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.
