A diode is an electronic two-terminal component that allows the electric current to flow through it in one direction. This term is generally used to refer to the semiconductor diode, the most common today. Consists of a piece of semiconductor crystal connected to two electrical terminals. The vacuum diode, which is no longer used, except for high power technologies, is a vacuum tube with two electrodes: a sheet as anode, and a cathode.
The first diodes were valves or vacuum tubes, also called thermionic valves made up of two electrodes surrounded by vacuum in a glass tube, looking similar to incandescent lamps. The invention was developed in 1904 by John Ambrose Fleming, an employee of the Marconi company, based on observations made by Thomas Alva Edison.