Classification
Thephotodetectorcanconvertopticalsignalsintoelectricalsignals.Accordingtothedifferentwaysinwhichthedevicerespondstoradiationortheworkingmechanismofthedevice,photodetectorscanbedividedintotwocategories:oneisphotondetectors;theotheristhermaldetectors.
Application
Thechoiceofapplicationofphotodetectordevicesisactuallysomeitemsorpointsofapplication.Inmanyapplicationswithlessstringentrequirements,anyphotodetectordevicecanbeused.However,insomecases,itismoreappropriatetochooseacertaindevice.Forexample,whenarelativelylargephotosensitiveareaisrequired,vacuumphototubescanbeused.Becauseoftheirwidespectralresponserange,vacuumphototubesarecommonlyusedinspectrophotometers.Whenthemeasuredradiationsignalisweakandtheresponsespeedishigh,thephotomultipliertubeismostsuitablebecauseitsamplificationfactorcanreachmorethan10^4～10^8.SuchhighgaincanmakethesignalexceedtheoutputandamplificationcircuitThenoisecomponentofthedetectorislimitedtothestatisticalchangeinthephotocathodecurrent.Therefore,photomultipliertubesarewidelyusedinastronomy,spectroscopy,laserrangingandscintillationcounting.
Solid-statephotodetectorsareveryversatile.CdSphotoresistorsareusedinbrightnesscontrol(suchasautomaticphotographicexposure)duetotheirlowcost;photocellsarethedeviceswiththelargestphotosensitiveareaamongsolidoptoelectronicdevices.Inadditiontobeingusedasdetectors,theycanalsobeusedassolarconverters;Siliconphotodiodesaresmallinsize,fastinresponse,highinreliability,andhavehighquantumefficiencyinthevisibleandnear-infraredbands,sotheyareusedinvariousindustrialcontrols.Siliconavalanchetubesarewidelyusedinlaserrangingandopticalfibercommunicationduetotheirhighgain,fastresponse,andlownoise.
photoconductivedetectorisakindofphotoconductivedetectormadeofthephotoconductiveeffectofsemiconductormaterials.Theso-calledphotoconductiveeffectreferstoaphysicalphenomenoninwhichtheconductivityoftheirradiatedmaterialchangesduetoradiation.Photoconductivedetectorsarewidelyusedinvariousfieldsofmilitaryandnationaleconomy.Inthevisibleornear-infraredband,itismainlyusedforraymeasurementanddetection,industrialautomaticcontrol,photometricmeasurement,etc.;intheinfraredband,itismainlyusedformissileguidance,infraredthermalimaging,andinfraredremotesensing.Anotherapplicationofthephotoconductoristouseitasthetargetsurfaceofthecameratube.Inordertoavoidtheimageblurringcausedbythediffusionofphoto-generatedcarriers,high-resistancepolycrystallinematerials,suchasPbS-PbO,Sb2S3,etc.,areusedforthecontinuousfilmtargetsurface.Othermaterialscanbeusedtoinlaythetargetsurface,theentiretargetsurfaceiscomposedofabout100,000individualdetectors.
In1873,BritishW.Smithdiscoveredthephotoconductivityeffectofselenium,butthiseffecthasbeeninthestageofexploratoryresearchforalongtimeandhasnotbeenappliedinpractice.AftertheSecondWorldWar,withthedevelopmentofsemiconductors,variousnewphotoconductivematerialscontinuedtoappear.Inthevisiblelightband,bythemid-1950s,good-performancecadmiumsulfide,cadmiumselenidephotoresistorsandinfraredbandleadsulfidephotodetectorshadbeenputintouse.Intheearly1960s,sensitiveGeandSidopedphotoconductivedetectorsinthemid-to-farinfraredbandweresuccessfullydeveloped.TypicalexamplesareGe:Au(germanium-dopedgold)andGe:Hgoperatinginthe3～5micronand8～14micronbands.Photoconductivedetector.Afterthelate1960s,theresearchofHgCdTe,PbSnTeandothervariablebandgapternarymaterialshasmadeprogress.WorkingprincipleandcharacteristicsThephotoconductiveeffectisakindofinternalphotoelectriceffect.WhentheenergyhvoftheirradiatedphotonisequaltoorgreaterthantheforbiddenbandwidthEgofthesemiconductor,thephotoncanexciteelectronsinthevalencebandtotheconductionband,therebygeneratingconductiveelectronandholepairs,whichistheintrinsicphotoconductivityeffect.HerehisthePlanckconstant,visthephotonfrequency,andEgisthebandgapofthematerial(inelectronvolts).Therefore,theresponselongwavelimitλcoftheintrinsicphotoconductorisλc=hc/Eg=1.24/Eg(μm)wherecisthespeedoflight.Thelong-wavelimitofintrinsicphotoconductivematerialsislimitedbytheforbiddenbandwidth.
