《光通信技术》课件：Lecture7

DepartmentofOpticalEngineeringAReviewCheckListTransmitterPartsLightsourceCouplingopticsDrivingelectronicsDataconversionNon-return-to-zerocodeReturn-to-zerocodeModulatorsMach-ZehnderElectroabsorptiveMonolithicLowopvoltageLED/LDbasicpropertiesFermilevel;FermiStatisticsInternalquantumefficiencyExternalquantumefficiencyE-OefficiencySlopeefficiencyDirectmodulationDelaytimeRecombinationtimeChirpMechanismPulsesourceNoisePhasefluctuationsIntensityfluctuationsEyediagramDistortionsJitterOvershootUndershoot……Principleofthep-njunctionPhotodiodeSchematicdiagramofareversebiasedp-njunctionphotodiodeSiO2Electrodernet–eNaeNdxxE(x)REmaxe–h+Iphhv

>EgWEnDepletionregionARcoatingVrElectrodeVoutNetspacechargeacrossthediodeinthe

depletionregion.

NdandNa：donorandacceptor

concentrationsinthepandnsides.

Thefieldinthedepletionregion.p+PhotocurrentisdependonnumberofEHPanddriftvelocity.Theelectrodedonotinjectcarriersbutallowexcesscarriersinthesampletoleaveandbecomecollectedbythebattery.Principleofpnjunctionphotodiode(a)Reversedbiasedp+njunctionphotodiode.Annularelectrodetoallowphotontoenterthedevice.Antireflectioncoating(Si3N4)toreducethereflection.Thep+-sidethickness<1μm.(b)Netspacechargedistribution,withinSCL.(c)TheEfieldacrossdepletionregion.Principleofthep-njunction(b)Energybanddiagramunderreversebias.(a)Cross-sectionviewofap-i-nphotodiode.(c)Carrierabsorptioncharacteristics.Operationofap-i-nphotodiode.Agenericphotodiode.Principleofthep-njunctionPhotodiodeVariationofphotonfluxwithdistance.Aphysicaldiagramshowingthedepletionregion.Aplotofthefluxasafunctionofdistance.ThereisalossduetoFresnelreflectionatthesurface,

followedbythedecayingexponentiallossduetoabsorption.Thephotonpenetrationdepthx0isdefinedasthe

depthatwhichthephotonfluxisreducedtoe-1ofitssurfacevalue.Principleofthep-njunctionPhotodiodeRAMO’sTheoremandExternalPhotocurrenttte-h+Iphoto(t)SemiconductorVx

lL

-

ltvhole0Lle–h+0t0Area=Charge=e

evh/Leve/LAnEHPisphotogeneratedatx=l.Theelectronandtheholedriftinopposite

directionswithdriftvelocitiesvh

andve.Theelectronarrivesattimetelectron

=(L-l)/ve

and

theholearrivesattimethole

=l/vh.photocurrentielectron(t)Etholetelectrontholeihole(t)i(t)telectrontholevelectroniphoto(t)Astheelectronandholedrift,eachgeneratesielectron(t)andihole(t).Thetotalphotocurrentisthesumofholeandelectronphotocurrentseachlastingadurationthandterespectively.RAMO’sTheoremandExternalPhotocurrentTransittimeRamo’sTheoremPhotocurrentThecollectedchargeisnot2ebutjust“oneelectron”.Ifachargeqisbeingdriftedwithavelocityvd(t)byafieldbetweentwobiasedelectrodesseparatedbyL,themotionofqgeneratesanexternalcurrent

givenbyAbsorptionCoefficientandPhotodiodeMaterialsAbsorbedPhotoncreatesElectron-HolePair.Cut-offwavelengthvs.EnergybandgapAbsorptioncoefficientIncidentphotonsbecomeabsorbedastheytravelinthesemiconductorandlightintensitydecaysexponentiallywithdistanceintothesemiconductor.AbsorptionCoefficientAbsorptioncoefficientαisamaterialproperty.Mostofthephotonabsorption(63%)occursoveradistance1/α(itiscalledpenetrationdepth

δ)DepartmentofOpticalEngineeringPhotodiodeFreeelectrons->electricconductionPhotonenergyabsorbedbyanelectrontoovercomeenergygapandbecomeafreeelectronDepartmentofOpticalEngineeringInput-outputCharacteristicInput: lightpowerPOutput: photocurrentIp->Ip=RPR:responsivity(A/W)DepartmentofOpticalEngineeringResponsivityv.s.WavelengthPhotocurrent:Ip=#ofelectrons/time=Ne/tLightpower:P=photons*energy/time=NpEp/tR=Ip/P=Ne/NpEp=(Ne/Np)(/hc)Quantumefficiency=Ne/Np

