band diagram for an n‐type semiconductor with negatively ... 14 semiconductor devices.pdf · has...
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Lecture14:SemiconductorDevices:DepletionLayer:n‐type:Assumethesurfaceofann‐typesemiconductorhasbeennegativelycharged.Thefreeelectronsnearthesurfacewillberepelled.Thus,theregionnearthesurfacehaslessfreeelectronsthantheinterior→Depletionlayer(space‐changeregion)
Banddiagramforann‐typesemiconductorwithnegativelychargedsurface.Thedepletionlayerisapotentialbarrierforelectrons.
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Banddiagramforap‐typesemiconductorwithpositivelychargedsurface.
Metalsemiconductorcontacts:n‐typemetal
1. SchottkyRectifier𝜑! > 𝜑!
2. Ohmiccontact𝜑! < 𝜑!
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Rectifyingcontacts(Schottkybarriercontacts):n‐type
𝜑! > 𝜑!
Banddiagramforametalandn‐typesemiconductor𝜑:𝑤𝑜𝑟𝑘 𝑓𝑢𝑛𝑐𝑡𝑖𝑜𝑛
𝜑! > 𝜑!Ifthemetalandsemiconductorarebroughtintocontact,electronsflowfromthesemiconductorsdownintothemetaluntiltheFermienergiesofbothsidesareequal.
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LooktothefollowingdrawingFig9‐16onbookThus,themetalwillbechargednegativelyandtheenergybandinthesemiconductorwillbelowered.Inequilibrium,electronsfrombothsidescrossthepotentialbarrier→DiffusionCurrentContactpotential:Thepotentialbarrierfortheelectronsdiffusingfromthesemiconductorintothemetal
𝜑! − 𝜑!Electronaffinity(X):Fromthebottomoftheconductionbandtotheionizationenergy.Driftcurrent:Whenanelectronholepairisthermallycreatedinornearthedepletionlayer.Theexcitedelectronsintheconductionbandissweptdownthebarrier,andsotheholes“thedriftcurrent”Note:thedriftcurrentisverysmallspecificallyforlargebandgap.Totalcurrent=diffusioncurrent+driftcurrent
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HowSchottkydiodeswork??“metalandn‐type”connectedtoD.CsourceReverseBias:Themetalisconnectedtothenegativeterminal,theelectronsinthesemiconductorwillberepelled,thedepletionlayer,onepotentialbarrierincreases,nodiffusioncurrent(negligible)However,thedriftcurrentdoesn’tdependonvoltage.ForwardBias:Metalisconnectedtothepositiveterminalofthebattery,thepotentialbarrierofsemiconductordecreases“becomesnarrower”CharacteristicsCurveofSchottkyDiodes
OhmicContact(Metallization)Similartometal‐metalcontact,the(I‐V)curveislinear(obeysOhm’sLaw)
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looktothefigure(9‐18)fromyourtextbook‐ metal‐ptype(if𝜑! > 𝜑!)
Metalp‐type:Similar
‐ Example:Al‐Siptype‐
Metaln‐type:Electronflowsfromthemetalintothesemiconductorandthebandsofsemiconductorbenddownward(nobarrier)electronflowsinthetwodirections.Thisconfigurationallowsthecurrenttoflowintoandoutofthesemiconductorwithoutpowerloss.
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Electronsflowfromthehigherlevel(n‐type)tothep‐typeThen‐typeloseselectronstothep‐type.
‐ Thep‐typelosesholestothen‐type‐ Then‐typebecomesnegativelychargedatthejunction
‐ Thep‐typebecomespositivelychargedatthejunction
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‐ AnelectricfieldisformedatthejunctionThisproceedsuntilequilibriumandbothFermienergiesareatthesamelevelNote:Thebandsmoveuptothep‐sideanddowntothen‐side
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Lecture14a:ContinuesemiconductordevicesLet’scontinueonp‐njunctionp‐njunction(Diode):Semipotentialbarriertoreactionisformed
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Electronsflowfromthehigherlevel(n‐type)tothep‐typeThen‐typeloseselectronstothep‐type.• Thep‐typelosesholestothen‐type• Then‐typebecomespositivelychargedatthejunction
• Thep‐typebecomesnegativelychargedatthejunction.
