Photodetectors Lecturer: Mauro Mosca (www.dieet.unipa.it/tfl) A.A. 2015-16 University of Palermo –DEIM Photodetector Thermal Thermoelectric Bolometers Pyroelectric Photonic Photomultipliers Photoconductors Photovoltaics Thermoelectric detectors - Principle of thermocouples large small electrical thermal conductivities minimize Joule heat heating effects conduction losses thermopile Bolometers aPt ~ aNi = = 0.005 K-1 Sensing element current flowing SMALL current must not raise temperature Why?... too much Pyroelectric detectors ferromagnetic material - lead zirconate - lithium tantalate molecules with a permanent electrical dipole Dispositivi emissivi: catodi NaKCsSb # emitted electron # absorbed photons (S20) = quantum yield lowest value for ef : Caesium (2.1 eV) Negative Electron Affinity (NEA) Photomultipliers + +++ ++ Dynodes (~ 100 V) ++ ++ ++ ++ ++ ++ ++ ++ Photoconductive detectors Photoconductive detectors: application circuits VO VBB RL RL RPC Plum VO VVBB BB se si è interessati solo alle variazioni di intensità radiante RPC RL k RL Plum RPC Se RL VBB RL VBB Plum k k RL Plum lum segnale d’uscita piccolo! (RL piccola) scarsa sensibilità! (RPC piccola) Photoconductive detectors : application circuits Photoconductive detectors : application circuits The most common method used to extract the signal is ????????????????????????????? to modulate the incident radiation at a specific frequency either by placing a mechanical chopper in front of the sensor or by electrically modulating the radiation source The signal due to radiation is now an AC signal while the dark current is a DC signal. The AC signal can be separated from the DC background signal using an AC amplifier Photoconductive detectors: gain I0 x = = Photoconductive detectors: gain I0 x photoconductive gain G = ratio of the rate of flow of electrons per second to the rate of generation of e--h+ pairs within the device Photoconductive detectors: gain Photoconductive detectors: gain Se consideriamo che: V v ( e h ) E ( e h ) L L t tr v G c t tr Photoconductive detectors: response rg c G ttr eP c I 0 i G e rg WDL G e WDL hD h t tr high c traps or sensitization centres poor response time Fotoresistors (LDR) Large surface Close electrodes Photoconductive detectors: pros and cons eP c i h ttr response c ttr sensitivity high low f Multiple-quantum well (MQW) detectors p-n junction detector (photodiode) - photovoltaic mode - photoconductive mode il I0 Silicon photodiode oppure… Silicon photodiode Silicon photodiode: responsivity i eP c h ttr i e eG G l P h hc Photodiode materials (near IR) • Ge lG = 1.88 m • InxGa1-xAs (x = 0.53) lG = 1.68 m lattice matched to InP wider bandgap with narrow bandgap materials: Why not homojunctions? - low breakdown voltages - large reverse leakage current Response time of photodiodes • transit time accross the depletion region • junction capacitance effects is minimized… Response time of photodiodes • carrier diffusion Noise in photodiodes Schottky photodiodes migliore risposta a l metallo più corte fotoeccitazione elettroni l più lunghe Metal-semiconductor-metal (MSM) photodetector capacità più piccole dispositivi più veloci Avalanche photodetectors (APD) The guard ring structure is a low doping region where depletion region extends an appreciable distance into it In the vicinity of guard ring the total depletion layer is greater (hence the maximum electric field is lower) than in the central region reduced breakdown no current leakage fron the edge Phototransistor VCB < 0 Maggiore sensibilità (mA) ma… IB IE = b IB Minore velocità (s contro i ns dei fotodiodi) Charge-Coupled Devices (CCD) Charge-Coupled Devices (CCD) Charge-Coupled Devices (CCD) CCD: read-out mechanisms CCD: read-out mechanisms
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