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Diode

In electronics, a diode is a two-terminal device (except that thermionic diodes may also have one or two ancillary terminals for a heater). Diodes have two active electrodes between which the signal of interest may flow, and most are used for their unidirectional current property. The varicap diode is used as an electrically adjustable capacitor.

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The directionalityof current flow most diodes exhibit is sometimes generically called the rectifying property. The most common function of a diode is to allow an electric current to pass in one direction (called the forward biased condition) and to block it in the opposite direction (the reverse biased condition). Thus, the diode can be thought of as an electronic version of a check valve. Real diodes do not displaysuch a perfect on-off directionality but have a more complex non-linear electrical characteristic, which depends on the particular type of diode technology. Diodes also have many other functions in which they are not designed to operate in this on-off manner.

Early diodes included “cat’s whisker” crystals and vacuum tube devices (also called thermionic valves). Today the most common diodes aremade from semiconductor materials such as silicon or germanium.

History

Although the crystal diode was popularized before the thermionic diode, thermionic and solid state diodes were developed in parallel. The principle of operation of thermionic diodes was discovered by Frederick Guthrie in 1873.[1] The principle of operation of crystal diodes was discovered in 1874 by the Germanscientist, Karl Ferdinand Braun.[2]
Thermionic diode principles were rediscovered by Thomas Edison on February 13, 1880 and he was awarded a patent in 1883 (U.S. Patent 307,031 ), but developed the idea no further. Braun patented the crystal rectifier in 1899 [1]. Braun's discovery was further developed by Jagdish Chandra Bose into a useful device for radio detection.
The first radio receiver using acrystal diode was built around 1900 by Greenleaf Whittier Pickard. The first thermionic diode was patented in Britain by John Ambrose Fleming (scientific adviser to the Marconi Company and former Edison employee[2]) on November 16, 1904 (U.S. Patent 803,684  in November 1905). Pickard received a patent for a silicon crystal detector on November 20, 1906 [3] (U.S. Patent 836,531 ).
At the time oftheir invention, such devices were known as rectifiers. In 1919, William Henry Eccles coined the term diode from Greek roots; di means "two", and ode (from odos) means "path".









Types of semiconductor diode


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There are several types of junction diodes, which either emphasize a different physical aspect of a diode often by geometric scaling, doping level, choosing theright electrodes, are just an application of a diode in a special circuit, or are really different devices like the Gunn and laser diode and the MOSFET:

Normal (p-n) diodes which operate as described above. Usually made of doped silicon or, more rarely, germanium. Before the development of modern silicon power rectifier diodes, cuprous oxide and later selenium was used; its low efficiency gave ita much higher forward voltage drop (typically 1.4–1.7 V per “cell”, with multiple cells stacked to increase the peak inverse voltage rating in high voltage rectifiers), and required a large heat sink (often an extension of the diode’s metal substrate), much larger than a silicon diode of the same current ratings would require. The vast majority of all diodes are the p-n diodes found in CMOSintegrated circuits, which include two diodes per pin and many other internal diodes.









Esaki or tunnel diodes
these have a region of operation showing negative resistance caused by quantum tunneling, thus allowing amplification of signals and very simple bistable circuits. These diodes are also the type most resistant to nuclear radiation.


Light-emitting diodes (LEDs)...