What is a diode?

Diode is a two-terminal electronic component, and the conduction current is mainly in one direction (asymmetric conductivity); It has low resistance in one direction (ideally zero) and high resistance in the other direction (ideally infinite). Diode vacuum tube or thermionic diode is a vacuum tube with two electrodes, a heated cathode and a plate, in which electrons can only flow from the cathode to the plate in one direction. A semiconductor diode, the most commonly used type today, is a semiconductor material with a pn junction connected to two electrical terminals.