A light sensor (photodetector) that allows current to flow in one direction from one side to the other when it absorbs photons (light). The more light, the more current. Used to detect light in camera sensors, optical fibers and other light-sensitive applications, a photodiode is the opposite of a light emitting diode (see
LED). Photodiodes detect light and let electricity flow; LEDs receive electricity and emit light.
Solar Cells Are Photodiodes
Solar cells are photodiodes that are chemically treated (doped) differently than the photodiode used as a switch or relay. When solar cells are struck by light, their silicon material is excited to a state where a small electrical current is generated. Many arrays of solar cell photodiodes are required to power a house. See
photocell and
phototransistor.