Electronic device that has two electrodes (anode and cathode) and that allows current to flow in only one direction, resisting current flow in the other. An applied voltage can cause electrons to flow in only one direction, from the cathode to the anode, and then back to the cathode through an external circuit. Diodes are used especially as rectifierswhich change alternating current into direct currentand to vary the amplitude of a signal in proportion to the voltage in a circuit, as in a radio or television receiver. The most familiar diodes are vacuum tubes and semiconductor diodes. Semiconductor diodes, the simplest of semiconductor devices, consist of two electrodes and a sandwich of two dissimilar semiconducting substances (a p-n junction). Such diodes form the basis for more complex semiconductor devices (including transistors) used in computers and other electronic equipment. Semiconductor diodes include light-emitting diodes and laser diodes; the latter emit laser light, useful for telecommunications via fibre optics and for reading compact discs.