Photodiode

A silicon photodiode is a semiconductor device that converts incident light into an electric current. Photodiodes are P-N Junction diode in which a small current is produced when photons are absorbed into the depletion region. When photons with sufficient energy (greater than 1.1eV) strike the diode, it creates an electron-hole pairs known as inner photoelectric effect. As a result, these carriers are swept away from the junction by the built-in electric field of the depletion region. The holes move toward the anode and the electrons move toward the cathode and produce photocurrent. Even at the absence of light, the movement of electrons and holes produce small current, known as dark current. So the total current produced by a photodiode is the sum of dark current plus photocurrent. 

A silicon photodiode can be operated in photovoltaic or photoconductive mode. In the photovoltaic mode, the photodiode is unbiased and the dark current is at its minimum. In photoconductive mode, an external reverse bias is applied and the photodiode exhibit its fastest switching speeds.