光电子学第5章-光电探测器课件

OptoelectronicsandPhotonics

PrinciplesandPracticesYangJunPhotonicsResearchCenter

SchoolofScienceHarbinEngineeringUniversity2008.12专业词汇选编Freeelectronholepairs(EHPs):自由电子空穴对Photodiode:光探测器Pyroelectric[pairo’ilectric]detector:热探测器Acceptor：受主Donor:施主Antireflectioncoating:抗反射膜、增透膜Depletionregion:耗尽区Spacechargelayer:空间电荷层Built-involtage:内建电场Neutralregions:中性区Photogenerate:光生Photocurrent:光电流专业词汇选编Driftvelocity:漂移速度Transittime:渡越时间Uppercut-offwavelength:长波截至波长Absorptioncoefficient:吸收系数Penetrationdepth:穿透深度Directbandgap:直接带隙Indirectbandgap:间接带隙PhononMomentum:声子动量Latticevibration:晶格振动Quantumefficiencyofthedetector:探测器的量子效率Externalquantumefficiency:外量子效率Responsivity:响应度Spectralresponsivity(radiantsensitivity)光谱响应度(辐射响应度)光纤技术中几种典型的光电探测器InGaAs-PIN光电二极管

PIN-TIA接收组件

Si-PIN光电二极管光探测器之所以能探测光辐射就是因为光辐射（即光频电磁波）传输能量。入射到光探测器上的光辐射使之产生光生载流子（或发射光电子）或使其本身的特性（如电阻、温度等）发生变化。根据上述光辐射响应方式或工作机理的不同，前者称之为光电效应，后者称之为光热效应，由此构成的光探测器分别称为光子探测器和热探测器。而光子探测器又分为：光电子发射探测器、光电导探测器、光伏探测器、光子牵引探测器；热探测器又分为：热探测器：测辐射热电偶、测辐射热计、热释电探测器、气动探测器。光电探测器(Photodetector)Photodetectors5.1PrincipleofthepnJunctionPhotodiode5.2Ramo’stheoremandexternalphotocurrent5.3AbsorptionCoefficientandPhotodiodeMaterials5.4QuantumEfficiencyandResponsivity5.5ThePINPhotodiode5.6AvalanchePhotodiode5.7HeterojunctionPhotodiodes5.8Phototransistors5.9NoiseinPhotodetectorsFigure5.1Aschematicdiagramofareversebiasedpnjunctionphotodiode.Netspacechargeacrossthediodeinthedepletionregion.Nd

andNaarethedonorandacceptorconcentrationsinthepandnsides.Thefieldinthedepletionregion.Thefigure5.1(a)showsthesimplifiedstructureofatypicalpnjunctionphotodiodethathasap+ntypeofjunction.Theilluminatedsidehasawindow,definedbyanannularelectrode,toallowphotonstoenterthedevice.Thereisanantireflectioncoating,typicallySi3N4,toreducelightreflections.Thepsideisgenerallyverythin(lessthanamicron)andisusuallyformedbyplanardiffusionintoann-typeepitaxiallayer.Figure5.2(b)showsthenetspacechargedistributionacrossthep+njunction.Thesechargesareinthedepletionregion,orinthespacechargelayer,andrepresenttheexposednegativelychargedacceptorsinthep+sideandexposedpositivelychargeddonorsinthen-side.Thedepletionregionextendsalmostentirelyintothelightlydopedn-sideand,itisafewmicrons.Thephotodiodeisnormallyreversebiased,theappliedreversebiasVr

dropsacrossthehighlyresistivedepletionlayerwidthWandmakesthevoltageacrossWequaltoVo+VrwhereVoisthebuilt-involtage.ThefieldisfoundbytheintegrationofthenetspacechargedensityρnetacrossWsubjecttoavoltagedifferenceofVo+Vr.Thefieldonlyexistsinthedepletionregionandisnotuniform.Itvariesacrosspenetratesintothen-side.Theregionsoutsidethedepletionlayeraretheneutralregionsinwhichtherearemajoritycarriers.Itissometimesconvenienttotreattheseneutralregionssimplyasresistiveextensionsofelectrodestothedepletionlayer.WhenaphotonwithanenergygreaterthanthebandgapEgisincident,itbecomesabsorbedtophotogenerateafreeEHP.Usually,thephotogenerationtakesplaceinthedepletionlayer.ThefieldEinthedepletionlayerseparatestheEHPanddriftstheminoppositedirectionsuntiltheyreachtheneutralregions.Driftingcarriersgenerateacurrent,calledphotocurrent

