^Photodiodes

Light incident near the junction of reverse biased (because the fractional change due to the photo-effects on the minority carrier dominated reverse bias current is more easily measurable than the fractional change in the forward bias current) diode produce electron hole pairs which are moved by the junction electric field (electrons towards n end & holes near the p end).

Thus p-side becomes positive and n-side becomes negative giving rise to emf. When an external load is connected, a current flows which can be controlled by changing the intensity of light falling on it. It is used in, light operated switches, optical counters, reproduction of sound from film in move projectors & in photo detectors to detect radiations.