Electro Optical Components, Inc. 5460 Skylane Boulevard, Santa Rosa, CA 95403 Toll Free: 855-EOC-6300 www.eoc-inc.com | [email protected] 5.1. Infrared (IR) Photodetectors Glossary noise, improve high frequency performance and increase the 5.1. Infrared (IR) Photodetectors dynamic range. Unfortunately, at the expense of flicker noise (1/f) in most cases. Infrared photodetectors are semiconductor electro-optical devices that convert infrared radiation into an electrical signal. Photovoltaic detectors are more vulnerable to electrostatic dis- charges than photoconductors. Hg1-xCdxTe Photoelectromagnetic detectors (PEM) Known also as Mercury Cadmium Telluride (MCT), HgCd- Photovoltaic detectors are based on the photoelectromagnetic Te, (Cd,Hg)Te or MerCadTel. It is a variable ba nd gap alloy, effect based on spatial separation of optically generated elec- commonly used for fabrication of photodetectors with tunable trons and holes in the magnetic field. The devices do not re- spectral response. quire electrical bias and show no flicker noise (1/f). The PEM 5 devices are typically used as fast, uncooled detectors of the long wavelength radiation. InAs1-xSbx InAs Sb also known as Indium Arsenide Antimonide and In- 1-x x Detector formats AsSb is another variable band gap alloy used for fabrication of Detector formats photodetectors with tunable spectral response. Square or rectangular formats are typically used for any IR detectors. Circular geometry is soSquaremetim ore rectangulars used fo formatsr pho tareod itypicallyodes. used for any IR de- Physical detector area A Detector formats tectors. Circular geometry is sometimes used for photodiodes. Square or rectangEuqlaur ivfoarmleantst samre atyllp siciagllny auls dede tfeocr taonry cIRir cduetiet ctors. Circular geometry is Active area of a detector where the incidentsom radiationetimes uiss eabd- fEorlEquivalent epchtortiocd ipordoeps small.e rties signal of so detectorme phot ocircuitdetec tors can be described by the small signal detector sorbed and sensed. ‧ Equivalent smallc sirigcunaitl dwehteicchto rc coirncsuisitt s of photocurrent source Iph =Ri P, detector dynamic resistance Rsh , Electric properties of some photodetectors can be described Electric propertiesc abyopf aasc osmallimtaen cpsignaleh oCto detector,d eatencdto rscircuitse rciaens which bre sdi seconsiststsacnribce d of R bphotocursy . tRhes iss-m tahlle s ipganaral sdietitce crteors istance of the devices that Optical detector area Ao ‧ circuit which conrseisdtus coef aphvoatiolacburlere npt hsootuorccuer rIephn =tR. i UPn, fdoerteucntaort edlyy,n athmeic sriemsipsltea nmceo dReshl , cannot accurately describe rent source Iph=RiP, detector dynamic resistance Rsh, capaci- capacitance C , andtance ser iC,es andres seriesistanc resistancee R . R Ris. Rth eis pa aparasiticrasitic rresistanceesistanc eof of the devices that The apparent optical area of the detector which is “seen”. It is properties of long wasvelsens gtsh photodiodes operating at near room temperatures. reduce available phtheoto devicescurren tthat. U nreducefortun availableately, th photocurrent.e simple mo Unfortunately,del cannot a ccurately describe equal to physical area of the detector active element unless an optical concentrator is used. The optical detectorproper tareaies ocanf lo beng wtheave simplelength model phot ocannotdiode accuratelys operati ndescribeg at ne apropertiesr room te ofm longper atures. significantly magnified in detectors supplied with optical con- wavelength photodiodes operating at near room temperatures. centrators, for i.e., immersion lenses (see Optical immersion chapter). Photoconductors (PC) Photoconductive detectors based on the photoconductive effect. Infrared radiation generates charge carriers in the semiconductor active region decreasing its resistance. The resistance change is sensed as a current change by applying a constant voltage bias. The devices are characterized by near linear current-voltage characteristics. The electric field in pho- Figure 1. Equivalent small signal circuit of toconductors is constant across the device. It equals ratio of an IR photodetector bias voltage and distance between contacts Figure 1. EquivaleFnti gsmuraell 1si.g nEaql ucivrcauliet notf small signal circuit of an IR photodetector E= V/L an IR photodetector Photocurrent The optimum bias voltage is specified in the final test report and depends on detector size, operating temperature and spectral Photocurrent is a current generated by IR radiation, which is response. Photocurrent not in thermal equilibrium with detector. For small irradiation, Photocurrent is thPe hthecou rtphotocurrentroecntu grerennerta tise proportionald by IR rad itoat iincidenton, wh iradiationch is no tpower in the Prmal equilibrium