Iph,intheexternalcircuitthatprovidestheelectricalsignal.ThephotocurrentIphdependsonthenumberofEHPsphotogeneratedandthedriftvelocitiesofthecarrierswhiletheyaretransitingthedepletionlayer.Thephotocurrentintheexternalcircuitisduetotheflowofelectrons,nottobothelectronsandholes.Photodetectors5.1PrincipleofthepnJunctionPhotodiode5.2Ramo’stheoremandexternalphotocurrent5.3AbsorptionCoefficientandPhotodiodeMaterials5.4QuantumEfficiencyandResponsivity5.5ThepinPhotodiode5.6AvalanchePhotodiode5.7HeterojunctionPhotodiodes5.8Phototransistors5.9NoiseInPhotodetectorsSupposethatasinglephotonisabsorbedstapositionx=lfromtheleftelectrodeandinstantlycreatesanelectronholepair.Transittime:isthetimeittakesforacarriertodriftfromitsgenerationpointtothecollectingelectrode.Figure5.2(b)Theelectronarrivesattimete

=(Ll)/ve

andtheholearrivesattimeth

=l/vh.Considerfirstonlythedriftingelectron.Supposethattheexternalphotocurrentduetothemotionofthiselectronisie(t).Workdone=eEdx=Vie(t)dtUsingE=V/Landve

=dx/dtwefindtheelectronphotocurrentThecurrentcontinuestoflowaslongastheelectronisdrifting.Itlastsforadurationteattheendofwhichtheelectronreachesthebattery.Thusalthoughtheelectronhasbeenphotogeneratedinstantaneously,theexternalphotocurrentisnotinstantaneousandhasatimespread.ElectronphotocurrentHolephotocurrentThetotalexternalcurrentwillbethesumofie(t)andih(t).EvaluatethecollectedchargeQcollected

Figure5.2(d)Thisresultcanbeverifiedbyevaluatingtheareaundertheiph(t)curveinFigure5.2(d).Ramo’stheorem5.3AbsorptioncoefficientandphotodiodematerialsThephotonabsorptionprocessforphotogeneration,thatisthecreationofEHPs,requiresthephotonenergytobeatleastequaltothebandgapenergyEg

ofthesemiconductormaterialtoexciteanelectronfromthevalenceband(VB)totheconductionband(CB).Theuppercut-offwavelength(orthethresholdwavelength)λgforphhotogenerativeabsorptionisthereforedeterminedbythebandgapenergyEg

ofthesemiconductorsothatorForexample:SiEg=1.12eV,λg

is1.11μm;GeEg=0.66eV,λg

is1.87μm;Fromabove,itiscleartatSiphotodiodescannotbeusedinopticalcommunicationsat1.3and1.55μmwhereasGephotodiodesarecommerciallyavailableforuseatthesewavelengths.D70.18InSbD3.50.35InAsI1.870.66GeD1.640.75In0.53Ga0.47AsD1.40.89In0.7Ga0.3As0.64P0.36I1.111.12SiD1.081.15GaAs0.88Sb0.12D0.911.35InPTypeλg(μm)Eg(eV)SemiconductorTABLE5.1BandgapenergyEgat300K,cut–offwavelengthλgandtypeofbandgap(D=DirectandI=Indirect)forsomephotodetectormaterials.Figure5.3Theabsorptioncoefficient(α)vs.wavelength(λ)forvarioussemiconductorsIndirectbandgapsemiconductorssuchasIII-Vsemiconductors(e.g.GaAs,InAs,InP,GaSb)andinmanyoftheiralloys(e.g.InGaAs,GaAsSb)thephotonabsorptionprocessisadirectprocessthatrequiresnoassistancefromlatticevibrations.Thephotonisabsorbedandtheelectronisexciteddirectlyfromthevalancebandtotheconductionbandwithoutachangeinitsk-vectorinasmuchasthephotonmomentumisverysmall.ThechangeintheelectronmomentumfromthevalencetotheconductionbandThisprocesscorrespondstoaverticaltransitionontheE-kdiagraminFigure5.4(a).Theabsorptionprocessissaidtobeindirectasitdependsonlatticevibrationswhichinturndependonthetemperature.Sincetheinteractionofaphotonwithavalenceelectronneedsathirdbody,alatticevibration,theprobabilityofphotonabsorptionisnotashighasinadirecttransition.Furthermore,thecut-offwavelengthisnotassharpasfordirectbandgapsemiconductors.Duringtheabsorptionprocess,aphononmaybeabsorbedoremitted.Ifυisthefrequencyofthelatticevibrationsthenthephononenergyishυ.Thephotonenergyis

