View source for Diode

{{Short description|Two-terminal electronic component}}
{{Other uses}}
{{Infobox electronic component
| name              = Diode
| image             = Diode-closeup.jpg
| caption           = Close-up view of a [[silicon]] diode. The anode is on the right side; the cathode is on the left side (where it is marked with a black band). The square [[Crystalline silicon|silicon crystal]] can be seen between the two [[Lead (electronics)|leads]].
| type              = Active
| inventor          = [[Jagadish Chandra Bose]]
| invention_Year    = 1901
| pins              = [[Anode]] and [[cathode]]
| symbol            = [[File:Diode symbol.svg|150px]]
}}

[[File:Dioden.JPG|thumb|Various semiconductor diodes. Left: A four-diode [[bridge rectifier]]. Next to it is a [[1N4148 signal diode]]. On the far right is a [[Zener diode]]. In most diodes, a white or black painted band identifies the [[cathode]] into which electrons will flow when the diode is conducting. Electron flow is the reverse of [[conventional current]] flow.<ref>{{cite book|url=https://books.google.com/books?id=NunPn6R__TAC&pg=PA81|title=Electronic Circuits: Fundamentals and Applications, 3rd Ed.|last=Tooley|first=Mike|publisher=Routledge|year=2013|isbn=978-1-136-40731-4|page=81}}</ref><ref name="Crecraft">{{cite book | last = Crecraft | first = Filip Mincic |author2=Stephen Gergely | title = Analog Electronics: Circuits, Systems and Signal Processing | publisher = Butterworth-Heinemann | year = 2002 | page = 110 | url = https://books.google.com/books?id=lS7qN6iHyBYC&pg=PA110 | isbn = 0-7506-5095-8}}</ref><ref name="Horowitz">{{cite book | last = Horowitz | first = Paul | author2=Winfield Hill | title = The Art of Electronics, 2nd Ed. | publisher = Cambridge University Press | year = 1989 | location = London | page = 44 | url = https://books.google.com/books?id=bkOMDgwFA28C&pg=PA44 | isbn = 0-521-37095-7}}</ref>]]

A '''diode''' is a two-[[Terminal (electronics)|terminal]] [[electronic component]] that conducts [[electric current]] primarily in [[One-way traffic|one direction]] (asymmetric [[electrical conductance|conductance]]). It has low (ideally zero) [[Electrical resistance and conductance|resistance]] in one direction and high (ideally infinite) resistance in the other.

A [[semiconductor]] diode, the most commonly used type today, is a [[Crystallinity|crystalline]] piece of semiconductor material with a [[p–n junction]] connected to two electrical terminals.<ref>{{cite web|url=https://www.element14.com/community/docs/DOC-22519/l/physical-explanation-general-semiconductors |title=Physical Explanation – General Semiconductors |date=2010-05-25 |access-date=2010-08-06}}</ref> It has an [[Exponential function|exponential]] [[current–voltage characteristic]]. Semiconductor diodes were the first [[Semiconductor device|semiconductor electronic devices]]. The discovery of asymmetric electrical conduction across the contact between a [[Crystal|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.<ref>{{cite web |url=http://www.element-14.com/community/docs/DOC-22518/l/the-constituents-of-semiconductor-components |archive-url=https://web.archive.org/web/20110710183421/http://www.element14.com/community/docs/DOC-22518/l/the-constituents-of-semiconductor-components |archive-date=2011-07-10 |title=The Constituents of Semiconductor Components |date=2010-05-25 |access-date=2010-08-06}}</ref>

The obsolete '''thermionic diode''' is a [[vacuum tube]] with two [[electrode]]s, a heated [[cathode]] and a [[plate electrode|plate]], in which [[Electron|electrons]] can flow in only one direction, from the cathode to the plate.

Among many uses, diodes are found in [[rectifier]]s to convert [[alternating current]] (AC) power to [[direct current]] (DC), [[demodulation]] in [[radio receiver]]s, and can even be used for [[Boolean Logic|logic]] or as [[temperature sensor]]s. 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 (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]].