with detector. For smaPll ihrroatdoiactuiornre, nthte i sp htohteoc cuurrrernetn ist pgreonpeortaiotneadl tboy i nIcRid reandt iraatdiioantio, nw phoicwhe ri sP not in thermal equilibrium with Photovoltaic detectors (PV, PVM) I = R ·P detector. For small irraphd iatiio n, the photocurrent is proportional to incident radiation power P where R is the current responsivity. Photovoltaic detectors (photodiodes) are semiconductori I = R ·P where R is the current responsivity. ph i structures with one (PV) or multiple (PVM),C homo-urren tor re heterospon-siwvihtye rRei Ri is the current responsivity. junctions. Absorbed photons produce charge carriers that are i Current responsivity (A/W) is the ratio of photocurrent and power of radiation. The current collected at the contacts, resulting in external photocurrent. CuCurrentrrent r eresponsivitysponsivity R Photodiodes have complex current voltageres pcharacteristics.onsivity is t ypically measured for moni ochromatic radiation (the spectral current responsivity) and blackbody radiation (the blackbody current responsivity). The responsivity The devices can operate either at flicker-free zero bias or with CuCurrentrrent responsivityresponsi v(A/W)ity ( Ais /aW ratio) i sof tphotocurrenthe ratio o andf p hpowoto- current and power of radiation. The current typically remains constant for weak radiation and tends to decrease with more strong reverse voltage. Reverse bias voltage is frequently applied to reersp ofo nradiation.sivity Theis tcurrentypica lresponsivityly meas uisr etypicallyd for measuredmonoc hromatic radiation (the spectral current increase responsivity, differential resistance,ra dreduceiation . the shot reforsp monochromaticonsivity) and radiation blackb (theod yspectral radia currenttion ( tresponsivihe blac-kbody current responsivity). The responsivity Current responsitvyiptyic-laelnlyg thre pmroadinusc t cRoi·Lns tant for weak radiation and tends to decrease with more strong The current responsivity of unbiased PEM, PVM and biased (with constant electric field E) 52 radiation. www.vigo.com.pl PC detectors is proportional to the reciprocal length. Therefore, the current responsivity Ri ·L is used to comparCe uderrveicnets roef svaproionus ifvoirtmya-ltse.n gth product Ri·L Another normalizTehd e ccuurrrernetn tr ersepsopnosnivsitiyv, ityR i ·oLf/ Eu, nbisi ausseedd PtEo Mc,o mPpVaMre arnedsp obniassiveitdy (owf ith constant electric field E) photoconductive devices of various format, and operating with different electric fields. PC detectors is proportional to the reciprocal length. Therefore, the current responsivity Ri ·L Maximum bias voislt augse dV tmoa xc ompare devices of various formats. The maximum voltage that can be applied to a photoconductor or p hotovoltaic detector Another normalized current responsivity, Ri ·L/E, is used to compare responsivity of without a risk of itsp hdaomtoacgoen. ductive devices of various format, and operating with different electric fields. Dark current I dark V The current that flMowasx iinm au pmho tboidaeste vctoolrt iang thee rmmaal xe quilibrium with its surrounding. The maximum voltage that can be applied to a photoconductor or photovoltaic detector without a risk of its damage. Dark current Idark The current that flows in a photodetector in thermal equilibrium with its surrounding. Detector formats DSeqtueacrteo ro fro rrmecattasn gular formats are typically used for any IR detectors. Circular geometry is S sqDouematreec ttiomorr e rfesoc rutmasneagdtus fl aorr fpohrmotaotds ioadree sty. pically used for any IR detectors. Circular geometry is sSoqmueatriem eosr urescetda nfogru plahro tfoodrmioadtess .a re typically used for any IR detectors. Circular geometry is Equivalent small signal detector circuit sometimes used for photodiodes. EEquleivcatrliecn pt rsompaelrlt iseisg noafl dseotmecet oprh coirtocudiet tectors can be described by the small signal detector Electric properties of some photodetectors can be described by the small signal detector Ecqiruciuviat lewnht iscmh acllo snisginsatsl doeft epchtorto cciurcrrueint t source I =R ‧P , detector dynamic resistance R , ‧ph i sh circuit which consists of photocurrent source Iph =Ri P, detector dynamic resistance Rsh , Electric proper tie s of some photodetector s ca n be described by the small signal detector capacitance C, and series resistan c e R s . Rs is the parasitic resistance of the devices that capacitance C, and series resistance Rs . Rs is the parasitic resistance of the devices that circuit which consists of photocurrent source I =R ‧P , detector dynamic resistance R , rerdeudcuec ea vaavilabilaleb pleh opthocoutorrceunrtr. eUnnt.f oUrtnufnoartteulny,a theely ,ps hitmhepil es imopdlee l mcaondneol t caacncnuorat tealcy cduersacterilbye dshescribe