where

isthephotonfrequency.ConservationofenergyrequiresthatThus,theonsetofabsorptiondoesnotexactlycoincidewithEg,buttypicallyitisveryclosetoEginasmuchas

issmall(<0.1eV).TheabsorptioncoefficientinitiallyriseslowlywithdecreasingwavelengthfromaboutλgasapparentinFigure5.3forGeandSi.

ChoiceofmaterialforphotodiodeThechoiceofmaterialforaphotodiodemustbesuchthatthephotonenergiesaregreaterthanEg.Further,atthewave-lengthofradiation,theabsorptionoccursoveradepthcoveringthedepletionlayersothatthephotogeneratedEHPscanbeseparatedbythefieldandcollectedattheelectrodes.Iftheabsorptioncoefficientistoolargethenabsorptionwilloccurverynearthesurfaceofthep+layerwhichisoutsidethedepletionlayer.First,theabsenceofafieldmeansthatthephotogeneratedelectroncanonlymakeittothedepletionlayertocrosstothen-sidebydiffusion.ChoiceofmaterialforphotodiodeSecondly,photogenerationnearthesurfaceinvariablyleadstorapidrecombinationduetosurfacedefectsthatactasrecombinationcenters.Ontheotherhand,iftheabsorptioncoefficientistoosmall,onlyasmallportionofthephotonswillbeabsorbedinthedepletionlayerandonlyalimitednumberofEHPscanbephotogenerated.Photodetectors5.1PrincipleofthepnJunctionPhotodiode5.2Ramo’stheoremandexternalphotocurrent5.3AbsorptionCoefficientandPhotodiodeMaterials5.4QuantumEfficiencyandResponsivity5.5ThepinPhotodiode5.6AvalanchePhotodiode5.7HeterojunctionPhotodiodes5.8Phototransistors5.9NoiseInPhotodetectors5.4QuantumEfficiencyandResponsivity

Thequantumefficiency

(QE)

η

isthenumberofthephoto-carrierpairsgeneratedperincidentphotonofenergyhvandisgivenbyThemeasuredphotocurrentIphintheexternalcircuitisduetotheflowofelectronspersecondtotheterminalsofthephotodiode.NumberofelectronscollectedpersecondisIph/e.IfPoistheincidentopticalpowerthenthenumberofphotonsarrivingpersecondisPo/hv.ThentheQEηcanalsobedefinedbyQEcanbeincreasedbyreducingthereflectionsatthesemiconductorsurface,increasingabsorptionwithinthedepletionlayerandpreventingtherecombinationortrappingofcarriersbeforetheyarecollected.Toachieveahighquantumefficiency,thedepletionlayermustbethicker.However,thethickerthedepletionlayer,thelongerittakesforthephoto-generatedcarrierstodriftacrossthereverse-biasedjunction.Compromisehastobemadebetweenresponsespeedandquantumefficiency.Theperformanceofaphotodiodeisoftencharacterizedbythespectralresponsivity

R.Thisisrelatedtothequantumefficiencyhby

Representativevaluesare0.65-A/WforSiat900-nmand

0.45-A/WforGeat1.3-µm.

