2. Bandgap (eV) 0.2 0.3 0.4 0.5 0.6 0.7 1.0 2.0 5.0 6.0 5.0 4.0 3.0 2.0 0.0 1.0 Lattice Constant (Å) Wavelength ( ㎛ ) GaN AlN InN 6H-SiC ZnO AlP GaP AlAs GaAs InP AlSb GaSb InSb E(eV)=1.24/ λ( ㎛ ) ZnS ZnSe CdS ZnTe CdSe CdTe Si Ge Direct gap Indirect gap InAs Al 2 O 3 III-Nitrides (c ~ 1.6 a 0 ) InN Theory AlN Theory Zincblend 3C-SiC Al 2 O 3 3.03.54.04.55.05.56.06.52.5 GaN Detector zoology

3. Photon detection devices (phototube) Photons to thermal energy Metal-Semicon. photoconductor (Schottky-barrier photodiode)

4. The External Photoeffect: Photoelectron Emission metal semiconductor  Photogenerated electrons escape from the material as free electrons.  photoelectrons

5. The Internal Photoeffect: Photoconductivity  Excited carriers remain within the material, serve to increase electrical conductivity. Generation: Absorbed photons generate free carriers (electrons and holes). Transport: An applied electric field induces these carriers to move, which results in a circuit current. Amplification: large electric fields enhance the responsivity of the detector. Photoconductors Photodiodes (PD) Avalanche photodiodes (APD) Here we will discuss three types of semiconductor photodetectors Quantum efficiency Responsivity Response time. Photon noise Photoelectron noise Gain noise

6. Quantum efficiency of photodetectors Internal Quantum Efficiency External Quantum Efficiency Fresnel loss Surface recombination effect Fraction absorbed in detection region

7. Responsivity and Response time Responsivity Transit time Holes and electrons move at different speed inside the material so that the transit time spreads so that, even if delayed, the response is not a delta in time. Then there is the intrinsic time of the junction related to the intrinsic capacitance (the junction is always equivalent to an RC circuit…)

8. Photoconductors

10. Photodiodes

11. P n + - i p

12. Two operation modes of PN photodiodes Open-circuit (photovotaic) operation of PDs Short-circuit (photoconductive) operation of PDs

13. Open-circuit (photovotaic) operation of PDs Photovoltage Vp across the device that increases with increasing photon flux. This mode of operation is used, for example, in solar cells Short-circuit operation of PDs

14. Reverse-biased PDs

15. p-i-n Photodiodes (PIN PDs)

16. Heterojunction Photodiodes

17. Simple to fabricate Quantum efficiency: Medium Problem: Shadowing of absorption region by contacts Capacitance: Low Bandwidth: High Can be increased by thinning absorption layer and backing with a non absorbing material. Electrodes must be moved closer to reduce transit time. To increase speed, decrease electrode spacing and absorption depth Absorption layer Non absorbing substrate Schottky-barrier Photodiodes (Metal-semiconductor PDs) Schottky-barrier Photodiodes (Metal-semiconductor PDs) A thin semitransparent metallic film is used in place of the p-type (or n-type) layer in the p-n junction photodiode.

18. Avalanche Photodiodes (APD)

19. APD with only one type of carrier (e or h) is desirable. High resistivity p-doped layer increases electric field across absorbing region High-energy electron-hole pairs ionize other sites to multiply the current Leads to greater sensitivity light absorption intrinsic region (very lightly doped p region) larger charge density High resistivity p region

20. APD with only one type of carrier (e or h) is desirable. e  h  e  ….. The ideal case of single-carrier multiplication is achieved when Ionization ratio : : ionization coefficients of e and h

21. APD gain