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Photodetectors Lecturer: Mauro Mosca (www.dieet.unipa.it/tfl) University of Palermo –DEIM A.A. 2014-15.

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Presentation on theme: "Photodetectors Lecturer: Mauro Mosca (www.dieet.unipa.it/tfl) University of Palermo –DEIM A.A. 2014-15."— Presentation transcript:

1 Photodetectors Lecturer: Mauro Mosca (www.dieet.unipa.it/tfl) University of Palermo –DEIM A.A

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

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

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

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

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

7 Negative Electron Affinity (NEA)

8 Photomultipliers Dynodes (~ 100 V)

9 Photoconductive detectors

10 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)

11 Photoconductive detectors : application circuits

12 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

13 Photoconductive detectors: gain I0I0 x = =

14 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

15 Photoconductive detectors: gain

16 Se consideriamo che:

17 Photoconductive detectors: response rgrg

18 Fotoresistors (LDR) Large surface Close electrodes

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

20 Multiple-quantum well (MQW) detectors

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

22 Silicon photodiode oppure…

23

24 Silicon photodiode: responsivity

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

26 Response time of photodiodes transit time accross the depletion region junction capacitance effects  is minimized…

27 Response time of photodiodes carrier diffusion

28 Noise in photodiodes

29 Schottky photodiodes fotoeccitazione elettroni metallo  più lunghe

30 Metal-semiconductor-metal (MSM) photodetector capacità più piccole dispositivi più veloci

31 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

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

33 Charge-Coupled Devices (CCD)

34

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36 CCD: read-out mechanisms

37


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