Working principle
A diode is formed where P-type and N-type semiconductor meet. At the junction, electrons and holes recombine and leave behind a depletion region containing an internal electric field, the built-in potential. That field is what makes the device one-way: it opposes further diffusion of carriers, so current can flow easily in one direction and is blocked in the other. The P side is the anode and the N side the cathode.
Forward and reverse bias
Connecting the anode positive narrows the depletion region, and once the applied voltage exceeds the forward voltage — roughly 0.7 V for silicon and 0.3 V for germanium — the diode conducts, with current rising exponentially for very small further increases in voltage. Reverse bias widens the depletion region and only a tiny leakage current flows. Push the reverse voltage far enough and the junction breaks down; in an ordinary rectifier that is destructive, while a Zener diode is designed to operate there safely.
The characteristic curve
A diode's I–V curve is strongly non-linear, which is precisely what makes it useful. The Shockley equation describes it: current depends exponentially on the junction voltage, with a saturation current and a temperature-dependent thermal voltage as the constants. One practical consequence is that a conducting diode holds an almost constant voltage across itself over a wide current range, so it must always be fed through a current-limiting element rather than connected straight to a voltage source.
Where it is used
Diodes rectify AC into DC in half-wave, full-wave and bridge configurations at the input of nearly every power supply. They clamp inductive spikes across relay coils and motor windings, protect circuits against reverse polarity, and form clipping and clamping networks in signal paths. Close relatives extend the idea: Schottky diodes for low forward drop and fast recovery, Zener diodes for voltage references, and LEDs for light emission.