Band diagram for an n‐type semiconductor with negatively ... 14 Semiconductor Devices.pdf · has been negatively charged. The free electrons near the surface ... Total current=diffusion

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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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Ohmiccontactcanoccurinmetal‐semiconductorcontactforthefollowingcases:

‐ metal‐ntype(if𝜑! < 𝜑!)

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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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p‐njunction(diode):Similarpotentialbarriertoreactionisformed

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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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Looktothenextpageformoredetails

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Breakdown(AvalancheandZenerDiode)

Looktothisphenomena,itoccursforthep‐ndiodewhenthereversevoltageincreasesmorethanacertainvalue(criticalvalue).Itsbreakdown,itoccursforavalanchingortunneling(Zenerdiode)

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‐UsesforZenerDiode

Photodiode(solarcell)

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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

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n‐p‐ntransistor