R=(/1248(eV/nm))Example:anInGaAsphotodiodew/q.e.70%,whatisR? Ans:energygapofInGaAs=0.75eV->1664nmR=(0.7/1248)1664=0.93A/WDepartmentofOpticalEngineeringAbsorptioncoefficientAbsorption:aprerequisiteofsignaldetectionBeer’slaw:Pabs=Pin(1-exp(-absw))DepartmentofOpticalEngineeringTwooperationmodesofphotodiodesPhotovoltaic->likeasolarcell,nobiasSlowresponsePhotoconductivemodeFastresponse(widebandwidth)AdvantagesofreversebiasingWithoutlight,nofreechargecarriers->notconductivelight->ehpairs,reversebiashelpstoseparatethepair->conductiveDriftcurrent(fast,b/cstrongelectricfield)indepletionregion;diffusioncurrent(slow)inpornregionDarkcurrent->thermallycreatedcarriersaresweptawaybyreversebiasingDepartmentofOpticalEngineeringBandwidthTwomechanismsrestrictingbandwidthTransittime

tr=w(10m)/vsat(105m/s)~100psInherentcapacitance

Cin=A/w~1-2pFRS=(Rs+RL)CinRLCin~100ps(50input)

BW=1/[2(tr+RS)]

~1Gbit/sToincreasebandwidth ->optimizewwiscontrolledbybiasvoltageHighw->longtransittime->lowerBW->lowcapacitance->increaseBWDepartmentofOpticalEngineeringSpecSheetParameterP/N

SuffixTest

ConditionsUnitsMin.Typ.Max.9µmfiber-5Lasersource

of10µWA/W.60----Spectral

Response-5--nm1150--1600Capacitance-5f=1MHzpF--.92.0Darkcurrent-5--nA--1.55FrequencyResponse-5--GHz1.0----Reliability--ID5nAhrs--2.0x108--

UnitsMin.Max.OperatingtemperatureC-4085StoragetemperatureC-40125ReversecurrentmA--1ThepinPhotodiodeThepnjunctionphotodiodehastwodrawbacks:Depletionlayercapacitanceisnotsufficientlysmalltoallowphotodetectionathighmodulationfrequencies(RCtimeconstantlimitation).NarrowSCL(spacechargeregion,atmostafewmicrons)longwavelengthsincidentphotonsareabsorbedoutsideSCLlowQEThepinphotodiodecansignificantlyreducetheseproblems.Intrinsiclayerhaslessdopingandwiderregion(5–50μm).

Reverse-biased

p-i-n

photodiodeThepinPhotodiode

pin

energy-band

diagram

pin

photodiode

circuit

SchematicdiagramofpinphotodiodeThepinPhotodiodeSmalldepletionlayercapacitancegiveshighmodulationfrequencies.HighQuantumefficiency.SiO2p+i-Sin+Electrodernet–eNaeNdxxE(x)RE0e–h+Iphhu

>EgWVrVoutElectrodeEIncontrasttopnjunctionbuilt-in-fieldisuniformAreversebiasedpinphotodiodeisilluminatedwithashortwavelengthphotonthatisabsorbedverynearthesurface.Thephoto-generatedelectronhastodiffusetothedepletionregionwhereitissweptintothei-

layeranddriftedacross.ThepinPhotodiodep-i-n

diodeThestructure;Equilibriumenergybanddiagram;Energybanddiagramunder

reversebias.ThepinPhotodiodeThepinPhotodiodeTheresponsivityofpinphotodiodesQuantumefficiencyversuswavelengthforvariousphotodetectors.PhotoconductiveDetectorsandPhotoconductivegain

Electricfieldofbiasedpin

Junction

capacitance

of

pinThepinPhotodiodeSmallcapacitance:HighmodulationfrequencyRCdeptimeconstantis

50psec.

Responsetime

Thespeedofpinphotodiodesareinvariablylimitedbythetransittimeofphotogeneratedcarriersacrossthei-Silayer.Fori-Silayerofwidth10m,thedrifttimeisaboutisabout0.1nsec.

