Diode

In [|electronics], a **diode** is a type of two- [|terminal] [|electronic component] with // [|nonlinear] // [|resistance and conductance] (i.e., a nonlinear [|current–voltage characteristic] ), distinguishing it from components such as two-terminal [|linear resistors] which obey [|Ohm's law]. A **semiconductor diode**, the most common type today, is a [|crystalline] piece of [|semiconductor] material connected to two electrical terminals. [|[1]] A **vacuum tube diode** (now rarely used except in some high-power technologies) is a [|vacuum tube] with two [|electrodes] : a [|plate] and a [|cathode]. The most common function of a diode is to allow an electric current to pass in one direction (called the diode's //forward//direction), while blocking current in the opposite direction (the //reverse// direction). Thus, the diode can be thought of as an electronic version of a [|check valve]. This unidirectional behavior is called [|rectification], and is used to convert [|alternating current] to [|direct current] , and to extract [|modulation] from radio signals in radio receivers—these diodes are forms of [|rectifiers]. However, diodes can have more complicated behavior than this simple on–off action. Semiconductor diodes do not begin conducting electricity until a certain threshold voltage is present in the forward direction (a state in which the diode is said to be [|//forward-biased//] ). The voltage drop across a forward-biased diode varies only a little with the current, and is a function of temperature; this effect can be used as a [|temperature sensor] or [|voltage reference]. Semiconductor diodes' nonlinear current–voltage characteristic can be tailored by varying the [|semiconductor materials] and introducing impurities into ( [|doping] ) the materials. These are exploited in special purpose diodes that perform many different functions. For example, diodes are used to regulate voltage ( [|Zener diodes] ), to protect circuits from high voltage surges ( [|avalanche diodes] ), to electronically tune radio and TV receivers ( [|varactor diodes] ), to generate [|radio frequency] [|oscillations] ( [|tunnel diodes], [|Gunn diodes] , [|IMPATT diodes] ), and to produce light ( [|light emitting diodes] ). Tunnel diodes exhibit [|negative resistance], which makes them useful in some types of circuits. Diodes were the first [|semiconductor electronic devices]. The discovery of [|crystals] ' [|rectifying] abilities was made by German physicist [|Ferdinand Braun] in 1874. The first semiconductor diodes, called [|cat's whisker diodes], developed around 1906, were made of mineral crystals such as [|galena]. Today most diodes are made of [|silicon], but other [|semiconductors] such as [|germanium] are sometimes used. [|[2]]

A [|diode], or "rectifier," is any device through which electricity can flow in only one direction. The first diodes were crystals used as rectifiers in home radio kits. A weak radio signal was fed into the crystal through a very fine wire called a [|cat's whisker]. The crystal removed the high frequency radio carrier signal, allowing the part of the signal with the audio information to come through loud and clear. The crystal was filled with impurities, making some sections more resistant to electrical flow than others. Using the radio required positioning the cat's whiskers over the right kind of impurity to get electricity to flow through the crystal to the output below it.

Signal diodes (small current)
Signal diodes are used to process information (electrical signals) in circuits, so they are only required to pass small currents of up to 100mA.

General purpose signal diodes such as the 1N4148 are made from silicon and have a forward voltage drop of 0.7V.


 * Germanium diodes** such as the OA90 have a lower forward voltage drop of 0.2V and this makes them suitable to use in radio circuits as detectors which extract the audio signal from the weak radio signal.

For general use, where the size of the forward voltage drop is less important, silicon diodes are better because they are less easily damaged by heat when soldering, they have a lower resistance when conducting, and they have very low leakage currents when a reverse voltage is applied.







Numbering and coding schemes
There are a number of common, standard and manufacturer-driven numbering and coding schemes for diodes; the two most common being the [|EIA]/[|JEDEC] standard and the European [|Pro Electron] standard:

EIA/JEDEC
A standardized 1N-series numbering system was introduced in the US by EIA/JEDEC (Joint Electron Device Engineering Council) about 1960. Among the most popular in this series were: 1N34A/1N270 (Germanium signal), 1N914/1N4148 (Silicon signal), [|1N4001]-1N4007 (Silicon 1A power rectifier) and 1N54xx (Silicon 3A power rectifier)[|[][|23][|]][|[][|24][|]][|[][|25][|]]

Pro Electron
The European [|Pro Electron] coding system for [|active components] was introduced in 1966 and comprises two letters followed by the part code. 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: As well as these common codes, many manufacturers or organisations have their own systems too — for example:
 * AA-series germanium low-power/signal diodes (e.g.: AA119)
 * BA-series silicon low-power/signal diodes (e.g.: BAT18 Silicon RF Switching Diode)
 * BY-series silicon rectifier diodes (e.g.: BY127 1250V, 1A rectifier diode)
 * BZ-series silicon Zener diodes (e.g.: BZY88C4V7 4.7V Zener diode)
 * GD-series germanium diodes (e.g.: GD9) — this is a very old coding system
 * OA-series germanium diodes (e.g.: OA47) — a coding sequence developed by [|Mullard], a UK company
 * HP diode 1901-0044 = JEDEC 1N4148
 * UK military diode CV448 = Mullard type OA81 = [|GEC] type GEX23