ForInGaAs,typicalvaluesare0.9-A/Wat1.3-µmand1.0-

A/Wat1.55-µm.FromthedefinitionofQE,itisclearthat,02004006008001000120001Wavelength(nm)SiPhotodiodelgResponsivity(A/W)IdealPhotodiodeQE=100%(h=1)Figure5.5Responsivity(R)vs.wavelength(λ)foranidealphotodiodewithQE=100%(η=1)andforatypicalcommercialSiphotodiode.Photodetectors5.1PrincipleofthepnJunctionPhotodiode5.2Ramo’stheoremandexternalphotocurrent5.3AbsorptionCoefficientandPhotodiodeMaterials5.4QuantumEfficiencyandResponsivity5.5ThepinPhotodiode5.6AvalanchePhotodiode5.7HeterojunctionPhotodiodes5.8Phototransistors5.9NoiseInPhotodetectors专业词汇选编P-intrinsic-ntypephotodiode：pin光电二极管depletionlayercapacitance：耗尽层电容Responsetime：响应时间Avalanchephotodiode(APD)：雪崩二极管Reach-throughAPD：通达型雪崩二极管Impact-ionize：碰撞电离Avalancheofimpactionizationprocesses：碰撞电离的雪崩过程internalgainmechanism：内增益机制Excessnoise：过剩噪声Avalanchemultiplicationfactor：雪崩倍乘因子Primary(unmultipied)photocurrent：初级(非倍增)电流Avalanchebreakdownvoltage：雪崩开启电压Guardring：地环5.5ThepinPhotodiodeThesimplepnjunctionphotodiodehastwomajordrawbacks:First:itsjunctionordepletionlayercapacitanceisnotsufficientlysmalltoallowphotodetectionathighmodulationfrequencies.ThisisanRCtimeconstantlimitation.Secondly:itsdepletionlayerisatmostafewmicrons.Thismeansthatatlongwavelengthswherethepenetrationdepthisgreaterthanthedepletionlayerwidth,themajorityofphotonsareabsorbedoutsidethedepletionlayerwherethereisnofieldtoseparatetheEHPsanddriftthem.TheQEiscorrespondinglylowattheselongwavelengths.Theseproblemsaresubstantiallyreducedinthepin(p-intrinsic-n-type)photodiode.Figure5.6Theschematicstructureofanidealizedpinphotodiode(b)Thenetspacechargedensityacrossthephotodiode.(c)Thebuilt-infieldacrossthediode.(d)Thepinphotodiodeinphotodetectionisreversebiased.Theseparationoftwoverythinlayersofnegativeandpositivechargesbyafixeddistance,widthWofthei-Si,isthesameasthatinaparallelplatecapacitor.Thejunctionordepletionlayercapacitanceofthepindiodeisgivenby

Aisthecrosssectionalarea,

ε

oε

risthepermittivityofthesemiconductor(Si).SinceWisfixedbythestructure,thejunctioncapacitancedoesnotdependontheappliedvoltage.

Cdep

istypicallyoftheorderofapicofaradinfastpin

photodiodessothatwitha50Ωresistor,theRCdep

timeconstantisabout50ps.WhenareversebiasvoltageVrisappliedacrossthepindevice,itdropsalmostentirelyacrossthewidthofi-Silayer.Thedepletionlayerwidthsofthethinsheetsofacceptoranddonorchargesinthep+andn+sidesarenegligiblecomparedwithW.ThereversebiasVrincreasesthebuilt-involtagetoVo+VrasshowninFigure5.6(d).ThefieldEinthei-SilayerisstilluniformandincreasestoThepinstructureisdesignedsothatphotonabsorptionoccursoverthei-Silayer.ThephotogeneratedEHPsinthei-SilayerarethenseparatedbythefieldEanddriftedtowardsthen+andp+sidesrespectivelyasillustratedinFigure5.6(d).Whilethephotogeneratedcarriersaredriftingthroughthei-SilayertheygiverisetoanexternalphotocurrentwhichisdetectedasavoltageacrossasmallresistorRinFigure5.6(d)TheresponsetimeofthepinphotodiodeisdeterminedbythetransittimesofthephotogeneratedcarriersacrossthewidthWofthei-Silayer.IncreasingWallowsmorephotonstobeabsorbedwhichincreasestheQE

butitslowsdownthespeedofresponseascarriertransittimesbecomelonger.Forachargecarrierthatisphotogeneratedattheedgeonthei-Silayer,thetransittimeordrifttimetdriftacrossthei-SilayerisWherevd

isitsdriftvelocity.Toreducethedrifttime,thatisincreasethespeedofresponse,wehavetoincreasevd

andthereforeincreasetheappliedfieldE.

