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.)
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.