Avalanche photodiode

Workingprinciple

Theavalanchephotodiodeisap-njunctionphotodetectiondiode,whichusestheavalanchemultiplicationeffectofcarrierstoamplifythephotoelectricsignaltoimprovethedetectionsensitivity.ItsbasicstructureoftenadoptstheReaddiodestructurewhichispronetoavalanchemultiplicationeffect(ieN+PIP+typestructure,P+sidereceiveslight),andalargereversebiasisappliedduringoperationtomakeitreachtheavalanchemultiplicationstate;itslightabsorptionThezoneisbasicallythesameasthemultiplicationzone(thePzoneandtheIzonewithhighelectricfield).

ThePNjunctionisappliedwithasuitablehighreversebiasvoltage,sothatthephotogeneratedcarriersinthedepletionlayerareacceleratedbyastrongelectricfieldtoobtainhighenoughkineticenergy,andtheycollidewiththelatticetoionizeandgeneratenewelectrons.Holepairs,thesecarrierscontinuetocausenewimpactionization,resultingincarrieravalanchemultiplicationandcurrentgain.Inthe0.6-0.9μmband,siliconAPDhasclosetoidealperformance.InGaAs(IndiumGalliumArsenide)/InP(IndiumPhosphorus)APDisanidealphotodetectorforlong-wavelength(1.3μn,1.55μm)opticalfibercommunication.ThelightabsorptionlayerusesInGaAsmaterial,whichhasahighabsorptioncoefficientforlightof1.3μmand1.55μn.InordertoavoidthetunnelbreakdownoftheInGaAshomojunctionbeforetheavalanchebreakdown,theavalanchezoneandtheabsorptionzoneareseparated,thatis,thePNjunctionismadeIntheInPwindowlayer.SincetheholeionizationcoefficientintheInPmaterialisgreaterthantheelectronionizationcoefficient,n-typeInPisselectedfortheavalanchezone.Thereisalargevalencebandbarrierattheheterogeneousinterfacebetweenn-InPandn-InGaAs,whichiseasytocausethecollapseofphoto-generatedholes,whichissandwichedbetweenEntertheInGaAsP(IndiumGalliumArsenidePhosphorus)transitionzonewithagradualbandgaptoformaSAGM(absorption,gradingandmultiplicationrespectively)structure.

InAPDmanufacturing,itisnecessarytoaddaprotectiveringonthesurfaceofthedevicetoimprovethereversewithstandvoltageperformance;thesemiconductormaterialispreferablySi(widelyusedtodetectlightbelow0.9um),butinthedetectionForlong-wavelengthlightabove1um,GeandInGaAsarecommonlyused(largernoiseanddarkcurrent).ThedisadvantageofthistypeofAPDisthatthereisaprocessoftunnelingcurrentmultiplication,whichwillproducelargershotnoise(reducingthep-zonedopingcanreducethetunnelingcurrent,buttheavalanchevoltagewillincrease).Animprovedstructureistheso-calledSAM-APD:themultiplicationzoneusesamaterialwithawiderbandgap(sothatitdoesnotabsorblight),andthelightabsorptionzoneusesamaterialwithanarrowbandgap;here,becauseoftheuseofheterojunction,youcanReducethedopingconcentrationofthemultiplicationregionwithoutaffectingthelightabsorptionregion,sothatthetunnelcurrentcanbereduced(ifitisamutationheterojunction,becauseoftheexistenceofΔEv,thephoto-generatedholeswillbeaccumulatedandaffectthedevice'sperformance.Responsespeed,atthistime,agradualchangelayercanbeinsertedinthemiddleofthemutationheterojunctiontoreducetheinfluenceofΔEv).

