Photodetectors

1. Photodetectors Lecturer: Mauro Mosca (www.dieet.unipa.it/tfl) last release: 02/10/2016 University of Palermo –DEIM

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

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

4. Sensing element Bolometers aPt~ aNi = = 0.005 K-1 current must not raise temperature too much Why?... current flowing SMALL

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

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

7. Negative Electron Affinity (NEA)

8. Photomultipliers +++ + ++ ++ ++ ++ ++ ++ ++ ++ ++ Dynodes (~ 100 V)

9. Photoconductive detectors

10. RPC VBB Photoconductive detectors: application circuits se si è interessati solo alle variazioni di intensità radiante segnale d’uscita piccolo! (RLpiccola) scarsa sensibilità! (RPCpiccola)

11. Photoconductive detectors : application circuits

12. 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

13. x Photoconductive detectors: gain I0 = =

14. x Photoconductive detectors: gain I0 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. Photoconductive detectors: gain Se consideriamo che:

17. rg Photoconductive detectors: response poor response time high tc high G traps or sensitization centres

18. Fotoresistors (LDR – Light Depending Resistor) Superficie ampia cattura un numero elevato di fotoni alta sensibilità Elettrodi vicini tempo di transito breve si evita la ricombinazione Materiali CdS, CdSe, InGaAs, InSb

19. c ttr high low f Photoconductive detectors:pros and cons response sensitivity

20. Multiple-quantum well (MQW) detectors

21. il I0 p-n junction detector (photodiode) • photovoltaic mode • photoconductive mode

22. Silicon photodiode oppure…

23. Silicon photodiode

24. Silicon photodiode: responsivity

25. Photodiode materials (near IR) • GelG= 1.88 mm lG= 1.68 mm • InxGa1-xAs (x = 0.53) lattice matched to InP with narrow bandgap materials: wider bandgap Why not homojunctions? - low breakdown voltages - large reverse leakage current

26. Response time of photodiodes • transit time accross the depletion region • junction capacitance effects tis minimized…

27. Response time of photodiodes • carrier diffusion

28. Noise in photodiodes

29. Schottky photodiodes migliore risposta a l più corte lpiù lunghe fotoeccitazione elettroni metallo

30. Metal-semiconductor-metal (MSM) photodetector necessaria polarizzazione 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 - internal amplification - operation under very high reverse bias reduced breakdown no current leakage fron the edge

32. IB IE = b IB Phototransistor VCB < 0 Maggiore sensibilità (mA) ma… Minore velocità (ms contro i ns dei fotodiodi)

33. Charge-Coupled Devices(CCD)

34. Charge-Coupled Devices(CCD)

35. Charge-Coupled Devices(CCD)

36. CCD: read-out mechanisms

37. CCD: read-out mechanisms