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A diode is an electronic device that has two electrodes arranged in such a way that electrons can flow in only one direction. Because of this ability to control the flow of electrodes, a diode is commonly used as a rectifier, a device that connects alternating current into direct current. 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.

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 types of diodes are made from n-type semi-conductors 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" 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.

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