Mainfeatures

Mainfeatures①AvalanchegaincoefficientM(alsocalledmultiplicationfactor),forsuddenchanges

Intheformula,Visthereversebiasvoltage,andVBisthebulkavalanchebreakdownvoltage;nisrelatedtothematerial,

devicestructureandincidentwavelength,etc.Itisaconstant,anditsvalueis1～3.②Gain-bandwidthproduct,whenthegainislargeandthefrequencyishigh,

M(ω)·ω

whereωistheangularfrequency;NisConstant,whichchangesslowlywiththeratiooftheionizationcoefficient;Wisthethicknessofthedepletionzone;Vsisthesaturationvelocity;αnandαparetheionizationcoefficientsofelectronsandholes,respectively,andthegainbandwidthproductisaconstant.Togetahighproduct,youshouldchoosealargeVs,asmallW,andasmallαn/αp(thatis,thedifferenceintheionizationcoefficientsofelectronsandholesshouldbelarge,andcarrierswithhigherionizationcoefficientsshouldbeinjectedintotheavalanchezone).③ExcessivenoisefactorF.Intheprocessofmultiplication,thenoisecurrentincreasesfasterthanthesignalcurrent.UseFtodenotethenoiseadditionF≈Mxcausedbytheavalancheprocess.Wherexistheexcessnoiseindex.ItisnecessarytoselecttheappropriatevalueofMtoobtainthebestsignal-to-noiseratioandmakethesystemreachthehighestsensitivity.④Temperaturecharacteristics,thecarrierionizationcoefficientdecreaseswiththeincreaseoftemperature,whichleadstothedecreaseofthemultiplicationfactorandtheincreaseofthebreakdownvoltage.ThetemperaturecoefficientofbreakdownvoltageisusedtodescribethetemperaturecharacteristicsofAPD.

β=

Intheformula,VBandVB0arethebreakdownvoltagewhenthetemperatureisTandT0,respectively.

Whenusing,thetemperatureoftheworkingpointshouldbecontrolled,andauniformP-Njunctionshouldbemadetopreventthelocaljunctionsurfacefrombeingbrokendown.

Avalanche photodiode

Material

Theoretically,anysemiconductormaterialcanbeusedinthemultiplicationzone:

Siliconmaterialissuitableforthedetectionofvisiblelightandnear-infraredlight,andhasrelativelyhighLowmultipliernoise(excessnoise).

Germanium(Ge)materialcandetectinfraredlightwithawavelengthnotexceeding1.7μm,butthemultiplicationnoiseisrelativelylarge.

InGaAsmaterialscandetectinfraredrayswithwavelengthsexceeding1.6μm,andthemultiplicationnoiseislowerthanthatofgermaniummaterials.Itisgenerallyusedasthemultiplicationregionofheterostructurediodes.Thismaterialissuitableforhigh-speedopticalfibercommunication,andthespeedofcommercialproductshasreached10Gbit/sorhigher.

Galliumnitridediodescanbeusedforultravioletdetection.

HgCdTediodescandetectinfraredrayswithawavelengthofupto14μm,buttheyneedtobecooledtoreducedarkcurrent.Usethisdiodetoobtainverylowexcessnoise.

Basicintroductionandapplicationrangeofbasicmodels

Eachmoduleincludesaphotodetector(fastphotodiodeoravalanchephotodiode)andatransimpedanceamplifier.Amplifierandphotodetectorarecombinedinthesamepackage,sothattheenvironmentalnoiseislowerandtheparasiticcapacitanceissmaller.

C30659seriesmodulesincludeanAPDconnectedtoalownoisetransimpedanceamplifier.Thereare4typesofsiliconcrystalavalanchephotodiodesand2typesofindiumgalliumarsenideavalanchephotodiodestochoosefrom.Thestandardbandwidthof50MHzand200MHzcanbeadaptedtoawiderangeofapplications.TherearealsotwoC30659modelsofavalanchephotodiodesequippedwiththermoelectriccooling(LLAMseries)tohelpimprovenoiseorkeeptheavalanchephotodiodesworkingataconstanttemperatureunderanyambienttemperature.

C30659modelcanchooseacustomizedbandwidthorcustomizedproductssuitableforspecialenvironmentalrequirementsaccordingtospecialapplicationneeds.Thereisalsoa14-pindualin-lineplug-inwithapigtailpackage,whichcanachievealmost100%couplingefficiency.

C30950EHisalow-costproductthatcanreplaceC30659.Theamplifierisusedtooffsettheinputcapacitanceofthevoltagegainamplifier.C30919EandC30950EHusethesamedesignstructure,withanadditionalhigh-voltagetemperaturecompensationcircuittomaintainthemodule'sresponsivenessconstantinawidetemperaturerange.TheothertwoHUVmodulescanbeusedforlowfrequencyandhighgainapplications,whichcoverawidespectrumfromultraviolettonearinfrared.