Beforetheearly1960s,nosuitablesemiconductormaterialswithnarrowbandgapwidthhadbeendeveloped,sopeopleusedtheextrinsicphotoconductivityeffect.ThereareimpuritylevelsofvariousdepthsintheforbiddenbandofGe,Siandothermaterials.Aslongastheirradiatedphotonenergyisequaltoorgreaterthantheionizationenergyoftheimpuritylevel,photo-generatedfreeelectronsorfreeholescanbegenerated.Theresponselong-wavelimitλoftheextrinsicphotoconductorisobtainedbythefollowingformula:λc=1.24/EiwhereEirepresentstheionizationenergyoftheimpuritylevel.Bythemiddleandlate1960s,ternarysemiconductormaterialssuchasHg1-xCdxTe,PbxSn1-xTe,PbxSn1-xSe,etc.weresuccessfullydevelopedandenteredthepracticalstage.Theirbandgapvarieswiththevalueofthecomponentx.Forexample,theHG0.8Cd0.2Tematerialwithx=0.2canbemadeintoaninfrareddetectorwitharesponsewavelengthof8to14micronsinanatmosphericwindow.ComparedwithGe:Hgdetectorsworkinginthesameband,ithasthefollowingadvantages:
Highworkingtemperature(above77K),easytouse,andGe:Hgworkingtemperatureis38K;intrinsicabsorptioncoefficientLarge,smallsamplesize;easytomanufacturemultipledevices.Table1andTable2respectivelylisttheEg,Eiandλcvaluesofsomesemiconductormaterials.
Generally,allsemiconductormaterialswithsuitablebandgaporimpurityionizationenergyhaveaphotoelectriceffect.However,factorssuchasperformance,process,andpricemustalsobeconsideredinthemanufactureofpracticaldevices.CommonlyusedphotoconductivedetectormaterialsincludeCdS,CdSe,CdTe,Si,Ge,etc.intherayandvisiblelightbands;inthenear-infraredbands:PbS,InGaAs,PbSe,InSb,Hg0.75Cd0.25Te,etc.;Thebandsare:Hg1-xCdxTe,PbxSn1-x,Te,Sidoping,Gedoping,etc.;CdS,CdSe,PbSandothermaterialscanbemadeintophotoconductivedetectorsintheformofpolycrystallinethinfilms.ThephotoconductivedetectorsCdS,CdSe,andCdTeinthevisiblelightbandhaveresponsebandsinthevisibleornear-infraredregion,andareusuallycalledphotoresistors.Theyhaveawideforbiddenbandwidth(muchgreaterthan1electronvolt)andcanworkatroomtemperature.Therefore,thedevicestructureisrelativelysimple.Generally,asemi-sealedbakeliteshellisused,withatransparentwindowatthefrontandtwopinsattheback.Asanelectrode.Thephotoconductivedetectorusedinhightemperatureandhighhumidityenvironmentcanadoptametalfullysealedstructure,andtheglasswindowandtheKovarmetalshellarefusedandsealed.
Thesensitivityofthedeviceisexpressedbythemagnitudeofthephotocurrentgeneratedperlumensofirradiationunderacertainbiasvoltage.Forexample,aCdSphotoresistor,whenthebiasvoltageis70V,thedarkcurrentis10-6～10-8A,andthelightsensitivityis3～10A/lumen.ThesensitivityofCdSephotoresistorsisgenerallyhigherthanthatofCdS.Anotherimportantparameterofthephotoresistoristhetimeconstantτ,whichrepresentstheresponsespeedofthedevicetolight.Afterthelightissuddenlyremoved,thetimerequiredforthephotocurrenttodropto1/e(about37%)ofthemaximumvalueisthetimeconstantτ.Thereisalsoacalculationofτaccordingtothephotocurrentfallingto10%ofthemaximumvalue;thetimeconstantsofvariousphotoresistorsareverydifferent.ThetimeconstantofCdSisrelativelylarge(ontheorderofmilliseconds).ThecommonresponsebandsofinfraredphotoconductivedetectorsPbSandHg1-xCdxTearethreeatmospherictransmissionwindowsof1-3microns,3-5microns,and8-14microns.Becausetheirbandgapisverynarrow,atroomtemperature,thermalexcitationissufficienttomakealargenumberoffreecarriersintheconductionband,whichgreatlyreducesthesensitivitytoradiation.
