Sentaurus教程：理想pn结的模拟

1G.BettiBeneventiTechnologyComputerAidedDesign(TCAD)LaboratoryLecture4,theidealdiode(pn-junction)GiovanniBettiBeneventiE-mail:[e-mail];[e-mail]:Engineeringfaculty,ARCESlab.(Ex.3.2room),vialedelRisorgimento2,BolognaPhone:[phone]AdvancedResearchCenteronElectronicSystems(ARCES)UniversityofBologna,Italy[Source:Synopsys]2G.BettiBeneventiOutline•Reviewofbasicpropertiesofthediode•SentaurusWorkbenchsetup(SWB)•ImplementationofInputfiles–SentaurusStructureEditor(SDE)commandfile–SentaurusDevice(SDevice)•commandfile•parameterfile•Runthesimulation•Post-processingofresults3G.BettiBeneventiOutlineReviewofbasicpropertiesofthediode•SentaurusWorkbenchsetup(SWB)•ImplementationofInputfiles–SentaurusStructureEditor(SDE)commandfile–SentaurusDevice(SDevice)•commandfile•parameterfile•Runthesimulation•Post-processingofresults4G.BettiBeneventiThediode:structureandapplicationsStructure:Simplestpossiblesemiconductordeviceismadebyaso-calledpn-junction.Itconsistoftworegionsofhomogeneoussemiconductor,onedopedwithacceptors(p-type)andtheotheronedopedwithdonors(n-type).IfthetransitionregionbetweenthetwosemiconductortypesisassumedtobethinthejunctionissaidtobeabruptorstepApplications:Logic:rarelyintegratedintheschematicofdigitalsystemsbutomnipresent,astheMetal-Oxide-SemiconductorField-Effect-Transistor(MOSFET)containsseveralreverse-biaseddiodes.Also,usedasadevicetoprotectICinputfromElectroStaticDischarges(ESD).Optoelectronicdevices:solarcells,lasers,LED.5G.BettiBeneventiThediode:physicsatequilibrium•Becauseoftheconcentrationgradientexperiencedbyelectronsandholesinthestructure(p-regionrichofholesanddepletedofelectrons,n-regionrichofelectronsanddepletedofholes),whenthejunctionisformed,electronstendtopassfromthen-tothep-region,andholesdotheopposite.•Asaresultofdiffusion,somedopantsnearthejunctionaredepletedoffreecharge(i.e.theybecomeionized),givingorigintotheso-called“space-charge-region”.•Thesefixedionchargescreateanelectrostaticpotentialwhichtendstohamperafurtherdiffusionofcarriers(theygiverisetotheso-called“built-in”potential)bysuperimposingtothediffusioncurrentadriftcomponent.•Atequilibriumdriftanddiffusionmicroscopicallycounterbalanceeachotherandnocurrentflowsinthedevice.inthisexample,p-dopingishigherthann-doping,givingrisetoathinnerdepletionregionatthep-side6G.BettiBeneventiThediode:built-inpotential•If𝑁𝐴istheconcentrationofacceptorsatthep-side,and𝑁𝐷istheconcentrationofdonorsartthen-side,itcanbeshownthat,ifthesemiconductorsarenon-degenerate,thebuilt-inpotential𝜑0isgivenbytheformula(1),wherethethermalvoltage𝜑𝑇=𝑘𝑇/𝑞=26mVatroomtemperature,andwhere𝑛𝑖isthesemiconductorintrinsiccarrierdensity,whichisamaterialproperty(dependsondensityofstatesandonband-gap)•Therefore,thehighertheproductofthepandndoping,thehigher𝝋𝟎•Thebuilt-inpotentialis“built-in”sinceitisanintrinsicpropertyofthejunction.Itrepresentstheenergybarrierthatmajoritycarriers(electroninn-region,holesinp-region)havetoovercomeinordertoobtainanetcurrentflowbydiffusion𝜑0=𝜑𝑇ln𝑁𝐴𝑁𝐷𝑛𝑖2Example(Silicon)If𝑁𝐴=1016𝑐𝑚−3,and𝑁𝐷=1016𝑐𝑚−3,𝑛𝑖≅1.5𝑥1010𝑐𝑚−3,𝜑0=697mV(1)•Bymodulatingthebuilt-involtageusinganexternalbatteryitispossibletomodulatethebehaviorofthedevice.Inparticular:•Theapplicationofapositivevoltageatthep-region(or,equivalently,anegativevoltageatthen-region)lowersthepotentialbarriers,andcurrentcanflowbydiffusionthroughthespace-chargeregion.Inthisregime,currentincreasesexponentiallywithvoltageandthediodeissaidtobeforwardbiased.•Theapplicationofanegativevoltageatthep-region(or,equivalently,apositivevoltageatthen-region)furtherincreasesthepotentialbarrier,currentcanflowonlybydriftofminoritycarriers,itisveryweakandalmostindependentbyvoltage.Inthisregime,thediodeissaidtobereversebiased.7G.BettiBeneventiThediodeinforwardbias•Inforwardbias,electronsfromthen-regioncanflowtowardthep-region,andholesfromthep-regioncanflowtowardthen-region,causinganexcess(comparedtotheequilibriumcondition,np0,pn0)ofelectronsinthep-regionandofholesinthen-region.Thisexcessofcarriersisnamed“minoritycarrierexcess”.Itcanbeshownthattheminoritycarrierconcentrationisexponentiallydecreasingfromthejunctionline.Theminoritycarrierprofileitisapproximatedwithalinearbehaviorifthereisnogeneration-recombination(i.e.notrapsorveryshortdiode)•Theseexcesscarriersgiverisetoalargecurrentwhichexponentiallyincreaseswithvoltage.Inforwardbiasthediodeisconducting,ideallyasashortcircuit.inthisexample,p-dopingishigherthann-doping,thustheamountofavailableholesishigherthantheamountofavailableelectrons[Logscale]8G.BettiBeneventiThediodeinreversebias•Inreversebias,minoritycarriersleavethespacechargeregionbecauseofthereversefield.Thespace-chargeregionwidthisincreased,andthereisadiminutionofminoritycarrierscomparedtotheequilibriumcondition.Theminoritycarrierprofileisagainexponential,butitisoftenapproximatedbyalinearbehaviorifthereisnogeneration-recombination(i.e.notrapsorveryshortdiode)•Theconcentrationofmovingcarriersinthespace-chargeregionisverylow,thereforeinreversebias