Photodiodes

Photodiodes converts light energy into electrical energy. We usually use a reverse biased p-n junction in photodiodes. Here current sensitivity to radiation is larger compared to forward biased p-n junction. Here photo induced current is a function of irradiance. Positive-Intrinsic-Negative (PIN) photodiode provides higher frequency response. Here a layer of intrinsic Silicon is added between the heavily doped p and n type Silicon materials. This reduces the diffusion time of photo induced electron-hole pairs.  PIN photodiode is also called as a constant current generator. Operation of a photodiode in its avalanche breakdown region increases current sensitivity by up to 100 times. A photodiode which operates in ita avalanche breakdown region is called as Avalanche photodiode (APD). The electron-hole pairs that are generated by incident photons are accelerated by electric field to kick new electrons from the valence to the conduction band. Excellent signal-to-noise ratio can be achieved by tuning operating frequency near 2GHz. APD is very expensive. N-p-n photo duodiodes are inexpensive but with higher current capabilities.