Photo Detectors Parameters: Parameters: Responsivity (Efficiency): Output current/input optical powerResponsivity (Efficiency): Output current/input optical power Spectral Response RangeSpectral Response Range Response timeResponse time Noise characteristicsNoise characteristics Basic Process: Basic Process: Photoconductive Mode: Reverse biased diodePhotoconductive Mode: Reverse biased diode Photovoltaic Mode: no external biasingPhotovoltaic Mode: no external biasing Dark current Dark current High speeds High speeds Linear response Linear response Less dark current Less dark current More sensitive More sensitive Amplification required Amplification required

pn DIODE Operation: Operation: Reverse pn diodeReverse pn diode Photons absorbedPhotons absorbed Current flowsCurrent flows Problems: Problems: Depletion region is thin, photons passes throughDepletion region is thin, photons passes through Absorption in doped region is slowAbsorption in doped region is slow PIN Diodes PIN Diodes Make depletion region thicker by adding an intrinsic material Make depletion region thicker by adding an intrinsic material

PIN Enhancements: Enhancements: More absorptions > better responsivityMore absorptions > better responsivity Junction wider > capacitance lower > fasterJunction wider > capacitance lower > faster Junction wider > drift is favored > fasterJunction wider > drift is favored > faster Spectrum Spectrum PIN will absorb photon with energy higher than gapPIN will absorb photon with energy higher than gap Cutoff wavelengthCutoff wavelength Lower bandgap energy > higher dark currentLower bandgap energy > higher dark current Efficiency Efficiency Quantum efficiencyQuantum efficiency Responsivity: A/WResponsivity: A/W

Schottky-Barrier PD Features: Features: More materialsMore materials Better responsivityBetter responsivity Faster operationFaster operation

APD Operation Operation Field is extremely high at n + p Field is extremely high at n + p Absorption and multiplication regions are separated Absorption and multiplication regions are separated Multiplication region is small Multiplication region is small Avalanche is controlled by: Avalanche is controlled by: Preferential Probability of ionization Preferential Probability of ionization

APD APD vs PIN: APD vs PIN: Higher reverse biased voltage Higher reverse biased voltage i-Zone > lightly p- doped -layer i-Zone > lightly p- doped -layer Features Features High sensitivityHigh sensitivity Avalanche build up limits speedAvalanche build up limits speed Gain BW product ~ 150 GHzGain BW product ~ 150 GHz

Rise Time Depends mainly on: Depends mainly on: Transit time of photocarriers in depletion region, t d =w/v d ~ 0.1 nsTransit time of photocarriers in depletion region, t d =w/v d ~ 0.1 ns Diffusion time of phtocarriers generated outside depletion region,Diffusion time of phtocarriers generated outside depletion region, RC time constantRC time constant Parameters responsible: Parameters responsible: Absorption coefficientAbsorption coefficient Depletion region widthDepletion region width CapacitanceCapacitance ResistancesResistances

Rise time PD response time to a step input Rise time for partially depleted PD A compromise between high frequency response and high quantum efficiency, 2/ s >W> 1/s

COMPARISON