Photodiode

A photodiode runs an LED in reverse: instead of turning current into light, it turns light into current. When a photon with enough energy strikes the semiconductor junction, it knocks an electron loose, creating an electron–hole pair; the diode's built-in field sweeps them apart, and that flow of freed charges is a tiny current proportional to how brightly the light shines. Shine more light, get more current — the device is an honest, linear light meter built from a single diode.

It is usually operated in reverse bias, which widens the depletion region (the 'collecting zone') and speeds the response, so photodiodes can follow light flickering billions of times a second — exactly what a fibre-optic receiver needs to decode laser pulses. Variants trade speed for sensitivity: a PIN diode for fast clean detection, an avalanche photodiode (APD) that multiplies each freed electron into an avalanche for faint-light work, and the humble solar cell, which is just a large photodiode optimised to harvest power rather than read signals.