== History ==
{{Further|Vacuum tube#History and development}}
[[File:Diode-english-text.svg|thumb|Structure of a [[vacuum tube]] diode. The filament itself may be the cathode, or more commonly (as shown here) used to heat a separate metal tube which serves as the cathode.]]

Thermionic ([[vacuum-tube]]) diodes and [[solid state (electronics)|solid-state]] (semiconductor) diodes were developed separately, at approximately the same time, in the early 1900s, as radio receiver [[detector (radio)|detectors]].<ref>{{Cite journal|last=Guarnieri|first=M.|date=2011|title=Trailblazers in Solid-State Electronics|journal=IEEE Ind. Electron. M.|volume=5|issue=4|pages=46–47|doi=10.1109/MIE.2011.943016|s2cid=45476055}}</ref> 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 (for example the [[12SQ7]] [[double diode triode]]), and vacuum-tube rectifiers and gas-filled rectifiers were capable of handling some high-voltage/high-current rectification tasks better than the semiconductor diodes (such as [[selenium rectifier]]s) that were available at that time.

In 1873, [[Frederick Guthrie (scientist)|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.<ref>Guthrie, Frederick (October 1873) [https://books.google.com/books?id=U08wAAAAIAAJ&pg=PA257 "On a relation between heat and static electricity,"] ''The London, Edinburgh, and Dublin Philosophical Magazine and Journal of Science'', 4th series, '''46''':  257–266.</ref><ref>[https://www.nobelprize.org/uploads/2018/06/richardson-lecture.pdf 1928 Nobel Lecture:] Owen W. Richardson, "Thermionic phenomena and the laws which govern them", December 12, 1929,</ref> In 1880, Thomas Edison observed unidirectional current between heated and unheated elements in a bulb, later called [[Thermionic emission#History|Edison effect]], and was granted a patent on application of the phenomenon for use in a [[direct current|DC]] [[voltmeter]].<ref>Edison, Thomas A. "Electrical Meter" {{US patent|307030}} Issue date: Oct 21, 1884</ref><ref>{{Cite journal |last=Redhead |first=P. A. |date=1998-05-01 |title=The birth of electronics: Thermionic emission and vacuum |journal=Journal of Vacuum Science & Technology A: Vacuum, Surfaces, and Films |volume=16 |issue=3 |pages=1394–1401 |doi=10.1116/1.581157 |issn=0734-2101 |bibcode=1998JVSTA..16.1394R}}</ref> About 20 years later, [[John Ambrose Fleming]] (scientific adviser to the [[Marconi Company]] and former Edison employee) realized that the Edison effect could be used as a [[Detector (radio)|radio detector]]. Fleming patented the first true thermionic diode, the [[Fleming valve]], in Britain on 16 November 1904<ref>{{cite web |url=http://www.jmargolin.com/history/trans.htm |title=Road to the Transistor |publisher=Jmargolin.com |access-date=2008-09-22}}</ref> (followed by {{US patent |803684}} in November 1905). 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.

