An electronic device with two (di) electrodes, originally in the form of a needle on a natural quartz crystal, and later an electron tube with an anode (positive terminal) and a cathode (negative terminal), but now usually in the form of a transistor with a p-n (positive-negative) junction between a positive and a negative layer of semiconducting materials. The negative layer is doped with impurities to create extra electrons, which are negatively charged.The positive layer is doped to create extra holes into which electrons can migrate when a charge is applied, which has the effect of adding extra positive particles. When current is applied and the electrons move across the junction, from the n semiconductor layer to the p semiconductor layer, and settle into the holes, they release energy in the form of photons, i.e., light. (Note:A diode conducts current in only one direction, like a one-way gate.) The composition of the semiconductor material determines the color of light, how much of it is absorbed, and how much of it is released. In electrical applications, a diode is primarily used as a rectifier, to convert alternating current (AC) to direct current (DC). In free space optics (FSO) and fiber optic transmission systems (FOTS), light sources variously use light-emitting diodes (LEDs) and diode lasers to convert electrical signals to optical signals. FSO and FOTS optical receivers variously use positive-intrinsic-negative (PIN) diodes and avalanche photodiodes (APDs) to convert light to electrical current. See also AC, APD, DC, diode laser, FOTS, FSO, LED, PIN, and rectifier.