Survey
* Your assessment is very important for improving the workof artificial intelligence, which forms the content of this project
* Your assessment is very important for improving the workof artificial intelligence, which forms the content of this project
Detector zoology AlN 6.0 III-Nitrides (c ~ 1.6 a0) Zincblend GaN ZnS GaN 3.0 2.0 0.4 InN Theory InN GaP ZnSe CdS ZnTe AlP AlAs CdSe AlSb InP CdTe Ge Al2O3 3C-SiC ZnO 1.0 6H-SiC GaAs Si GaSb InAs InSb 3.0 3.5 4.0 4.5 Lattice Constant (Å) 5.0 5.5 6.0 0.5 0.6 0.7 1.0 2.0 5.0 6.5 Wavelength (㎛) 0.3 4.0 Al2O3 Bandgap (eV) E(eV)=1.24/ λ(㎛) AlN Theory 5.0 0.0 2.5 0.2 Direct gap Indirect gap Photon detection devices Photons to thermal energy (phototube) Metal-Semicon. photoconductor (Schottky-barrier photodiode) The External Photoeffect: Photoelectron Emission Photogenerated electrons escape from the material as free electrons. photoelectrons metal < Phototube > semiconductor < Photomultiplier tube (PM tube) > 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. Here we will discuss three types of semiconductor photodetectors Photoconductors Photodiodes (PD) Avalanche photodiodes (APD) Quantum efficiency Responsivity Response time. Photon noise Photoelectron noise Gain noise Quantum efficiency of photodetectors Internal Quantum Efficiency int Number of Collected electrons 1 e d Number of Photons *Entering* detector External Quantum Efficiency ext i /q Number of Collected electrons 1 RF 1 e d ph Number of Photons *Incident* on detector Po / h Fresnel loss Fraction absorbed in detection region Surface recombination effect Responsivity and Response time Responsivity R i Photo Current (Amps) q ph ext Incident Optical Power (Watts) Po h Photocurrent : i ph RPo 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…) Photoconductors Photoconductors Photodiodes Photodiodes n P + - ip Two operation modes of PN photodiodes Short-circuit (photoconductive) operation of PDs Open-circuit (photovotaic) operation of PDs 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 Reverse-biased PDs p-i-n Photodiodes (PIN PDs) Heterojunction Photodiodes 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. •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 Avalanche Photodiodes (APD) 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) High resistivity p region larger charge density APD with only one type of carrier (e or h) is desirable. : ionization coefficients of e and h Ionization ratio : e h e ….. The ideal case of single-carrier multiplication is achieved when APD gain