Figure5.7showsthevariationofthedriftvelocityofelectronsandholeswiththefieldinSi.Driftvelocityvs.electricfieldforholesandelectronsinSi.102103104105107106105104Electricfield(Vm-1)ElectronHoleDriftvelocity(ms-1)Example5.5.1~5.5.3Example5.5.1,P228;Example5.5.2,P228;Example5.5.3,P229;Photodetectors5.1PrincipleofthepnJunctionPhotodiode5.2Ramo’stheoremandexternalphotocurrent5.3AbsorptionCoefficientandPhotodiodeMaterials5.4QuantumEfficiencyandResponsivity5.5ThepinPhotodiode5.6AvalanchePhotodiode5.7HeterojunctionPhotodiodes5.8Phototransistors5.9NoiseInPhotodetectors5.6AvalanchePhotodiodeAsimplifiedschematicdiagramofaSireach-throughAPDisshownFigure5.9(a).Then+sideisthinanditisthesidethatisilluminatedthroughawindow.Therearethreep-typelayersofdifferentdopinglevelsnexttothen+layertosuitablymodifythefielddistributionacrossthediode.Thefirstisathinp-typelayerandthesecondisathicklightlyp-typedoped(almostintrinsic)π-layerandthethirdisaheavilydopedp+layer.Thediodeisreversebiasedtoincreasethefieldsinthedepletionsregions.ThenetspacechargedistributionacrossthediodeduetoexposeddopantionsisshowninFigure5.9(b).Underzerobiasthedepletionlayerinthep-regiondoesnotnormallyextendacrossthislayertotheπ-layer.TheelectricfieldisgivenbytheintegrationofthenetspacechargedensityρnetacrossthediodesubjecttoanappliedvoltageVracrossthedevice.ThevariationinthefieldacrossthediodeisshowninFigure5.9(c).Thefieldlinesstartantpositiveionsandendatnegativeionswhichexistthroughthep,πandp+

layers.šp+SiO2ElectrodernetxxE(x)REhu>EgpIphe–h+AbsorptionregionAvalancheregion(a)(b)(c)Electroden+Figure5.9(a)Aschematicillustrationofthestructureofanavalanchephotodiode(APD)biasedforavalanchegain.(b)Thenetspacechargedensityacrossthephotodiode.(c)Thefieldacrossthediodeandtheidentificationofabsorptionandmultiplicationregions.Theabsorptionofphotonsandhencephotogenerationtakesplacemainlyinthelongπ-layer.Thenearlyuniformfieldhereseparatestheelectronholepairs(EHPs)anddriftsthematvelocitiesnearsaturationtowardsthen+andp+sidesrespectively.Whenthedriftingelectronsreachthep-layer,theyexperienceevengreaterfieldsandthereforeacquiresufficientkineticenergytoimpact-ionizesomeoftheSicovalentbondsandreleaseEHPsasillustratedinFigure5.10.ThesegeneratedEHPsthemselvescanalsobeacceleratedbythehighfieldsinthisregiontosufficientlylargekineticenergiestofurthercauseimpactionizationandreleasemoreEHPswhichleadstoanavalancheofimpactionizationprocesses.h+Ešn+pe–Avalancheregione–h+EcEv(a)(b)EFigure5.10(a)ApictorialviewofimpactionizationprocessesreleasingEHPsandtheresultingavalanchemultiplication.(b)Impactofanenergeticconductionelectronwithcrystalvibrationstransferstheelectron'skineticenergytoavalenceelectronandtherebyexcitesittotheconductionband.Fromasingleelectronenteringthep-layeronecangeneratealargenumberofEHPsallofwhichcontributetotheobservedphotocurrent.ThephotodiodepossessesaninternalgainmechanisminthatasinglephotonabsorptionleadstoalargenumberofEHPsgenerated.ThephotocurrentintheAPDinthepresenceofavalanchemultiplicationthereforecorrespondstoaneffectivequantumefficiencyinexcessofunity.Thereasonforkeepingthephotogenerationwithintheπ-regionandreasonablyseparatefromtheavalanchep-regioninFigure5.9(a)isthatavalanchemultiplicationisastatisticalprocessandhenceleadstocarriergenerationfluctuationwhichleadstoexcessnoiseintheavalanchemultipliedphotocurrent.ThemultiplicationofcarriersintheavalancheregiondependsontheprobabilityofimpactionizationwhichdependsstronglyonthefieldinthisregionandhenceonthereversebiasVr.TheoveralloreffectiveavalanchemultiplicationfactorMofanAPDisdefinedasWhereIphistheAPDphotocurrentthathasbeenmultipliedandIphoistheprimaryorunmultipliedphotocurrent,thephotocurrentthatismeasuredintheabsenceofmultiplication,forexample,underasmallreversebiasVr.ThemultiplicationMisastrongfunctionofthereversebiasandalsothetemperature.MultiplicationFactorThemultiplicationMcanempiricallybeexpressedasWhereVbrisaparametercalledtheavalanchebreakdownvoltage

