固态电子器件-2013 -2014chapter

Chapter8OptoelectronicdevicesPhotonsSemiconductors(electrons)PhotodiodeEmittersofphotonsphotodiode光(电)二极管detector探测器coherentsource相干光源solarcell太阳能电池dispersion色散light-emittingdiode(LED)发光二极管incoherentsource非相干光源photovoltaiceffect光生伏特（打）效应electroluminescence电致发光photoluminescence光致发光attenuationoflights光衰减indexofrefraction折射率graded-indexfiber变型光纤stepindexfiber突变型光纤multimodefibers多模光纤single-modefibers单模光纤spontaneousemission自发辐射stimulatedemission受激辐射infrared红外8.1photodiodesPhotons(光子）8.1.1Currentandvoltageinanilluminatedjunction•••EHP°electronsholesIfasteadylightisshoneonlyonthejunctionatanopticalgenerationrategop,Thegenerationcurrent

willbeIfthesteadylightisshoneonthewholesampleatgop,theexcesscarrierconcentrationoneachsideofjunctioncanbewrittenas

Theminoritycarrierswithinadiffusionlengthwilldiffusetowardtothejunctionandwillbeswepttotheothersideofjunctionbytheelectricfield.OnnsideOnpsideBackTheshort-circuitcurrent+-vphotovoltaiceffectAforwardvoltageappearsacrossanilluminatedjunctionTheopen-circuitvoltagegthrepresentstheequilibriumthermalbinationrateForasymmetricaljunction,pn=np,τp=τn,pn/τn=gthSeeP125-131,Atequilibrium,thegenerationrateofexcesscarriersgi=rn0p0,andthelifetimeoftheexcesscarrieris=1/r(n0+p0)AstheminoritycarrierconcentrationisincreasedbyopticalgenerationofEHP,thelifetimenesshorter,andgtheslarger.Therefore,Voccannotincreaseindefinitelywithincreasedgenerationrate.VocislimitedbycontactpotentialV0.Powerisdeliveredfromthejunctiontotheexternalcircuit.SolarcellCurrentdoesnotdependonthebiasing,butdependsonthegenerationgopPhotodetectorFig.8-3cappearsdoubtfulthatmuchpowercanbedeliveredbyanindividualdevice.Becausethevoltageisrestrictedtothevaluelessthanthecontactpotential.8.1.2SolarcellsIfmanysuchdevicesareused,theresultingpowercanbesignificant.Arraysofp-njunctionsolarcellsarecurrentlyusedtosupplyelectricpowerformanyspacesatellites.LargeareajunctionReducereflectionandsurfacebinationThejunctiondepthd<LpP-region:ThethicknessofthepregionthediffusionlengthLnTheopticalpenetrationdepthLargecontactpotential—V0---NaNdButtooheavilydopedlifetimeisreduced,andshortLnSeriesresistancefillfactor填充因子conversionefficiency转换效率:Shockley–Queisserlimit~33.7%Awell-madesinglecrystalSicellcanhaveabout~22-25%efficiency,220W/cm2,butlosingefficiencyathightemperatureGaAsandrelatedcompoundscanbeusedat100℃orhigher.Inphysics,theShockley–Queisserlimitordetailedbalancelimitreferstothemaximumtheoreticalefficiencyofasolarcellusingap-njunctiontocollectpowerfromthecell.ItwasfirstcalculatedbyWilliamShockleyandHansQueisseratShockleySemiconductorin1961.[1]Thelimitisoneofthemostfundamentaltosolarenergyproduction,andisconsideredtobeoneofthemostimportantcontributionsinthefield.[2]Thelimitplacesmaximumsolarconversionefficiencyaround33.7%assumingap-njunctionbandgapof1.1eV(typicalforsilicon).Thatis,ofallthepowercontainedinsunlightfallingonasiliconsolarcell(about1000W/m²),only33.7%ofthatcouldeverbeturnedintoelectricity(337W/m²).Moderncommercialsingle-crystallinesolarcellsproduceabout22-25%conversionefficiency,thedifferenceduelargelytopracticalconcernslikereflectionoffthefrontsurfaceandlightblockagefromthethinwiresonitssurface.Shockley–Queisserlimit:

solarconversionefficiency~33.7%forsinglep-njunctionbandgapof1.1eV(typicalforsilicon).Therearethreeprimaryconsiderationsforenergyloss:Blackbodyradiationloss:~7%binationloss:~10%Spectrumlossesloss:~50%Solutionstoexceedthislimit?Tandemcells!atwo-layercellcanreach42%efficiency,three-layercells49%,andatheoreticalinfinity-layercell68%.8.1.3Photodetectors

