Editing Diode (section)

== Main functions ==

=== Unidirectional current flow ===
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 it in the opposite direction (the ''reverse'' direction). Its [[hydraulic analogy]] is a [[check valve]]. This unidirectional behavior can convert alternating current (AC) to direct current (DC), a process called [[rectification (electricity)|rectification]]. As rectifiers, diodes can be used for such tasks as extracting [[modulation]] from [[radio signal]]s in [[radio receiver]]s.

=== Threshold voltage ===
[[File:Siirded forward diode-characteristics.svg|thumb|Forward [[Current–voltage characteristic|current–voltage curve]] of 4 common diodes.]]
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:
{| class="wikitable"
|+Forward threshold voltage for various semiconductor diodes
!Diode Type
!Forward threshold voltage
|-
|Silicon [[Schottky diode|Schottky]]
|0.15 V to 0.45 V
|-
|Germanium [[p–n diode|p–n]]
|0.25 V to 0.3 V
|-
|Silicon p–n
|0.6 V to 0.7 [[volt|V]]
|-
|[[Infrared]] ([[Gallium arsenide|GaAs]]) p–n
|~1.2 V
|-
|[[Light-emitting diode]]s (LEDs)
|1.6 V (red) to 4 V (violet). {{Slink|Light-emitting diode physics|Materials}} 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'' (V<sub>F</sub>) for a specified current and temperature (e.g. 20&nbsp;mA and 25&nbsp;'''°'''C for LEDs),<ref>{{Cite web |title=All About LEDs |url=https://learn.adafruit.com/all-about-leds/forward-voltage-and-kvl |access-date=2023-01-19 |website=Adafruit Learning System |language=en-US}}</ref> 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&nbsp;V to 1.5&nbsp;V is typical at full rated current for silicon power diodes. (See also: {{Slink|Rectifier|Rectifier voltage drop}})

However, a semiconductor diode's [[Exponential function|exponential]] [[current–voltage characteristic]] is really more gradual than this simple on–off action.<ref name="Turner">{{cite book |last1=Turner |first1=L. W. |url=https://books.google.com/books?id=2N0gBQAAQBAJ&pg=PA14 |title=Electronics Engineer's Reference Book, 4th Ed. |date=2015 |publisher=Butterworth-Heinemann |isbn=978-1483161273 |pages=8.14–8.22}}</ref> Although an exponential function may appear to have a definite "[[knee of a curve|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]] (using a [[logarithmic scale]] for current and a linear scale for voltage), 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 measurements|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 ([[Zener diode]]s) or to protect circuits from high voltage surges ([[avalanche diode]]s).

=== Other functions ===
A semiconductor diode's current–voltage characteristic can be tailored by selecting the [[semiconductor materials]] and the [[doping (semiconductor)|doping]] impurities introduced into the materials during manufacture.<ref name="Turner" /> These techniques are used to create special-purpose diodes that perform many different functions.<ref name="Turner" /> For example, to electronically tune radio and TV receivers ([[varactor diode]]s), to generate [[radio frequency|radio-frequency]] [[oscillation]]s ([[tunnel diode]]s, [[Gunn diode]]s, [[IMPATT diode]]s), and to produce light ([[light-emitting diode]]s). 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 [[Noise generator#Shot noise generator|shot-noise generators]].