A photodiode is a p-n junction diode designed to detect light.

• It is operated in reverse bias.
• When light with energy $h\nu >E_g$ falls on the depletion region, it creates electron-hole pairs.
• The electric field sweeps these carriers across the junction, creating a current called photocurrent.
• More incident photons (higher intensity) create more carriers per second, making the current directly proportional to the light intensity. This allows it to act as an accurate light sensor.