nisacharacteristicindexthatprovidesthatthebestfittotheexperimentaldata.Misastrongfunctionofbothreversebiasvoltageandtemperature.M~100(SiAPD),M~10(GeAPDs).structureofpracticalSiAPDSiO2GuardringElectrodeAntireflectioncoatingnnn+p+špSubstrateElectroden+p+špSubstrateElectrodeAvalanchebreakdown(a)(b)Figure5.11(a)ASiAPDstructurewithoutaguardring.(b)AschematicillustrationofthestructureofamorepracticalSiAPD.Oneofthedrawbacksofthesimplereach-throughAPDstructureisthatthefieldaroundthen+pjunctionperipheraledgereachesavalanchebreakdownbeforethen+pregionsundertheilluminatedareaasillustratedinFigure5.11(a).Ideallytheavalanchemultiplicationshouldoccuruniformlyintheilluminatedregiontoencouragetheavalanchemultiplicationoftheprimaryphotocurrentratherthanthemultiplicationofthedarkcurrent.InapracticalSiAPD,ann-typedopedregionactingasaguardringsurroundsthecentraln+regionasshownFigure5.11(b)sothatthebreakdownvoltagearoundtheperipheryisnowhigherandavalancheisconfinedmoretoilluminatedregion.TypicalCharacteristicsofDifferentPDsPhotodetectors5.1PrincipleofthepnJunctionPhotodiode5.2Ramo’stheoremandexternalphotocurrent5.3AbsorptionCoefficientandPhotodiodeMaterials5.4QuantumEfficiencyandResponsivity5.5ThepinPhotodiode5.6AvalanchePhotodiode5.7HeterojunctionPhotodiodes5.8Phototransistors5.9NoiseInPhotodetectors5.7HeterojunctionPhotodiodesIII-VcompoundbasedAPDshavebeendevelopedforuseatthecommunicationswave-lengths1.3μmand1.55μm.Asinthereach-throughSiAPD,theabsorptionorphotogenerationregionisseparatedfromtheavalancheormultiplicationregionwhichallowsthemultiplicationtobeinitiatedbyonetypeofcarrier.Figure5.12isasimplifiedschematicdiagramofthestructureofanInGaAs-InPAPDwithaseparateabsorptionandmultiplication(SAM).A.SeparateAbsorptionandMultiplication(SAM)APDFigure5.12Simplifiedschematicdiagramofaseparateabsorptionandmultiplication(SAM)APDusingaheterostructurebasedonInGaAs-InP.PandNrefertopandn-typewider-bandgapsemiconductor.ThereareanumberofpracticalfeaturesthatarenotshowninthehighlysimplifieddiagraminFigure5.12.Photogeneratedholesdriftingfromn-InGaAstoN-InPbecometrappedattheinterfacebecausethereisasharpincreaseinthebandgapandasharpchangeΔEv

inEv

(valencebandedge)betweenthetwosemiconductorsandholescannoteasilysurmountthepotentialenergybarrierΔEv

asdepictedinFigure5.13(a).Thisproblemisovercomebyusingthinlayersofn-typeInGaAstoInPasdepictedinFigure5.13(b).InPInGaAsh+e–