{{Anchor|Crystal diode}}In 1874, German scientist [[Karl Ferdinand Braun]] discovered the "unilateral conduction" across a contact between a metal and a [[mineral]].<ref>Braun, Ferdinand (1874) [http://gallica.bnf.fr/ark:/12148/bpt6k152378/f580.image.langEN "Ueber die Stromleitung durch Schwefelmetalle"] (On current conduction in metal sulphides), ''Annalen der Physik und Chemie'', '''153''' : 556–563.</ref><ref>[https://web.archive.org/web/20060211010305/http://chem.ch.huji.ac.il/~eugeniik/history/braun.htm Karl Ferdinand Braun]. chem.ch.huji.ac.il</ref> Indian scientist [[Jagadish Chandra Bose]] was the first to use a crystal for detecting radio waves in 1894.<ref name="Sarkar">{{Cite book |last = Sarkar |first= Tapan K. |title = History of wireless  |publisher = John Wiley and Sons |year = 2006 |location = US |pages = 94, 291–308 |url = https://books.google.com/books?id=NBLEAA6QKYkC&pg=PA291 |isbn = 0-471-71814-9}}</ref> 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.<ref>Pickard, G. W., "Means for receiving intelligence communicated by electric waves" {{US patent|836531}} Issued: August 30, 1906</ref> 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.<ref name="Scaff_Ohl_01">Scaff, J. H., Ohl, R. S. [https://archive.org/details/bstj26-1-1 "Development of Silicon Crystal Rectifiers for Microwave Radar Receivers"], ''The Bell System Technical Journal'', Vol. 24, No. 1, January 1947. pp. 1 - 30</ref> Researchers at [[Bell Labs]], [[Western Electric]], [[MIT]], [[Purdue]] and in the [[UK]] intensively developed point-contact diodes (''crystal rectifiers'' or ''crystal diodes'') during World War II for application in radar.<ref name="Scaff_Ohl_01"/> 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.<ref>Cornelius, E. C. [http://www.americanradiohistory.com/Archive-Electronics/40s/Electronics-1946-02.pdf "Germanium Crystal Diodes"], ''Electronics'', February 1946, p. 118</ref><ref>{{cite web |url = http://www.bunkerofdoom.com/tubes/syl43/DATA/1949/1N34.GIF |title = Sylvania 1949 data book page |archive-url=https://web.archive.org/web/20180525062316/http://www.bunkerofdoom.com/tubes/syl43/DATA/1949/1N34.GIF |archive-date=25 May 2018 |url-status=dead}}</ref><ref>Sylvania, [http://n4trb.com/AmateurRadio/SemiconductorHistory/40%20Uses%20for%20Germanium%20Diodes.pdf ''40 Uses for Germanium Diodes''], Sylvania Electric Products Co., 1949, p. 9</ref> During the early 1950s, junction diodes were developed.

==Etymology==
{{Main article|Rectifier}}

At the time of their invention, asymmetrical conduction devices were known as [[rectifier]]s. In 1919, the year [[tetrode]]s were invented, [[William Henry Eccles]] coined the term ''diode'' from the [[Greek and Latin roots|Greek roots]] ''di'' (from ''δί''), meaning 'two', and ''ode'' (from ''οδός''), meaning 'path'. The word ''diode'' however was already in use, as were ''[[triode]], [[tetrode]], [[pentode]], [[hexode]]'', as terms of [[Time-division multiplexing|multiplex]] [[telegraphy]].<ref>{{cite web| url = https://books.google.com/books?id=ifA2AQAAMAAJ&pg=PA252| title = W. H. Preece, "Multiplex Telegraphy", ''The Telegraphic Journal and Electrical Review'', Vol. XIX, September 10, 1886, p. 252| year = 1886}}</ref>

Although all diodes ''rectify'', ''"rectifier"'' usually applies to diodes used for [[power supply]], to differentiate them from diodes intended for [[Signal (electrical engineering)|small signal]] circuits.

==Vacuum tube diodes==
{{Infobox electronic component
| name              = Thermionic diode
| image             = 2-50A 2 (2).JPG
| image_size        = 225px
| caption           = A high power vacuum tube diode used in [[radio]] equipment as a [[rectifier]]
| type              = Active
| working_principle = [[Thermionic emission]]
| inventor          = [[John Ambrose Fleming]]
| invention_Year    = 1904
| pins              = [[Plate electrode|Plate]] and [[Hot cathode|cathode]]
| symbol            = [[File:Vacuum diode.svg|100px]]
| symbol_caption    = The symbol for an indirectly heated vacuum tube diode. From top to bottom, the element names are: ''plate'', ''cathode'', and ''heater''.
}}

A '''thermionic diode''' is a [[vacuum tube]] consisting of a sealed, evacuated glass or metal envelope containing two [[electrode]]s: a [[hot cathode|cathode]] and a [[plate electrode|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 {{cvt|800–1000|C}}. 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.

