Diode Image

A two-terminal semiconductor (rectifying) device that exhibits a nonlinear current-voltage characteristic. The function of a diode is to allow current in one direction and to block current in the opposite direction. The terminals of a diode are called the anode and cathode. There are two kinds of semiconductor diodes: a P-N junction diode, which forms an electrical barrier at the interface between N- and P-type semiconductor layers, and a Schottky diode, whose barrier is formed between metal and semiconductor regions.

But this discussion really ought to start with a bit about semiconductors as materials.

Semiconductors are crystals that, in their pure state, are resistive (that is, their electrical properties lie between those of conductors and insulators) -- but when the proper impurities are added (this process is called doping) in trace amounts (often measured in parts per billion), display interesting and useful properties.

Diodes can perform switching and digital logic operations. Forward and reverse bias switch a diode between the low and high impedance states, respectively. Thus, it serves as a switch.
Logic
Diodes can perform digital logic functions: AND, and OR. Diode logic was used in early digital computers. It only finds limited application today. Sometimes it is convenient to fashion a single logic gate from a few diodes.