The discovery of asymmetric electrical conduction across the contact between a crystalline mineral and a metal was made by German physicist Ferdinand Braun in 1874.

Tunnel, Gunn and IMPATT diodes exhibit negative resistance, which is useful in microwave and switching circuits.

Throughout the vacuum tube era, valve diodes were used in almost all electronics such as radios, televisions, sound systems, and instrumentation.

They slowly lost market share beginning in the late 1940s due to selenium rectifier technology and then to semiconductor diodes during the 1960s.

In 1874, German scientist Karl Ferdinand Braun discovered the "unilateral conduction" across a contact between a metal and a mineral.

During the 1930s understanding of physics advanced and in the mid-1930s researchers at Bell Telephone Laboratories recognized the potential of the crystal detector for application in microwave technology.

[19] After World War II, AT&T used these in its microwave towers that criss-crossed the United States, and many radar sets use them even in the 21st century.

A thermionic diode is a thermionic-valve device consisting of a sealed, evacuated glass or metal envelope containing two electrodes: a cathode and a plate.

The operating temperature of the cathode causes it to release electrons into the vacuum, a process called thermionic emission.

Point-contact diodes were developed starting in the 1930s, out of the early crystal detector technology, and are now generally used in the 3 to 30 gigahertz range.

The metal side is the pointed end of a small diameter wire that is in contact with the semiconductor crystal.

[27] In the welded contact type, a small P region is formed in the otherwise N-type crystal around the metal point during manufacture by momentarily passing a relatively large current through the device.

Impurities are added to it to create a region on one side that contains negative charge carriers (electrons), called an n-type semiconductor, and a region on the other side that contains positive charge carriers (holes), called a p-type semiconductor.

When the n-type and p-type materials are attached together, a momentary flow of electrons occurs from the n to the p side resulting in a third region between the two where no charge carriers are present.

The junction does not allow the flow of electrons in the opposite direction when the potential is applied in reverse, creating, in a sense, an electrical check valve.

If an external voltage is placed across the diode with the same polarity as the built-in potential, the depletion zone continues to act as an insulator, preventing any significant electric current flow (unless electron–hole pairs are actively being created in the junction by, for instance, light; see photodiode).

However, if the polarity of the external voltage opposes the built-in potential, recombination can once again proceed, resulting in a substantial electric current through the p–n junction (i.e. substantial numbers of electrons and holes recombine at the junction) that increases exponentially with voltage.

[32] The odd terms can be omitted because they produce frequency components that are outside the pass band of the mixer or detector.

3 Following the end of forwarding conduction in a p–n type diode, a reverse current can flow for a short time.

The magnitude of such a reverse current is determined by the operating circuit (i.e., the series resistance) and the diode is said to be in the storage-phase.

Their low efficiency required a much higher forward voltage to be applied (typically 1.4 to 1.7 V per "cell", with multiple cells stacked so as to increase the peak inverse voltage rating for application in high voltage rectifiers), and required a large heat sink (often an extension of the diode's metal substrate), much larger than the later silicon diode of the same current ratings would require.

The symbol used to represent a particular type of diode in a circuit diagram conveys the general electrical function to the reader.

The first letter represents the semiconductor material used for the component (A = germanium and B = silicon) and the second letter represents the general function of the part (for diodes, A = low-power/signal, B = variable capacitance, X = multiplier, Y = rectifier and Z = voltage reference); for example: Other common numbering/coding systems (generally manufacturer-driven) include: In optics, an equivalent device for the diode but with laser light would be the optical isolator, also known as an optical diode, that allows light to only pass in one direction.

The diode rectifies the AM radio frequency signal, leaving only the positive peaks of the carrier wave.

In microwave and millimeter wave technology, beginning in the 1930s, researchers improved and miniaturized the crystal detector.

Automotive alternators are a common example, where the diode, which rectifies the AC into DC, provides better performance than the commutator or earlier, dynamo.

Since most electronic circuits can be damaged when the polarity of their power supply inputs are reversed, a series diode is sometimes used to protect against such situations.

Many integrated circuits also incorporate diodes on the connection pins to prevent external voltages from damaging their sensitive transistors.

In electronics, cosmic rays and other sources of ionizing radiation cause noise pulses and single and multiple bit errors.

These semiconductor radiation detectors need efficient and uniform charge collection and low leakage current.