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(dī`ōd), two-terminal electronic device that permits current flow predominantly in only one direction. Most diodes are semiconductor devices; diode electron tubeselectron tube,
device consisting of a sealed enclosure in which electrons flow between electrodes separated either by a vacuum (in a vacuum tube) or by an ionized gas at low pressure (in a gas tube).
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 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 capacitancecapacitance,
in electricity, capability of a body, system, circuit, or device for storing electric charge. Capacitance is expressed as the ratio of stored charge in coulombs to the impressed potential difference in volts.
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 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 tunnelingtunneling,
quantum-mechanical effect by which a particle can penetrate a barrier into a region of space that would be forbidden by ordinary classical mechanics. Tunneling is a direct result of the wavelike properties of particles; the wave associated with a particle "decays"
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). Gunn diodes are negative-resistance diodes that are the basis of some microwave oscillatorsoscillator, electronic
, electronic circuit that produces an output signal of a specific frequency. An oscillator generally consists of an amplifier having part of its output returned to the input by means of a feedback loop; the necessary and sufficient condition for
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. 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. The first visible-light (red) LEDs were developed in the 1960s; these were initially used in indicator lights and alphanumeric displays. The development of green and, later and more importantly, blue LEDs made possible their use to produce white light for ordinary, energy-efficient lighting. In 2014 Isamu Akasaki, Hiroshi Amano, and Shuji Nakamura were awarded the Nobel Prize in Physics for their development of practical blue LEDs in the 1990s. LEDs are now used in computer monitors and television screens (where they provide the backlight for liquid crystalliquid crystal,
liquid whose component particles, atoms or molecules, tend to arrange themselves with a degree of order far exceeding that found in ordinary liquids and approaching that of solid crystals.
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 displays), 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. They are used in small portable devices such as smartphones and digital cameras and increasingly in television and computer screens.



a two-electrode vacuum-tube, ion (gas-discharge), or semiconductor device with unidirectional conduction of electric current. Vacuum-tube and gas-discharge diodes have a cathode (an electron emitter), which is heated directly or indirectly, and an anode (an electron receiver). In a vacuum-tube diode, if there is a positive potential at the anode, an electron current passes between the electrodes; in a gas-filled tube, which is filled with an inert gas, hydrogen, or mercury vapor at low pressure, both electron and ion currents will flow. When there is a negative potential at the anode, no current will flow through such diodes. Unidirectional conductivity exists in semiconductor diodes because a p-n junction is created either within the semiconductor or in the interface between a metal and the semiconductor.

Diodes are used in radio engineering, electronics, and power engineering, mainly for the rectification of alternating current, for detection, for frequency conversion and multiplication, and in the switching of electrical circuits.



A two-electrode electron tube containing an anode and a cathode.


A two-terminal electron device exhibiting a nonlinear current-voltage characteristic. Although diodes are usually classified with respect to the physical phenomena that give rise to their useful properties, in this article they are more conveniently classified according to the functions of the circuits in which they are used. This classification includes rectifier diodes, negative-resistance diodes, constant-voltage diodes, light-sensitive diodes, light-emitting diodes, and capacitor diodes.

A circuit element is said to rectify if voltage increments of equal magnitude but opposite sign applied to the element produce unequal current increments. An ideal rectifier diode is one that conducts fully in one direction (forward) and not at all in the opposite direction (reverse). This property is approximated in junction and thermionic diodes. Processes that make use of rectifier diodes include power rectification, detection, modulation, and switching. See Rectifier

Negative-resistance diodes, which include tunnel and Gunn diodes, are used as the basis of pulse generators, bistable counting and storage circuits, and oscillators. See Negative-resistance circuits, Oscillator, Tunnel diode

Breakdown-diode current increases very rapidly with voltage above the breakdown voltage; that is, the voltage is nearly independent of the current. In series with resistance to limit the current to a nondestructive value, breakdown diodes can therefore be used as a means of obtaining a nearly constant reference voltage or of maintaining a constant potential difference between two circuit points, such as the emitter and the base of a transistor. Breakdown diodes (or reverse-biased ordinary junction diodes) can be used between two circuit points in order to limit alternating-voltage amplitude or to clip voltage peaks.

Light-sensitive diodes, which include phototubes, photovoltaic cells, photodiodes, and photoconductive cells, are used in the measurement of illumination, in the control of lights or other electrical devices by incident light, and in the conversion of radiant energy into electrical energy. Light-emitting diodes (LEDs) are used in the display of letters, numbers, and other symbols in calculators, watches, clocks, and other electronic units. See Light-emitting diode, Photoconductive cell, Photoelectric devices

Semiconductor diodes designed to have strongly voltage-dependent shunt capacitance between the terminals are called varactors. The applications of varactors include the tuning and the frequency stabilization of radio-frequency oscillators. See Microwave solid-state devices, Varactor


1. a semiconductor device containing one p-n junction, used in circuits for converting alternating current to direct current
2. the earliest and simplest type of electronic valve having two electrodes, an anode and a cathode, between which a current can flow only in one direction. It was formerly widely used as a rectifier and detector but has now been replaced in most electrical circuits by the more efficient and reliable semiconductor diode


(hardware, electronics)
A semiconductor device which conducts electric current run in one direction only. This is the simplest kind of semiconductor device, it has two terminals and a single PN junction. One diode can be used as a half-wave rectifier or four as a full-wave rectifier.


(1) An electronic component that acts like a one-way valve. As a discrete component or built into a chip, it is used in a variety of functions. Used as a rectifier, it is a key element in changing AC to DC by limiting current flow to a single direction. Diodes are used as temperature and light sensors and light emitters (LEDs). In communications, they filter out analog and digital signals from carriers and modulate signals onto carriers. In digital logic, they're used as one-way valves and as switches similar to transistors. See laser diode and PN junction.

(2) A type of vacuum tube used in electronic circuits as a rectifier or radio frequency detector. Modern applications of tube diodes are generally limited to rectifiers in high-end audio amplifiers and other specialized high-voltage circuits.

The tube diode uses two active elements (cathode and plate) and one passive element (the filament or heater). In typical operation, the cathode is heated by the filament, and the AC voltage is applied to the cathode. The heated cathode releases excited electrons that flow to the plate (anode) and become the rectified current. The diode allows current flow in only one direction. For example, if current were applied to the plate, electron flow could not occur, because the plate's electrons are not heated by the filament.

In some instances, the filament is also the cathode. This is accomplished by connecting the AC voltage source to one of the filament's leads. See triode, tetrode, pentode, magnetron and klystron.

The Diode Is the Simplest
Widely used to convert AC to DC (a rectifier), the diode is the least complicated tube type with no grids between the cathode and anode.