchottkydiodeisgivenbyVonVBDIV(A:area;A*:RichardsonConstant)ECEVV=0fBVbiVIncreasingfBSchottkyDiodeThecurrent-voltagebehaviorofaSchottkydiodeissimilartothatofapnjunctionECEVV=0fBVbiECEVV<0BarrierisfBVbi+VECEVV>0BarrierisVbi-VElectronflowMetal-n-S.MContactTheenergyfmrequiredtoremoveanelectronwiththeFermienergytoapointoutsidethemetalwithzero.Whenelectronsarethermallyexcitedoutofasemiconductor,theeffectiveworkfunctionfsofthesemiconductoristheenergydifferencebetweentheFermienergyandthevacuumlevel.OhmicContactTheaimofanohmiccontactprovidethecurrentthatthedevicerequiresthevoltagedropacrossthecontactshouldbelessthanthevoltagedropacrossthedeviceTherearetwoimpedimentsforcurrentflow:thebarrierheightandthebarrierwidthItisalmostimpossibletochangethebarrierheight,butwecanchangethebarrierwidthWbyincreasingthedopingdensityNDECEFEVWThermionicemission(overthebarrier)SchottkyDiodeECEFEVWFieldemission(tunneling)(throughthebarrier)OhmicContactOhmicContactI-VchangessubstantiallyasthebarrierwidthchangesASchottkydiodeturnsintoanohmiccontactIVOhmicContactNDincreasesSchottkyDiodeOhmicContactCompletebanddiagramofapnjunctionIhavenevereverseenanenergybanddiagramlikethisbefore!p+pnn+OhmicTunnelContactOhmicTunnelContactTunnelingTunnelingAnimportantpropertyofEfisthat,forasysteminthermalequilibrium,itisconstantthroughoutthesystem.ApplicationsMetal-semiconductordevicesfindapplicationsas:NonlinearSchottkydiodesRectifiers(canvarytheonvoltage)Highfrequencydetectors(majoritycarrierdevices)Gatesofcertainfield-effecttransistors(metal-semiconductorFETs)OhmiccontactsOhmiccontactsSummaryForward-biasandreverse-biaspropertiesofthep-njunctionimplythatitcanbeusedasadiode.Ap-njunctiondiodeallowselectricchargestoflowinonedirection,butnotintheoppositedirection;negativecharges(electrons)caneasilyflowthroughthejunctionfromntopbutnotfromptonandthereverseistrueforholesWhenthep-njunctionisforward-biased,electricchargeflowsfreelyduetoreducedresistanceofthep-njunctionWhenthep-njunctionisreverse-biased,however,thejunctionbarrier(andthereforeresistance)becomesgreaterandchargeflowisminimal....SemiconductivityinOxidesModelsforoxidessystemaremuchmorecomplexthanforsiliconbecauseofPurityLimitationinOxidesElectronic/IonicCompensationofSolidsInacovalentS.M,theeffectivenessofdonorsandacceptorsissolelydeterminedbytheirionizationenergiesInoxideS.M,theydependsontheextentofoxidationandreductionAnaliovalentsoluteinanioniccompoundcanbecharge-compensatedbyionicdefectsorbyelectrons/holes,ormixedcompensationLargevariationsinelectricalconductivitycanresultwithchangesinPO2andTataconstantdopinglevel,duetochangesinthecompensationmechanismNote:Thetermionizationenergyismostcommonlyusedtorefertotheenergyrequiredtoremove(toinfinity)theoutermostelectronintheatomormoleculewhenthegasatomormoleculeisisolatedinfreespaceandisinitsgroundelectronicstateExample:IncorporationofNb2O5intoTiO2IoniccompensationElectronicCompensationOxidation/reductionequilibriumAtlowerNbconcentrations/higherT./lowerO2pressure,whichisdominant?AthigherNbconcentrations/lowerT./higherO2pressure,whichisdominant?SimultaneousDefectEquilibria:BrouwerDiagramDefectformationreactionsofinterestwillgenerallyincludethosePredominantintrinsicionicdefects(SchottkyorFrenkel)IntrinsicelectronicdefectsOxidationandreductionIncorporationofanysignificantsolutesorimpuritiesThesereactionswillinvolveatotalofNdefects,however,concentrationofwhichareNvariablestobesolvedforExample:IntrinsicMgOTherearefourdefectsofinterestRelevantdefect-formingreactionsare:Therespectiveequilibriumconstant:Oneadditionalequationisnecessarycalledelectron-neutralityconditionBrouwerApproximationAssumingasingledefectofeachsignhasaconcentrationmuchhigherthanothersofthesamesign,thereisjustonepositivedefectandonenegativedefectForpureMgO,fourBrouwerapproximationsarepossibleOnlyoneisimportantin(3)and(4)Athightemperature,(3)isdominantAtthelowestrangeofoxygenpressure,(1)islikelyimportantAtthehighestrangeofoxygenpressure,(2)islikelyimportantDrawingBrouwerDiagraminthecenterregiondefinedbyOverthisregionKi1/2liesbelowKs1/2WithsufficientreductionThewidthofeachBrouwerregimeistemperature-dependentSimpleProceduresforConstructingaBrouwerDiagramDeterminetherelevantdefectspecies/crystalstructure,soluteconcentration,andelectricalconductivityordiffusionratesWriteindependentdefectchemicalreactionsandtheirequilibriumconstantsGettheelectronneutralityapproximationwithonlyonenegativeandonepositivedefectUsetheBrouwerapproximationinthemass-actionrelationshiptosolveforPO2ObservewhichdefectconcentrationsareincreasingwithdecreasingorincreasingPO2InteractionsBetweenPointDefectsandInterfacesgVxgVmFormationofanionandcationvacanciesatafreesurface.AdifferenceingVmandgVxcanleadtoaNonstoichiometriccation/anionratioatsurfaceandformationofexcesssurfacechargeApplicationsofpointdefectschemistryinBaTiO3:EffectsofoxygenpressureConductivityisntypeatlowpO2,ConductivityisptypeathighpO2nmContainingpredominantlyacceptorsasnaturalimpuritiesAssumption:1)theobservedconductivityisdeterminedbyeandhconc.;2)theeandhmobilitiesareindependentofpO2DifferentiateK(T)canbeestimatedfromtheminimaintheconductivityisothermsandaknowledgeofthemobilities.Atlowoxygenpressurestheacceptor-compensatingoxygenvacancyconcentrationisregardedasinsignificantcomparedwiththatarisingthroughlossofoxygen:Oxygenvacancyconcentrationnowdeterminedbytheacceptorimpurityconcentration[A]notPO2P-typecontributionPO2>105PaIntentionallyLa2O3donor-dopedBaTiO3IntentionallyLa2O3donor-dopedBaTiO3AtlowestpO2values(AB)lossofoxygenfromthecrystal/withtheformationofandelectronsBandModelforBaTiO3