Diode
A diode is a two-terminal electronic component that conducts current readily in one direction and blocks it in the other, behaving like a one-way valve for electric charge. The two terminals are the anode, where conventional current enters when the diode conducts, and the cathode, marked by a stripe on most packages. The first diode was a glass vacuum tube; today virtually every diode is a small piece of semiconductor, and billions of them sit inside every processor as protection structures and as the junctions of transistors.
The vacuum diode came first, in 1904, and it conducts for a different reason than the crystal. John Ambrose Fleming, working on the Marconi company's wireless problem, put a heated filament and a metal plate in an evacuated bulb. The hot filament emits electrons; when the plate is positive they flow across the gap, and when it is negative they are repelled, so alternating current passes in only one direction. Fleming called it a valve, a name that persists in British usage. Its one-way action is built into the geometry, since only the cathode is hot enough to emit, and that is why it can be made to carry a large forward current but also needs a filament power supply and warms up.
The semiconductor diode gets its one-way behavior from a pn junction or a metal-semiconductor contact. In a pn junction, p-type material with mobile holes is joined to n-type material with mobile electrons; at the boundary a depletion region forms that holds a built-in potential. Forward bias, with positive voltage on the p side, shrinks the barrier and current rises exponentially with voltage; reverse bias widens it and only a tiny leakage current flows. In prose, the current-voltage curve has three regions. Below roughly 0.6 to 0.7 volts for silicon (about 0.3 volts for germanium) almost nothing flows. Above that knee the current climbs steeply while the voltage across the diode barely changes. In reverse, the current stays nearly flat at a very small value until the breakdown voltage, where it surges.
Several specialized diodes simply exploit a different part of that curve. The light-emitting diode, first demonstrated in visible red form by Nick Holonyak at General Electric in 1962, recombines carriers in a direct-gap compound semiconductor and emits photons; its forward drop is larger, from about 1.8 volts for red to over 3 volts for blue. The Zener diode, named for the physicist Clarence Zener who analyzed electrical breakdown in solids in the 1930s, is built to run safely in reverse breakdown and so clamps a voltage near a fixed value. The Schottky diode, named for Walter Schottky, uses a metal-semiconductor junction without minority-carrier storage; its forward drop is only about 0.2 to 0.4 volts and it switches very fast, which suits switching power supplies and radio-frequency mixers. Its ancestor is the point-contact crystal detector of the early radio era.
Diodes are used wherever current must be forced to go one way. Rectification turns alternating current into pulsating direct current. A diode placed across a relay or motor coil, a flyback diode, gives the collapsing inductive current somewhere to go and prevents a damaging voltage spike. Clamp diodes on chip pins steer electrostatic discharge into the supply rails before it can reach the thin gate oxide of the transistors. A diode in series with a battery input guards against reversed polarity, and photodiodes in cameras and fiber-optic receivers run the same junction in reverse to turn light into current. The modern importance is largely unseen: every input and output pad of a processor has diodes protecting it.
The ideal one-way valve is a useful picture but a misleading one. A real silicon diode drops about 0.7 volts when conducting, so it dissipates power equal to that drop times the current, and this is why high-current rectifiers get hot and why designers turn to Schottky diodes or to transistors acting as switches. In reverse a real diode leaks a little, leakage rises steeply with temperature, and after being conducting it takes a short reverse recovery time to turn off. Another pitfall is treating the diode as a resistor: its voltage barely rises with current, so connecting one across a supply without a current-limiting resistor can destroy it, and an LED with no series resistor is the classic casualty.
Read diode as the smallest possible rule of electronics: a junction or contact that lets charge pass only one way, from which rectifiers, light emitters, voltage references and, by adding a third terminal, the transistor itself all follow.