[[File:5U4GB.agr.jpg|thumb|upright|100px|left|A vacuum tube containing two power diodes]]
The operating temperature of the cathode causes it to release [[electron]]s into the vacuum, a process called [[thermionic emission]]. The cathode is coated with [[oxide]]s of [[alkaline earth metal]]s, such as [[barium]] and [[strontium]] [[oxide]]s. 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.

==Semiconductor diodes==<!-- This section is linked from [[Boltzmann constant]] -->
[[File:EFD108_Point_Contact_Germanium_Diode.jpg|thumb|upright=0.75|Close-up of an EFD108 germanium point-contact diode in DO7 glass package, showing the sharp metal wire (''cat whisker'') that forms the semiconductor junction.]]

===Point-contact diodes===
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.<ref name="Scaff_Ohl_01"/><ref name="SG">{{cite web| url = https://www.semigen.net/point-contact-diodes/| title = SemiGen Inc.}}</ref><ref name="AS">{{cite web| url = http://www.advancedsemiconductor.com/pdf/diodes/SiliconPointContactMixer.pdf| title = Advanced Semiconductor, Inc.| access-date = 2018-05-24| archive-date = 2023-05-21| archive-url = https://web.archive.org/web/20230521233514/http://www.advancedsemiconductor.com/pdf/diodes/SiliconPointContactMixer.pdf| url-status = dead}}</ref><ref name="MB">{{cite web| url = https://massbaytech.com/point-contact-diodes/| title = Massachusetts Bay Technologies}}</ref> Point-contact diodes use a small diameter metal wire in contact with a semiconductor crystal, and are of either ''non-welded'' contact type or ''welded contact'' type. Non-welded contact construction utilizes the Schottky barrier principle. The metal side is the pointed end of a small diameter wire that is in contact with the semiconductor crystal.<ref name="HC">{{cite web| url = https://www.scribd.com/document/37134001/MIT-Radiaton-Lab-Series-V15-Crystal-Rectifiers| title = H. C. Torrey, C. A. Whitmer, ''Crystal Rectifiers'', New York: McGraw-Hill, 1948}}</ref> 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.<ref>{{cite web| url = https://patentimages.storage.googleapis.com/fe/87/8a/7e1064ddfc7d8a/US2704818.pdf| title = H. Q. North, ''Asymmetrically Conductive Device'', U.S. patent 2,704,818}}</ref><ref>{{cite web| url = https://archive.org/stream/neetsmodules_202003/NEETS%20MOD%2011%20NAVEDTRA%2014183A#page/n181/mode/2up| title = U. S. Navy Center for Surface Combat Systems, ''Navy Electricity and Electronics Training Series, Module 11'', 2012, pp. 2-81–2-83}}</ref> Point contact diodes generally exhibit lower capacitance, higher forward resistance and greater reverse leakage than junction diodes.

===Junction diodes===

====p–n junction diode====
{{Main article|p–n diode}}
A p–n junction diode is made of a crystal of [[semiconductor]], usually silicon, but [[germanium]] and [[gallium arsenide]] are also used. Impurities are added to it to create a region on one side that contains negative [[charge carrier]]s (electrons), called an [[n-type semiconductor]], and a region on the other side that contains positive charge carriers ([[Electron hole|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. This region is called the [[depletion region]] because there are no charge carriers (neither electrons nor holes) in it. The diode's terminals are attached to the n-type and p-type regions. The boundary between these two regions, called a [[p–n junction]], is where the action of the diode takes place. When a sufficiently higher [[Electric potential|electrical potential]] is applied to the P side (the [[anode]]) than to the N side (the [[cathode]]), it allows electrons to flow through the depletion region from the N-type side to the P-type side. 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]].

