tunnel diode: see diode.

light-emitting diode: see diode.

diode, 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. 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 finding use in small portable devices such as telephones, digital cameras, and PDAs and in small screen televisions.

Zener diode: see diode.

Semiconductor diode that produces visible or infrared light when subjected to an electric current, as a result of electroluminescence. Visible-light LEDs are used in many electronic devices as indicator lamps (e.g., an on/off indicator) and, when arranged in a matrix, to spell out letters or numbers on alphanumeric displays. Infrared LEDs are used in optoelectronics (e.g., in auto-focus cameras and television remote controls) and as light sources in some long-range fibre-optic communications systems. LEDs are formed by the so-called III-V compound semiconductors related to gallium arsenide. They consume little power and are long-lasting and inexpensive.

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Electronic device that has two electrodes (anode and cathode) and that allows current to flow in only one direction, resisting current flow in the other. An applied voltage can cause electrons to flow in only one direction, from the cathode to the anode, and then back to the cathode through an external circuit. Diodes are used especially as rectifiers—which change alternating current into direct current—and to vary the amplitude of a signal in proportion to the voltage in a circuit, as in a radio or television receiver. The most familiar diodes are vacuum tubes and semiconductor diodes. Semiconductor diodes, the simplest of semiconductor devices, consist of two electrodes and a sandwich of two dissimilar semiconducting substances (a p-n junction). Such diodes form the basis for more complex semiconductor devices (including transistors) used in computers and other electronic equipment. Semiconductor diodes include light-emitting diodes and laser diodes; the latter emit laser light, useful for telecommunications via fibre optics and for reading compact discs.

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