Diode, electronic device that allows the passage of current in only one direction. The first such devices were vacuum-tube diodes, consisting of an evacuated glass or steel envelope containing two electrodes—a cathode and an anode. Because electrons can flow in only one direction, from cathode to anode, the vacuum-tube diode could be used in rectification. The diodes most commonly used in electronic circuits today are semiconductor diodes. The simplest of these, the germanium point-contact diode, dates from the early days of radio, when the received radio signal was detected by means of a germanium crystal and a fine, pointed wire that rested on it. In modern germanium (or silicon) point-contact diodes, the wire and a tiny crystal plate are mounted inside a small glass tube and connected to two wires that are fused into the ends of the tube.

Junction-type diodes consist of a junction of two different kinds of semiconductor material. The zener diode is a special junction-type diode, using silicon, in which the voltage across the junction is independent of the current through the junction. Because of this characteristic, zener diodes are used as voltage regulators. Another special junction-type diode is used in solar cells; a voltage appears spontaneously when the junction is illuminated. In light-emitting diodes (LEDs), on the other hand, a voltage applied to the semiconductor junction results in the emission of light energy. LEDs are used in numerical displays such as those on electronic digital watches and pocket calculators. See Photoelectric Effect.