WhenaphotodiodeisoperatedinthethirdquadrantofitsI-Vcharacteristic.Thecurrentisessentiallyindependentofvoltage,butisproportionaltotheopticalgenerationrate.Suchadeviceprovidesausefulmeansofmeasuringilluminationlevelsorofconvertingtime-varyingopticalsignalsintoelectricalsignals.Depletionlayerphotodiodep-i-nphotodiodeAvalanchephotodiode(APD)MainparametersforphotodetectorsResponsespeed响应速度Operationwavelength工作波长Quantumefficiency量子效率Responsivity响应度Sensitivity灵敏度Darkcurrent暗电流Responsespeed(time)响应速度(时间）Responsespeed~t1+t2t1:thetimeperiodexcessminoritycarriersdiffusetothejunction;t2:excesscarriersbesweptacrosstotheothersideofthejunction.LargerWsmalljunctioncapacitance.

Butontheotherhand,Wmustnotbesowidethatthetimerequiredfordriftoutofthedepletionregionisexcessive.

So,WshouldbelargeenoughsothatmostofthephotonsareabsorbedwithinWratherthaninneutralpandnregion.Operationwavelength

Forfiberopticsystems(1.55μm)Si适合于0.85μm短波长区；Ge可用于长波长波段，大的暗电流限制了应用III-V族材料用于长波长区，量子效率~75%~100%，工作电压~5V,理论带宽~15GHzQuestion:IsGaAsdetectorsuitableforlongdistancefiberopticcommunications?ThebandgapofGaAsis1.43eV.IfthephotonsaremuchmoreenergeticthanEg,theywillbeabsorbedverynearthesurface,wherethebinationrateishigh.WidebandgapInP(1.35eV)reducessurfacebinationeffects(seeFig.8-8)Quantumefficiency量子效率Responsivity(响应度）:R=Ip/Pin(A/W)Sensitivity(接收灵敏度)：达到指定信噪比或误码率时的最小接收光功率，常以dBm表示。PIN:-18dBm,-23dBm功率以dBm表示时DarkCurrent在正常工作电压下，没有光照射时光电探测器产生的电流，叫做暗电流。DepletionlayerphotodiodeResponsespeed:diffusionofminorityneartheedgeNarrowneutralregionWidedepletionregionIncidentphotonswillbeabsorbedinthedepletionSmalljunctioncapacitanceToowidedepletionresultsinlongdrifttimeCarriersaregeneratedprimarilywithinW,thedetectoriscalledSomekindsofphotodetectors:p-i-nphotodiodeOneconvenientmethodofcontrollingthewidthofthedepletionregionistobuildap-i-nphoto-detector.Theresistivityofiregionishigh.

Theappliedreversevoltageappearsalmostentirelyacrosstheiregion.p-i-nphoto-detectorIflow-levelopticalsignalsaretobedetected,itisoftendesirabletooperatethephotodiodeintheavalancheregionofitscharacteristic.Inthismode,eachphotogeneratedcarrierresultsinasignificantchangeinthecurrentbecauseofavalanchemultiplication.Avalanchephotodiode(APD)SensitivityofAPDforfiberopticsystems-32dBmorhigher

雪崩光电二极管（AvalanchePhotodiodes,APD)雪崩效应：耗尽区的高电场使光功率产生的（一次）电子或空穴加速，高速电子或空穴与原子碰撞，使其电离，产生新的电子空穴对；新的电子空穴对也被加速，并与原子碰撞，产生更多的电子空穴对。倍增增益具有统计特征，通常取其平均值为倍增增益：＜g＞=G.g决定于电离系数，雪崩区宽度，反偏电压等。电离系数：一个载流子在渡越单位距离所产生的电子空穴对数。电子的电离系数αe，空穴的电离系数β，两者差别越大，APD的噪声越小，增益越高，带宽越宽。Si-APD,αe，β两者差别大，增益~100~1000；GeorInGaAs增益小~15GainofAPDisdependonappliedvoltageWhere,Vbisbreakdownvoltage,RiistheresistoroftheAPD,IMiscurrent,aisaconstantwhichdependsonthematerial,dopingandwavelength.参见《光纤通信原理与系统》第四章张明德、孙小菡东南大学出版社P198-200Ingeneral,thecriticalreversevoltageforbreakdownincreasewiththebandgapofthematerial.Vbrdecreaseasthedopingincreases.Si-APD,αe，β两者差别大，增益~100~1000；InGaAs增益小~15L