====Schottky diode====
{{Main article|Schottky diode}}
Another type of junction diode, the [[Schottky diode]], is formed from a [[metal–semiconductor junction]] rather than a p–n junction, which reduces capacitance and increases switching speed.<ref name="skyworks_01">{{cite web| url = http://www.skyworksinc.com/uploads/documents/200826A.pdf| title = Skyworks Solutions, Inc., ''Mixer and Detector Diodes''}}</ref><ref>{{cite web| url = https://www.microsemi.com/product-directory/rf-microwave-a-millimeter-wave/1575-diodes-schottky| title = Microsemi Corporation Schottky web page}}</ref>

===Current–voltage characteristic===
A semiconductor diode's behavior in a circuit is given by its [[current–voltage characteristic]]. The shape of the curve is determined by the transport of charge carriers through the so-called ''[[depletion region|depletion layer]]'' or ''[[depletion region]]'' that exists at the [[p–n junction]] between differing semiconductors. When a p–n junction is first created, conduction-band (mobile) electrons from the N-[[dopant|doped]] region diffuse into the P-[[dopant|doped]] region where there is a large population of holes (vacant places for electrons) with which the electrons "recombine". When a mobile electron recombines with a hole, both hole and electron vanish, leaving behind an immobile positively charged donor (dopant) on the N side and negatively charged acceptor (dopant) on the P side. The region around the p–n junction becomes depleted of [[charge carrier]]s and thus behaves as an [[insulator (electricity)|insulator]].

However, the width of the depletion region (called the [[depletion width]]) cannot grow without limit. For each [[electron–hole pair]] recombination made, a positively charged [[dopant]] ion is left behind in the N-doped region, and a negatively charged dopant ion is created in the P-doped region. As recombination proceeds and more ions are created, an increasing electric field develops through the depletion zone that acts to slow and then finally stop recombination. At this point, there is a "built-in" potential across the depletion zone.

[[File:PN band.gif|thumb|upright=2.8<!--max normally 1.8 but WP:IAR-->|none|A [[p–n junction]] diode in low forward bias mode. The [[depletion width]] decreases as voltage increases. Both p and n junctions are doped at a 1e15/cm3 [[doping (semiconductor)|doping]] level, leading to built-in potential of ~0.59V. Observe the different [[quasi Fermi level]]s for conduction band and valence band in n and p regions (red curves).]]

====Reverse bias====
{{See also|p–n diode#Reverse bias}}
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 pair]]s are actively being created in the junction by, for instance, light; see [[photodiode]]).

====Forward bias====
{{See also|p–n diode#Forward bias}}
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.

====Operating regions====
[[File:Diode current wiki.png|thumb|upright=1.4|[[Current–voltage characteristic]] of a p–n junction diode showing three regions: '''breakdown''', '''reverse''' biased, '''forward''' biased. The exponential's "knee" is at ''V''<sub>d</sub>. The leveling off region which occurs at larger forward currents is not shown.]]

A diode's [[current–voltage characteristic]] can be approximated by four operating regions. From lower to higher bias voltages, these are:

* '''Breakdown''': At very large reverse bias, beyond the [[peak inverse voltage]] (PIV), a process called reverse [[avalanche breakdown|breakdown]] occurs that causes a large increase in current (i.e., a large number of electrons and holes are created at, and move away from the p–n junction) that usually damages the device permanently. The [[avalanche diode]] is deliberately designed for use in that manner. In the [[Zener diode]], the concept of PIV is not applicable. A Zener diode contains a heavily doped p–n junction allowing electrons to tunnel from the valence band of the p-type material to the conduction band of the n-type material, such that the reverse voltage is "clamped" to a known value (called the ''Zener voltage''), and avalanche does not occur. Both devices, however, do have a limit to the maximum current and power they can withstand in the clamped reverse-voltage region. Also, following the end of forwarding conduction in any diode, there is reverse current for a short time. The device does not attain its full blocking capability until the reverse current ceases.
* '''Reverse biased''': For a bias between breakdown and 0 V, the reverse current is very small and asymptotically approaches -''I''<sub>s</sub>. For a normal P–N rectifier diode, the reverse current through the device is in the micro-ampere (μA) range. However, this is temperature dependent, and at sufficiently high temperatures, a substantial amount of reverse current can be observed (mA or more). There is also a tiny surface leakage current caused by electrons simply going around the diode as though it were an imperfect insulator.[[File:DiodeGenCharacteristics1.jpg|right|thumb|500x500px|[[Semi-log]] I–V (logarithmic current vs. linear voltage) graph of various diodes.]]
* '''Forward biased''': The current–voltage curve is [[Exponential function|exponential]], approximating the [[Shockley diode equation]]. When plotted using a linear current scale, a smooth "[[Knee of a curve|knee]]" appears, but no clear threshold voltage is visible on a semi-log graph.
* '''Leveling off''': At larger forward currents the current–voltage curve starts to be dominated by the ohmic resistance of the bulk semiconductor. The curve is no longer exponential, it is asymptotic to a straight line whose slope is the bulk resistance. This region is particularly important for power diodes and can be modeled by a ''Shockley ideal diode'' in series with a fixed resistor.

