Diode

A diode is an electronic device through which electrons can flow in only one direction. Because of its ability to control current flow, a diode is commonly used as a rectifier, a device that transforms alternating current into direct current.

In general, two types of diodes exist. Until the invention of solid-state devices such as the transistor, diodes were vacuum tubes containing two metal components. Later diodes are solid state devices consisting of adjacent regions of n-type and p-type (negatively-doped and positively-doped) semiconductor.

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.

If the metal plate is maintained at a positive potential difference compared to the cathode, electrons will flow from the cathode to the plate. If the plate is negative compared to the cathode, however, electrons are repelled and there is no electrical current from cathode to plate. Thus, the diode acts as a rectifier, allowing the flow of electrons in only one direction, from cathode to plate.

One use of such a device is to transform alternating current to direct current. Alternating current is current that flows first in one direction and then the other. But alternating current fed into a diode can move in one direction only, thereby converting the current to a one-way or direct current.

Newer diodes are made from n-type semiconductors and p-type semiconductors. N-type semiconductors contain small impurities that provide an excess of electrons (negative charge carriers, hence n-type) with the capability of moving through a system. P-type semiconductors contain small impurities that provide an excess of positively charged “holes”—gaps in the atomic structure where electrons would normally be— capable of moving through the semiconductor crystal (positive charge carriers, hence p-type).

A semiconductor diode is made by joining a sandwich of n-type semiconductor and 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-type semiconductor corresponds to the cathode and the p-type 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 Electrical conductivity; Electric current.