Thelongertheresponsewavelengthis,themoreobvioustheelectricalconductoris.Amongthem,thedetectorinthe1-3micronbandcanworkatroomtemperature(thesensitivityisslightlyreduced).Therearethreetypesofdetectorsinthe3～5micronband:
Workingatroomtemperature,butthesensitivityisgreatlyreduced,thedetectionrateisgenerallyonly1～7×108cm·W-1·H;thermoelectriccoolingWorkingattemperature(about-60℃),thedetectiondegreeisabout109cm·W-1·Hz;workingat77Korlowertemperature,thedetectiondegreecanreachmorethan1010cm·W-1·Hz.Detectorsinthe8-14micronbandmustworkatlowtemperatures,sothephotoconductormustbekeptinavacuumDewar.Therearetwocoolingmethods:liquidnitrogenfillingandmicro-refrigerator.
Thetimeconstantoftheinfrareddetectorismuchsmallerthanthatofthephotoresistor.ThetimeconstantofthePbSdetectorisgenerally50～500microseconds,andthetimeconstantoftheHgCdTedetectoris10-6～10-8seconds.class.Infrareddetectorssometimesneedtodetectveryweakradiationsignals,suchas10-14watts;theoutputelectricalsignalsarealsoverysmall,sospecialpreamplifiersarerequired.
Intermsofdynamiccharacteristics(iefrequencyresponseandtimeresponse),photomultipliertubesandphotodiodes(especiallyPINtubesandavalanchetubes)arethebest;intermsofphotoelectriccharacteristics(ielinearity),Photomultipliertubes,photodiodesandphotocellsarethebest;intermsofsensitivity,photomultipliertubes,avalanchephotodiodes,photoresistorsandphototransistorsarethebest.Itisworthpointingoutthathighsensitivitydoesnotnecessarilymeanhighoutputcurrent.Deviceswithhighoutputcurrentincludelarge-areaphotocells,photoresistors,avalanchephotodiodes,andphototransistors;thelowestappliedbiasvoltageisphotodiodes,phototransistors,andphotocells.Anexternalbiasisrequired;intermsofdarkcurrent,thephotomultipliertubeandphotodiodearethesmallest,thereisnodarkcurrentwhenthephotocellisnotbiased,andthedarkcurrentafterreversebiasislargerthanthatofthephotomultipliertubeandphotodiode;stablelong-termworkIntermsofperformance,photodiodesandphotocellsarethebest,followedbyphotomultipliertubesandphototransistors;intermsofspectralresponse,photomultipliertubesandCdSephotoresistorsarethewidest,butphotomultipliertubesrespondintheUVdirection,whilephotoresistorsResponsetopartialinfrareddirection.
Workingprinciple
Theworkingprincipleofthephotodetectorisbasedonthephotoelectriceffect.Thethermaldetectorisbasedonthematerialthatabsorbsthelightradiationenergyandthetemperaturerises,therebychangingitselectricalperformance.Thebiggestfeaturethatdistinguishesitfromphotondetectorsisthatithasnoselectivitytothewavelengthoflightradiation.
Photoelectronemissiondevice:photoelectrictubeandphotomultipliertubearetypicalphotoelectronemission(externalphotoelectriceffect)detectordevices.Itsmainfeaturesarehighsensitivity,goodstability,fastresponsespeedandlownoise.Itisacurrentamplifierdevice.Inparticular,thephotomultipliertubehasahighcurrentgain,whichisparticularlysuitablefordetectingweaklightsignals;butithasacomplexstructure,ahighworkingvoltageandalargevolume.
Photomultipliertubesaregenerallyusedtomeasureweakradiationandrequirehighresponsespeed,suchassatellitelaserrangefinders,lightradars,etc.