Drift

velocity

vs.electricfieldforholesandelectronsinSilicon.102103104105107106105104Electricfield(Vm-1)ElectronHoleDriftvelocity(msec-1)The

pin

PhotodiodeDepartmentofOpticalEngineeringPINphotodiodeWhydoweneedp-i-njunction?

PowerandBWarehigh!MostwidelyusedinfiberopticsDepletionregioniscontrolledbyintrinsiclayerthickness,notbybiasElectron-holepairscreatedinilayer,highquantumefficiencyNofreechargeinilayer->efficientseparationofehpairs->highdriftspeedLowdarkcurrent:b/cthickdepletionregionLowbiasVerysmalldiffusioncurrent->highBW,b/cthinpandnlayersPINPD:thickdepletionregionType:FrontorRearilluminatedCompromisebetweenpowerefficiencyandbandwidthBW~1/trFrontIlluminationRearIlluminationHeavilydopedNtype,transparenttolightDepartmentofOpticalEngineeringPINphotodiodeExample:SiPIN(850nm)InGaAsPIN(1550nm)

abs~103cm-1abs~105cm-1

w~40mw~4m

vdrift=105m/sBW(Si)=w/2v=0.4Gbit/sBW(InGaAs)=w/2v=4Gbit/sDepartmentofOpticalEngineeringAvalanchePhotodiodeAPD:aspecialPINPDwithveryhighsensitivityHighreversebias~20VElectronsorholesareacceleratedtogainenergyeorhstrikeneutralatomsproducingsecondaryeorh->avalancheeffectOnephoton->10-100ehpairsQ.E.>1Gain-bandwidthproduct MxBW=1/2ee=kAtr

p+SiO2ElectrodernetxxE(x)Rhu>EgpIphotoe–h+AbsorptionregionAvalancheregionElectroden+EAvalanchePhotodiode(APD)h+n+pe–AvalancheregionEe-h+EcEvImpactionizationprocessesresultingavalanchemultiplicationImpactofanenergetic

electron'skineticenergyexcitesVBelectrontotheCV.SiO2GuardringElectrodeAntireflectioncoatingnnn+p+pSubstrateElectroden+p+pSubstrateElectrodeAvalanchebreakdownSiAPDstructurewithoutaguardring

MorepracticalSiAPDAvalanchePhotodiode(APD)SchematicdiagramoftypicalSiAPD.

Breakdownvoltagearoundperipheryishigherandavalancheis

confinedmoretoilluminatedregion(n+pjunction).

ENnElectrodexE(x)RhuIphAbsorptionregionAvalancheregionInPInGaAsh+e–InPP+n+VrVoutEAPDPhotodetectorsHeterojunctionPhotodiodeSeparateAbsorptionandMultiplication(SAM)APDInGaAs-InPheterostructure

SeparateAbsorptionandMultiplication

APDPandNrefertop-andn-typewider-bandgapsemiconductor.InPInGaAsh+e–EcEvEcEvInPInGaAsEvEvh+DEv

E

PhotodetectorsHeterojunction+SeparateAbsorptionandMultiplication(SAM)APDInGaAsPgradinglayer(a)Energybanddiagramfora

SAMheterojunctionAPDwhere

thereisavalencebandstep

DEv

fromInGaAstoInPthatslows

holeentryintotheInPlayer.(b)Aninterposinggradinglayer

(InGaAsP)withanintermediate

bandgapbreaksDEvandmakesit

easierfortheholetopasstotheInP

layer.xRe–h+iphhu

>EgWVrPhotogeneratedelectronconcentrationexp(-ax)attimet=0BAvdeEAPDPhotodetector:PhotogeneratedelectronconcentrationDepartmentofOpticalEngineeringTypicalcharacteristicsofPINsandAPDsDepartmentofOpticalEngineeringNoisesourcesinphotodetectorsNoisesourcesShotnoise(unit:amperespersquarerootofhertz)Deviationofactualnumberofelectronsfromtheaverage

ThermalnoiseThedeviationsofaninstantaneousnumberofelectronsfromtheiraveragevaluebecauseoftemperaturechange

DarkcurrentnoiseThesameasshotnoiseI->Idark1/fnoiseNoiseisnotfindependent(whitenoise),=2,=1-1.5,f=modulationfreq.(Operationfreq.)

DepartmentofOpticalEngineeringNoisecalculationAPINPDInput0.1W,BW=2.5Gbit/s,R=1.0A/W,T=300KAnalyzeitsnoises:Ans:1.P