雪崩光电二极管(APD).ppt

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1、雪崩光电二极管（APD）探测器1TheMultiplicationProcessAvalanchePhotodiodeDesignsAvalanchePhotodiodeBandwidthAvalanchePhotodiodeNoiseAvalanchePhotodiodeDetectors2345678Measuredvaluesofionisationcoefficientseandhforsomecommonsemiconductormaterials,plottedversus(1/E).TheMultiplicationProcess9TheMultiplicationProc

2、essWemaydefineionisationcoefficientsforelectronsandholes,eandhrespectively,astheprobabilitythatagivencarrierwillexciteanelectron-holepairinunitdistance.Thecoefficientsincreasesorapidlywithincreasingelectricfieldstrength,thatitisoftenconvenienttothinkintermsofabreakdownfield,EB,atwhichavalancheexci

3、tationbecomescritical,saybecomesoftheorder105–106m-1.Graphsofeandhversuselectricfieldareplottedforanumberofsemiconductorsknowntobeofinterestasdetectormaterials.Thecurvesrefertoroomtemperature.Asthetemperatureincreases,theionisationcoefficientsdecrease,becausethegreaternumberofscatteringcollisions

4、reducesthehigh-energytailofthecarrierenergydistributionandhencereducestheprobabilityofexcitation.Insomematerialse>h,inothersh>e,whileingalliumarsenideandindiumphosphidethetwocoefficientsareapproximatelythesame.Theratiok=h/eisfoundtolieintherange0.01to100.10TheMultiplicationProcess-Experimental

5、BehaviourTwofactorslimittheincreaseofMe,themultiplicationfactorfortheinjectedelectronsandhenceIastheappliedvoltageapproachesthebreakdownvoltage,VB,atwhichthevalueseandhsatisfytheconditionforbreakdown,thatisM->.Thefirstistheseriesresistanceofthebulksemiconductor,RS,betweenthejunctionandthediodeter

6、minals.Thesecondistheeffectoftheriseintemperatureresultingfromtheincreaseddissipationasthecurrentrises.Thisreducesthevaluesofeandhandraisesthebreakdownvoltage.Italsoincreasestherateofthermalgenerationofcarriersandhencethedarkcurrent.Multiplicationfactorsmeasuredasafunctionoftheappliedterminalvolta

7、ge,V,canusuallybefittedtotheformM=1/

8、1-(V-IR’)/VB

9、nWhereR’=RS+RThisthesumoftheseriesresistance,RS,andaneffectiveresistance,RTh,whichderivesfromtheriseintemperature.Theindex,n,isafunctionofthedetailedd