Diode

A diode is an electronic device that has two electrodes (conductors of electrical currents) arranged in such a way that electrons (subatomic particle having a negative charge) can flow in only one direction. Because of this ability to control the flow of electrons, a diode is commonly used as a rectifier—a device that converts alternating current into direct current. (Alternating current is an electric current that flows first in one direction and then in the other. But alternating current fed into a diode can move in one direction only, thereby converting the current to a one-way flow known as a direct current.)

Types of diodes

In general, two types of diodes exist. Older diodes were vacuum tubes containing two metal components, while newer diodes are solid-state devices consisting of one n-type and one p-type semiconductor. (Solid-state devices are electronic devices that take advantage of the special conducting properties of solids. Semiconductors are substances that conduct an electric current but do so very poorly.)

Vacuum tube diode. The working element in a vacuum tube diode is a metal wire or cylinder known as the cathode. Surrounding the cathode or placed at some distance from it is a metal plate. The cathode and plate are sealed inside a glass tube from which all air is removed. The cathode is also attached to a heater which, when turned on, causes the cathode to glow. As the cathode glows, it emits electrons. The diode acts as a rectifier, allowing the flow of electrons in only one direction, from cathode to plate.

Semiconductors. Newer types of diodes are made from n-type semiconductors and p-type semiconductors. N-type semiconductors contain small impurities that provide an excess of electrons with the capability of moving through a system. P-type semiconductors contain small impurities that provide an excess of positively charged "holes" that are capable of moving through the system.

A semiconductor diode is made by joining an n-type semiconductor with a p-type semiconductor through an external circuit containing a source of electrical current. The current is able to flow from the n-semiconductor to the p-semiconductor, but not in the other direction. In this sense, the n-semiconductor corresponds to the cathode and the p-semiconductor to the plate in the vacuum tube diode. The semiconductor diode has most of the same functions as the older vacuum diode, but it operates much more efficiently and takes up much less space than does a vacuum diode.

[See also Cathode; Electrical conductivity; Electric current ]

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. 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.

diode Electronic component with two electrodes, used as a rectifier to convert alternating current (AC) to direct current (DC). Semiconductor diodes have largely replaced electron-tubes, and allow electric current to flow freely in only one direction; only a small current flows in the reverse direction. A Zener diode blocks current until a critical voltage is reached.

diode An electronic device, generally of semiconductor material, that has two terminals and is capable of allowing current flow in one direction only. The terminals are called the anode and cathode. The diode presents a very low (high) impedance when a forward bias (reverse bias) is applied.

di·ode / ˈdīˌōd/ • n. Electr. a semiconductor device with two terminals, typically allowing the flow of current in one direction only. ∎  a thermionic tube having two electrodes (an anode and a cathode).

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