diode (dĪˈōd) [key], two-terminal electronic device that permits current flow predominantly in only one direction. Most diodes are semiconductor devices; diode electron tubes are now used only for a few specialized applications. A diode has a low resistance to electric current in one direction and a high resistance to it in the reverse direction. This property makes a diode useful as a rectifier, which can convert alternating current (AC) into direct current (DC). An arrangement of four diodes, called a diode bridge, transforms AC into DC using both phases of the alternating current. When the voltage applied in the reverse direction exceeds a certain value, a semiconductor diode "breaks down" and conducts heavily in the direction of normally high resistance. When the reverse voltage at which breakdown occurs remains nearly constant for a wide range of currents, the phenomenon is called avalanching. A diode using this property, called a Zener diode, can be used to regulate the voltage in a circuit.
Semiconductor diodes can be designed to have a variety of characteristics. A thermistor is a special semiconductor diode whose conductivity increases with the diode temperature. A varactor, or varicap, exhibits a capacitance that is dependent upon the voltage across it. In an Esaki, or tunnel, diode, the current through the device decreases as the voltage is increased within a certain range; this property, known as negative resistance, makes it useful as an amplifier (see tunneling). Gunn diodes are negative-resistance diodes that are the basis of some microwave oscillators. Light-sensitive, or photosensitive, diodes can be used to measure illumination; the voltage drop across them depends on the amount of light that strikes them. Photodiodes, which respond to being struck by packets of light, or photons, can be used as solar cells. Schottky diodes are used in low voltage circuits and batteries. Snap diodes provide very fast voltage transitions. A light-emitting diode (LED) produces light as current passes through it; a specialized LED, called a laser diode, emits laser light, useful for telecommunications through optical fibers. LEDs are used in computer monitors and television screens, in flashlights, and in lighting. Organic light-emitting diodes (OLEDs) are made with plastics rather than silicon and other traditional semiconductor materials; color OLEDs are thinner, lighter, brighter, and use less power than color LEDs and are used in small portable devices such as smartphones, digital cameras, and PDAs and in small screen televisions.