Diode

A diode is a two-terminal electronic component that conducts electric current primarily in one direction. It has low resistance in one direction and high resistance in the other.
A semiconductor diode, the most commonly used type today, is a crystalline piece of semiconductor material with a p–n junction connected to two electrical terminals. It has an exponential current–voltage characteristic. Semiconductor diodes were the first semiconductor electronic devices. 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. Today, most diodes are made of silicon, but other semiconducting materials such as gallium arsenide and germanium are also used.
The obsolete thermionic diode is a vacuum tube with two electrodes, a heated cathode and a plate, in which electrons can flow in only one direction, from the cathode to the plate.
Among many uses, diodes are found in rectifiers to convert alternating current power to direct current, demodulation in radio receivers, and can even be used for logic or as temperature sensors. A common variant of a diode is a light-emitting diode, which is used as electric lighting and status indicators on electronic devices.

Main functions

Unidirectional current flow

The most common function of a diode is to allow an electric current to pass in one direction, while blocking it in the opposite direction. Its hydraulic analogy is a check valve. This unidirectional behavior can convert alternating current to direct current, a process called rectification. As rectifiers, diodes can be used for such tasks as extracting modulation from radio signals in radio receivers.

Threshold voltage

A diode's behavior is often simplified as having a forward threshold voltage or turn-on voltage or cut-in voltage, above which there is significant current and below which there is almost no current, which depends on a diode's composition:

Diode Type	Forward threshold voltage
Silicon Schottky	0.15 V to 0.45 V
Germanium p–n	0.25 V to 0.3 V
Silicon p–n	0.6 V to 0.7 V
Infrared p–n	~1.2 V
Light-emitting diodes	1.6 V to 4 V. has a complete list.

This voltage may loosely be referred to simply as the diode's forward voltage drop or just voltage drop, since a consequence of the steepness of the exponential is that a diode's voltage drop will not significantly exceed the threshold voltage under normal forward bias operating conditions. Datasheets typically quote a typical or maximum forward voltage for a specified current and temperature, so the user has a guarantee about when a certain amount of current will kick in. At higher currents, the forward voltage drop of the diode increases. For instance, a drop of 1 V to 1.5 V is typical at full rated current for silicon power diodes.
However, a semiconductor diode's exponential current–voltage characteristic is really more gradual than this simple on–off action. Although an exponential function may appear to have a definite "knee" around this threshold when viewed on a linear scale, the knee is an illusion that depends on the scale of y-axis representing current. In a semi-log plot, the diode's exponential curve instead appears more like a straight line.
Since a diode's forward-voltage drop varies only a little with the current, and is more so a function of temperature, this effect can be used as a [|temperature sensor] or as a somewhat imprecise voltage reference.

Reverse breakdown

A diode's high resistance to current flowing in the reverse direction suddenly drops to a low resistance when the reverse voltage across the diode reaches a value called the breakdown voltage. This effect is used to regulate voltage or to protect circuits from high voltage surges.

Other functions

A semiconductor diode's current–voltage characteristic can be tailored by selecting the semiconductor materials and the doping impurities introduced into the materials during manufacture. These techniques are used to create special-purpose diodes that perform many different functions. For example, to electronically tune radio and TV receivers, to generate radio-frequency oscillations, and to produce light. Tunnel, Gunn and IMPATT diodes exhibit negative resistance, which is useful in microwave and switching circuits.
Diodes, both vacuum and semiconductor, can be used as shot-noise generators.

History

Thermionic diodes and solid-state diodes were developed separately, at approximately the same time, in the early 1900s, as radio receiver detectors. Until the 1950s, vacuum diodes were used more frequently in radios because the early point-contact semiconductor diodes were less stable. In addition, most receiving sets had vacuum tubes for amplification that could easily have the thermionic diodes included in the tube, and vacuum-tube rectifiers and gas-filled rectifiers were capable of handling some high-voltage/high-current rectification tasks better than the semiconductor diodes that were available at that time.

Thermionic

In 1873, Frederick Guthrie observed that a grounded, white-hot metal ball brought in close proximity to an electroscope would discharge a positively charged electroscope, but not a negatively charged electroscope. In 1880, Thomas Edison observed unidirectional current between heated and unheated elements in a bulb, later called Edison effect, and was granted a patent on application of the phenomenon for use in a DC voltmeter.
About 20 years later, John Ambrose Fleming realized that the Edison effect could be used as a radio detector. Fleming patented the first true thermionic diode, the Fleming valve, in Britain on 16 November 1904. 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. Today they are still used in a few high power applications where their ability to withstand transient voltages and their robustness gives them an advantage over semiconductor devices, and in musical instrument and audiophile applications.

Semiconductor

In 1874, German scientist Karl Ferdinand Braun discovered the "unilateral conduction" across a contact between a metal and a mineral. C. E. Fitts made a selenium current rectifier, but his work did not result in any practical devices until the 1930s. Indian scientist Jagadish Chandra Bose was the first to use a crystal for detecting radio waves in 1894, and he filed a U.S. patent for detecting radio signals with a galena crystal point-contact semiconductor diode in 1901.
The crystal detector was developed into a practical device for wireless telegraphy by Greenleaf Whittier Pickard, who invented a silicon crystal detector in 1903 and received a patent for it on 20 November 1906. He subsequently founded a company to market "cat’s whisker" crystal radio detectors; probably the first to make and sell commercial silicon semiconductor devices. Other experimenters tried a variety of other minerals as detectors.
Semiconductor principles were unknown to the developers of these early rectifiers. 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. Researchers at Bell Labs, Western Electric, MIT, Purdue and in the UK intensively developed point-contact diodes during World War II for application in radar. 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. In 1946, Sylvania began offering the 1N34 crystal diode. During the early 1950s, junction diodes were developed.

Etymology

At the time of their invention, asymmetrical conduction devices were known as rectifiers. In 1919, the year tetrodes were invented, William Henry Eccles coined the term diode from the Greek roots di, meaning "two", and ode, meaning "path". The word diode however was already in use, as were triode, tetrode, pentode, hexode, as terms of multiplex telegraphy.
Although all diodes rectify, "rectifier" usually applies to diodes used for power supply, to differentiate them from diodes intended for small signal circuits.

Vacuum tube diodes

A thermionic diode is a vacuum tube consisting of a sealed, evacuated glass or metal envelope containing two electrodes: a cathode and a plate. The cathode is either indirectly heated or directly heated. If indirect heating is employed, a heater is included in the envelope.
In operation, the cathode is heated to red heat, around. A directly heated cathode is made of tungsten wire and is heated by a current passed through it from an external voltage source. An indirectly heated cathode is heated by infrared radiation from a nearby heater that is formed of Nichrome wire and supplied with current provided by an external voltage source.
The operating temperature of the cathode causes it to release electrons into the vacuum, a process called thermionic emission. The cathode is coated with oxides of alkaline earth metals, such as barium and strontium oxides. These have a low work function, meaning that they more readily emit electrons than would the uncoated cathode.
The plate, not being heated, does not emit electrons; but is able to absorb them.
The alternating voltage to be rectified is applied between the cathode and the plate. When the plate voltage is positive with respect to the cathode, the plate electrostatically attracts the electrons from the cathode, so a current of electrons flows through the tube from cathode to plate. When the plate voltage is negative with respect to the cathode, no electrons are emitted by the plate, so no current can pass from the plate to the cathode.
Types of vacuum tube diodes include the mercury vapor diode, the xenon gas diode, the Cold-cathode rectifier, and the magnetron.