雪崩光電二極管(APD)課件

1雪崩光電二極管

（APD）探測器

2TheMultiplicationProcessAvalanchePhotodiodeDesignsAvalanchePhotodiodeBandwidthAvalanchePhotodiodeNoiseAvalanchePhotodiodeDetectors3456789Measuredvaluesofionisationcoefficientseandhforsomecommonsemiconductormaterials,plottedversus(1/E).TheMultiplicationProcess10TheMultiplicationProcessWemaydefineionisationcoefficientsforelectronsandholes,eandhrespectively,astheprobabilitythatagivencarrierwillexciteanelectron-holepairinunitdistance.Thecoefficientsincreasesorapidlywithincreasingelectricfieldstrength,thatitisoftenconvenienttothinkintermsofabreakdownfield,EB,atwhichavalancheexcitationbecomescritical,saybecomesoftheorder105

–106m-1.Graphsofeandhversuselectricfieldareplottedforanumberofsemiconductorsknowntobeofinterestasdetectormaterials.Thecurvesrefertoroomtemperature.Asthetemperatureincreases,theionisationcoefficientsdecrease,becausethegreaternumberofscatteringcollisionsreducesthehigh-energytailofthecarrierenergydistributionandhencereducestheprobabilityofexcitation.Insomematerialse>h,inothersh>e,whileingalliumarsenideandindiumphosphidethetwocoefficientsareapproximatelythesame.Theratiok=h/eisfoundtolieintherange0.01to100.11TheMultiplicationProcess-ExperimentalBehaviourTwofactorslimittheincreaseofMe,themultiplicationfactorfortheinjectedelectronsandhenceIastheappliedvoltageapproachesthebreakdownvoltage,VB,atwhichthevalueseandhsatisfytheconditionforbreakdown,thatisM->.Thefirstistheseriesresistanceofthebulksemiconductor,RS,betweenthejunctionandthediodeterminals.Thesecondistheeffectoftheriseintemperatureresultingfromtheincreaseddissipationasthecurrentrises.Thisreducesthevaluesofeandhandraisesthebreakdownvoltage.Italsoincreasestherateofthermalgenerationofcarriersandhencethedarkcurrent.Multiplicationfactorsmeasuredasafunctionoftheappliedterminalvoltage,V,canusuallybefittedtotheformM=1/|1-(V-IR’)/VB|nWhereR’=RS+RThisthesumoftheseriesresistance,RS,andaneffectiveresistance,RTh,whichderivesfromtheriseintemperature.Theindex,n,isafunctionofthedetaileddesignandthematerialofthediode.SometypicalcurvesofM(V)forasiliconAPDareshowninthefigure.1213142023/6/61516APDBandwidthInthissectionweavoidadetailedanalysisoftheconsequencesofsinusoidalmodulationoftheincidentlightbutconcentrateinsteadontheresponseofanAPDtoanopticalpulse.Thefulltheory,whichhasmuchincommonwiththetheoryofIMPATTandTRAPATToscillatorsiscomplex,sowelimitthediscussiontothegeneralphysicalprinciplesandtoestimatetheorderofmagnitudeofanbandwidthlimitation.Inthen+-p--p+typeofAPDillustratedpreviouslytheoverallresponseismadeupofthreeparts:A)theelectrontransittimeacrossthedriftregion,(ttr)e=w2/se,B)thetimerequiredfortheavalanchetodevelop,tA,C)thetransittimeofthelastholesproducedintheavalanchebackacrossthedriftspace,(ttr)h=w2/sh.PartsB)andC)representdelaysadditionaltothoseexperiencedinanon-avalanchingdiode.17APDBandwidthTheavalanchedelaytime,tA,isafunctionoftheratiooftheionisationcoefficients,k.Thedistance-timediagramstofollowgiveagraphicillustrationofthis.Whenk=0,theavalanchedevelopswithinthenormalelectrontransittimeacrosstheavalancheregion(wA/se).WeassumewA<<w2.Whenk>0,theavalanchedevelopsinmultiplepassesacrosstheavalancheregionandathighlevelsofmultiplication,with0<k<1,tAMkwA/seTheoverallresponsetime,,thenbecomes(w2+MkwA)/se+(w2+wA)/vshAndweshouldexpectthe(-3dB)bandwidthtobegivenapproximatelybyf(-3dB)0.44/18APDDistance-TimeDiagramsAvalanchebuild-upshownondistance-timediagrams:a)k=0,M=16;b)k=0.37,M=241920212223242526APDNoiseThevalueofthenoisefactor,F,anditsvariationwiththemultiplicationfactor,M,areclearlymatterswhichbearontheoptimisationoftheopticalreceiver.Forpurposesofsystemevaluationtheapproximation:FMxHasoftenbeenused.Theindex,x,typicallytakesonvaluesbetween0.2and1.0dependingonthematerialandthetypeofcarrierinitiatingtheavalanche.Aswewillsee,FMx,maybereasonablyvalidoveralimitedrangeofvaluesofM.AtheoreticaltreatmentbyMcIntyre,yieldsthefollowingmorecomplexexpressions.WhenthemultiplicationisinitiatedbyelectronsFe