Photodetectors Lecturer: Mauro Mosca ( University of Palermo –DEIM A.A

Photodetector Thermal Thermoelectric Thermoelectric Bolometers Bolometers Pyroelectric Pyroelectric Photonic Photonic Photomultipliers Photomultipliers Photoconductors Photoconductors Photovoltaics Photovoltaics

Thermoelectric detectors - Principle of thermocouples large electrical conductivities thermopile minimize Joule heating effects small thermal conductivities minimize heat conduction losses

Bolometers current flowing SMALL Why?... current must not raise temperature too much  Pt ~  Ni = = K -1 Sensing element

Pyroelectric detectors ferromagnetic material molecules with a permanent electrical dipole - lead zirconate - lithium tantalate

Dispositivi emissivi: catodi # emitted electron # absorbed photons = quantum yield lowest value for e  : Caesium (2.1 eV) NaKCsSb (S20)

Negative Electron Affinity (NEA)

Photomultipliers Dynodes (~ 100 V)

Photoconductive detectors

V BB R PC Photoconductive detectors: application circuits se si è interessati solo alle variazioni di intensità radiante segnale d’uscita piccolo! (R L piccola) scarsa sensibilità! (R PC piccola)

Photoconductive detectors : application circuits

The most common method used to extract the signal is to modulate the incident radiation at a specific frequency by placing a mechanical chopper in front of the sensor or by electrically modulating the radiation source ????????????????????????????? either 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 I0I0 x = =

I0I0 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

Se consideriamo che:

Photoconductive detectors: response rgrg

Fotoresistors (LDR) Large surface Close electrodes

response sensitivity Photoconductive detectors: pros and cons c t tr high low f

Multiple-quantum well (MQW) detectors

p-n junction detector (photodiode) - photovoltaic mode - photoconductive mode i II

Silicon photodiode oppure…

Silicon photodiode: responsivity

Photodiode materials Ge G = 1.88  m In x Ga 1-x As (x = 0.53) G = 1.68  m lattice matched to InP

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 fotoeccitazione elettroni metallo  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 V CB < 0 IBIB I E =  I B Maggiore sensibilità (mA) ma… Minore velocità (s contro i ns dei fotodiodi)

Charge-Coupled Devices (CCD)

CCD: read-out mechanisms