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• Anelectricfieldisformedatthejunction
ThisproceedsuntilequilibriumisreachedandbothFermienergiesareatthesamelevel.Note:Thebandsmoveuptothep‐sideanddowntothen‐side
Afterthepotentialbarrieriscreated,(atequilibrium):Attheconductionband,electronsfromn‐regionfindapotentialbarrierthatreducesitsdiffusiontop‐region.Electronsinp‐regioncaneasilydiffusedownthepotentialbarrierton‐region,howevertheirnumberissmall(theyareonlyduetoexcitation).Thus,thenumberofelectronscrossingthejunctioninbothdirectionsisequal.Applyingexternalpotentialto(p‐n)junctionWewillobtainsimilareffectstoSchottkydiode
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Breakdown(AvalancheandZenerDiode)
Looktothisphenomena,itoccursforthep‐ndiodewhenthereversevoltageincreasesmorethanacertainvalue(criticalvalue).Itsbreakdown,itoccursforavalanchingortunneling(Zenerdiode)
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Consistsofp‐njunction.Iflightfallsonornearthedepletedlayer,electronsareliftedfromthevalencebandtotheconductionband(electron‐holepairsarecreated).Let’slooktothebanddiagram
Theelectronswillmovetothen‐regionandtheholeswillmovetothep‐region.‐ wecandetectthesechargecarriersintwoways:
1. OpenCircuit(photovoltaicmodeofoperation)Anexternalpotentialwillappear
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2. ShortCircuitthedevice(photo‐conductivemodeofoperation)
Anexternalcurrentwillflow
Thep‐regionismadeverythin(≈ 1nm),sothelightcanreachthedepletedlayer.Note:Defectsplayaseriousrole
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AvalanchePhotodiode:p‐nphotodiodeoperatedinahighreversebias‐mode(nearbreakdownvoltage)Light→createelectron‐holepairs→areacceleratedthroughthedepletedregiontohighvelocity→ionizelatticeatomsandgeneratemorehole‐electronpairs=photocurrentgainTunnelDiode:Let’sdiscussfirst“degeneratesemiconductors”Atveryhighleveldoping,theFermilevelmovesupintotheconductionbandinthen‐typematerialandmovesdowntothevalenceinp‐typematerial.
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Thephotodiodeisdegeneratep‐typeanddegeneraten‐typesemiconductor.Looktothefollowingfigure:
Anotherphenomena:Electronscantunnelthroughthepotentialbarrierinbothdirections.Inequilibriumthenettunnelcurrentiszero.LooktotheFermienergyineverymodeofoperationinthenextfigureandlooktotheresultantI‐Vcharacteristiccurveoftunneldiode
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ReverseBias:PotentialbarrierisincreasedtheFermienergyinthep‐areaisraised.Electronsflowfromp‐typeton‐type.ForwardBias(small):Potentialbarrierisdecreased.Electronsflowfromn‐typetop‐typeForwardBias(Medium):Theareaopposite(facing)thefilledconductionbandisforbiddenthecurrentdecreasesForwardBias(normal):Electronsintheconductionbandgetenoughenergytoclimbthepotentialbarrierofp‐sideTransistorsThemostimportantelectronicdeviceBipolartransistorUnipolartransistor(fieldeffecttransistor)Bipolartransistor:twojunctions
n‐p‐ntransistorp‐n‐ptransistor
Bipolartransistor:carriersaremajorityandminorscurrentcontrolFieldeffectresistor:carriersareonlymajorityvoltagecontrolBipolarJunctionTransistor(n‐p‐n)
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Banddiagramofanunbiasedn‐p‐nbiopolarjunctiontransistorE:emitterB:baseC:collectorNote:ThebaseisthincomparedtotheemitterandcollectorAlso,Emitter→ℎ𝑒𝑎𝑣𝑖𝑙𝑦 𝑑𝑜𝑝𝑒𝑑Base→𝑙𝑖𝑔ℎ𝑡𝑙𝑦 𝑑𝑜𝑝𝑒𝑑Collector→𝑙𝑖𝑔ℎ𝑡𝑙𝑦 𝑑𝑜𝑝𝑒𝑑Itcanworkas:• Signalamplifier• Switch
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SignalAmplification:Emitterandbasediode:forwardbiasedBase‐collectordiode:reversebiasedLooktothefigureof:“Biasingann‐p‐ntransistor”‐ BecauseofforwardingbiasingtheE‐Bdiode,thebarrierwillbereduced,alargeelectronflowtothebase.
Whythebaseisthingandlightlydoped?Reversebiasingthebase‐collector→CausestheelectronstoacceleratedowntothecollectorSwitchingBasevoltagecanstoptheelectronflowfromtheemittertocollector