Photodiodes

A photodiode is a p-n junction diode that converts an optical signal INTO an electrical signal -- the operating principle is essentially the inverse of the LED's, and (unlike the LED) a photodiode is operated in REVERSE bias. Structurally it is a p-n junction of photosensitive material housed inside a plastic case with a small transparent window that lets incident light reach the junction; its circuit symbol shows arrows pointing TOWARDS the diode (light going in), the mirror image of the LED's symbol. When a photon of sufficient energy $h\nu$ strikes the depletion region, it elevates a valence-band electron into the conduction band, simultaneously creating a hole in the valence band -- exactly the electron-hole pair generation process of an intrinsic semiconductor, but now driven by absorbed light rather than heat. The reverse bias's electric field then sweeps these newly created carriers across the junction (holes towards the n-side, electrons towards the p-side) before they can recombine, and once an external circuit is completed, this constitutes a measurable photocurrent, proportional to the intensity of incident light -- which is why photodiodes are used as light sensors. Even with zero incident light, the ordinary thermally generated reverse saturation curren …