Photoconductivedevice:Aphotodetectormadeofasemiconductormaterialwithaphotoconductiveeffectiscalledaphotoconductivedevice,usuallycalledaphotoresistor.Therearesuitablephotoresistorsinthevisiblelightbandandtheseveralwindowsthroughwhichtheatmospherepasses,namelythenear-infrared,mid-infraredandfar-infraredbands.Photoresistorsarewidelyusedinphotoelectricautomaticdetectionsystems,photoelectrictrackingsystems,missileguidance,infraredspectroscopysystems,etc.
CadmiumsulfideCdSandcadmiumselenideCdSephotoresistorsarethetwomostcommonlyusedphotoresistorsinthevisiblelightband;leadsulfidePbSphotoresistorsarethefirstinfraredtransparentThemainphotoresistorthroughthewindow,thePbSphotoresistorworkingatroomtemperaturehasaresponsewavelengthrangeof1.0～3.5microns,andapeakresponsewavelengthofabout2.4microns;indiumantimonideInSbphotoresistorsaremainlyusedtodetectthesecondinfraredtransmissionwindowoftheatmosphere,anditsresponsewavelength3～5μm;Thespectralresponseofthemercurycadmiumtelluridedeviceis8～14μm,anditspeakwavelengthis10.6μm,whichmatchesthelaserwavelengthoftheCO2laser.Itisusedtodetectthethirdwindowoftheatmosphere(8～14μm)°
Technicalrequirements
Inordertoimprovethetransmissionefficiencyandtransformthephotoelectricsignalwithoutdistortion,thephotodetectormustnotonlymatchthemeasuredsignalandopticalsystem,butalsohavethecharacteristicsofthesubsequentelectroniccircuit.Matchwiththeworkingparameters,sothateachinterconnecteddeviceisinthebestworkingcondition.Themainpointsofapplicationselectionofthephotodetectoraresummarizedasfollows:
Thephotodetectormustmatchthespectralcharacteristicsoftheradiationsignalsourceandopticalsystem.Ifthemeasurementwavelengthisintheultravioletband,chooseaphotomultipliertubeoraspecialultravioletphotoelectricsemiconductordevice;ifthesignalisvisiblelight,youcanchooseaphotomultipliertube,photoresistor,andSiphotoelectricdevice;ifitisaninfraredsignal,chooseaphotoresistor.Siphotoelectricdevicesorphotomultipliertubesareusedforinfrared;
Thephotoelectricconversioncharacteristicsofthephotodetectormustmatchtheincidentradiationenergy.Amongthem,thefirstthingtopayattentiontoisthatthephotosensitivesurfaceofthedeviceshouldmatchtheirradiatedlight,becausethelightsourcemustilluminatetheeffectivepositionofthedevice.Iftheilluminatingpositionchanges,thephotoelectricsensitivitywillchange.Ifthephotoresistorisavariableresistor,theresistanceofthepartwithlightisreduced.Thelightmustbeshiningonalltheresistorsbetweenthetwoelectrodesinordertoeffectivelyuseallthephotosensitivesurfaces.Thephotosensitivesurfaceofthephotodiodeandphototransistorisonlyaverysmallareanearthejunction,sothelensisgenerallyusedasthelightentrancewindow,andthefocalpointofthelensshouldbealignedwiththesensitivepointofthelight.Thecenterofchangeoftheincidentfluxmustbewithinthelinearrangeofthephotoelectriccharacteristicsofthedetectiondevicetoensureagoodlinearoutput.Forweakopticalsignals,thedevicemusthaveappropriatesensitivitytoensureacertainsignal-to-noiseratioandoutputasufficientlystrongelectricalsignal;
Thephotodetectormustbeconsistentwiththemodulationform,signalfrequencyandwaveformoftheopticalsignal.Matchingtoensuretheoutputwaveformwithoutfrequencydistortionandgoodtimeresponse.Inthiscase,themainchoiceistoselectadevicewithashortresponsetimeorahighupperlimitfrequency,butalsopayattentiontomatchingthedynamicparametersinthecircuit;
Thephotodetectormustbewellmatchedwiththeinputcircuitintermsofelectricalcharacteristics.Inordertoensurealargeenoughconversionfactor,linearrange,signal-to-noiseratioandfastdynamicresponse,etc.;
Inordertoenablethedevicetoworkstablyandreliablyforalongtime,attentionmustbepaidtotheselectionofthedevice’sspecificationsandtheenvironmentinwhichitisusedConditions,andthedeviceshouldbeusedunderratedconditions;