EEcEvEcEvInPInGaAsEvEvInGaAsPgradinglayerh+DEv(a)EnergybanddiagramforaSAMheterojunctionAPDwherethereisavalencebandstepDEvfromInGaAstoInPthatslowsholeentryintotheInPlayer.(b)Aninterposinggradinglayer(InGaAsP)withanintermediatebandgapbreaksDEv

andmakesiteasierfortheholetopasstotheInPlayer(a)(b)Figure5.13EffectivelyΔEv

hasbeenbrokenupintotwosteps,theholehassufficientenergytoovercomethefirststepandentertheInGaAsPlayer.ItdriftsandacceleratesintheInGaAsPlayertogainsufficientenergytosurmountthesecondstep.Thesedevicesarecalledseparateabsorption,gradignandmultiplication(SAGM)APDs.BoththeInPlayersaregrownepitaxiallyonanInPsubstrate.ThesubstrateitselfisnotuseddirectlytomaketheP-Njunctiontopreventcrystaldefectsinthesubstrateappearinginthemultiplicationregionandhencedeterioratingdeviceperformance.TheschematicdiagramofamorepracticalSAGMAPDisdepictedinFigure5.14.P+–InPSubstrateP+–InP(2-3mm)BufferepitaxiallayerN–InP(2-3mm)Multiplicationlayer.Photonn–In0.53Ga0.47As(5-10mm)AbsorptionlayerGradedn–InGaAsP(<1mm)ElectrodeElectrodeFigure5.14Simplifiedschematicdiagramofamorepracticalmesa-etchedSAGMlayeredAPD.Asmentionedpreviously,APDsexhibitexcessnoiseinthephotocurrentduetoinherentstatisticalvariationsintheavalanchemultiplicationprocess.Thisexcessavalanchenoiseisreducedtominimumwhenonlyonetypeofcarrier,forexampletheelectron,isinvolvedinimpactionizations.Onemethodifachievingsinglecarriermultiplicationisbyfabricationmultilayerdevicesthathavealternatinglayersofdifferentbandgapsemiconductors,asinmultiplequantumwelldevices.B.SuperlatticeAPDsThemultilayeredstructureconsistingofmanyalternatinglayersofdifferentbandgapsemiconductorsiscalledasuperlattice.Figure5.15(a)showstheenergybanddiagramofastaircasesuperlatticeAPDinwhichthebandgapisgradedwithineachlayer.ThebandgapineachlayerchangesfromaminimumEg1toamaximumEg2whichismorethantwiceEg1.Inverysimpleterms,asillustratedinFigure5.15(b),thephotogeneratedelectroninitiallydriftsinthegradelayerconductionban.huh+e–n+EcEv10–20nmp+EEg1Eg2DEcFigure5.15EnergybanddiagramofastaircasesuperlatticeAPD(a)Nobias.(b)Withanappliedbias.(a)(b)Photodetectors5.1PrincipleofthepnJunctionPhotodiode5.2Ramo’stheoremandexternalphotocurrent5.3AbsorptionCoefficientandPhotodiodeMaterials5.4QuantumEfficiencyandResponsivity5.5ThepinPhotodiode5.6AvalanchePhotodiode5.7HeterojunctionPhotodiodes5.8Phototransistors5.9NoiseInPhotodetectors5.9NoiseInPhotodetectorsThelowestsignalthataphotodetectorscandetectisdeterminedbytheextentofrandomfluctuationsinthecurrentthroughthedetectorandthevoltageacrossitasaresultofvariousstatisticalprocessesinthedevice.WhenapnjunctionisreversebiasedthereisstilladarkcurrentIdpresentwhichismainlyduetothermalgenerationofelectron-holepairs(EHPs)inthedepletionlayerandwithindiffusionlengthstothedepletionlayer.Ifthedarkcurrentwereabsolutelyconstantwithnofluctuationsthenanychangeinthediodecurrent,howeversmall(evenatinyfractionofId),duetoanopticalsignalcouldbeeasilydetectedbyblockingorremovingId.A.ThepnJunctionandthepinPhotodiodesVoutCurrentTimeIdVrpnPoDarkIlluminatedId

+IphId+Iph+inRAFigure5.19Inpnjunctionandpindevicesthemainsourceofnoiseisshotnoiseduetothedarkcurrentandphotocurrent.ThedarkcurrenthoweverexhibitsshotnoiseorfluctuationsaboutId,asshowninFigure5.19.Thisshotnoiseisduetothefactthatelectricalconductionisbydiscretechargeswhichmeansthatth