===Shockley diode equation===
{{main article|Shockley diode equation}}
The ''Shockley ideal diode equation'' or the ''diode law'' (named after the [[bipolar junction transistor]] co-inventor [[William Shockley|William Bradford Shockley]]) [[Conceptual model|models]] the [[Exponential function|exponential]] [[Current–voltage characteristic|current–voltage (I–V) relationship]] of diodes in moderate forward or reverse bias. The article [[Shockley diode equation]] provides details.

===Small-signal behavior===
At forward voltages less than the saturation voltage, the voltage versus current characteristic curve of most diodes is not a straight line. The current can be approximated by <math>I = I_\text{S} e^{V_\text{D}/(n V_\text{T})}</math> as explained in the [[Shockley diode equation]] article.

In detector and mixer applications, the current can be estimated by a Taylor's series.<ref name="Giacoletto_1977">{{cite book |author-first=Lawrence Joseph |author-last=Giacoletto |title=Electronics Designers' Handbook |location=New York |publisher=[[McGraw-Hill]] |date=1977 |pages=24–138}}</ref> The odd terms can be omitted because they produce frequency components that are outside the pass band of the mixer or detector. Even terms beyond the second derivative usually need not be included because they are small compared to the second order term.<ref name="Giacoletto_1977" /> The desired current component is approximately proportional to the square of the input voltage, so the response is called ''[[square-law detector|square law]]'' in this region.<ref name="HC" />{{rp|p. 3}}

===Reverse-recovery effect===
Following the end of forwarding conduction in a p–n type diode, a reverse current can flow for a short time. The device does not attain its blocking capability until the mobile charge in the junction is depleted.

The effect can be significant when switching large currents very quickly.<ref>[http://ecee.colorado.edu/~ecen5817/hw/hw1/Diode%20reverse%20recovery%20in%20a%20boost%20converter.pdf Diode reverse recovery in a boost converter] {{Webarchive|url=https://web.archive.org/web/20111007214034/http://ecee.colorado.edu/~ecen5817/hw/hw1/Diode%20reverse%20recovery%20in%20a%20boost%20converter.pdf |date=2011-10-07 }}. ECEN5817. ecee.colorado.edu</ref> A certain amount of "reverse recovery time" {{mvar|t}}<sub>r</sub> (on the order of tens of nanoseconds to a few microseconds) may be required to remove the reverse recovery charge {{mvar|Q}}<sub>r</sub> from the diode. During this recovery time, the diode can actually conduct in the reverse direction. This might give rise to a large current in the reverse direction for a short time while the diode is reverse biased. 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.<ref>{{Cite journal | doi = 10.1109/LED.2014.2353301| title = Gate-Controlled Reverse Recovery for Characterization of LDMOS Body Diode| journal = IEEE Electron Device Letters| volume = 35| issue = 11| page = 1079| year = 2014| last1 = Elhami Khorasani | first1 = A. | last2 = Griswold | first2 = M. | last3 = Alford | first3 = T. L.|bibcode = 2014IEDL...35.1079E | s2cid = 7012254}}</ref> <!-- That is to say, current will effectively flow from the cathode to the anode! --> In certain real-world cases it is important to consider the losses that are incurred by this non-ideal diode effect.<ref>[http://ecee.colorado.edu/~ecen5797/course_material/SwLossSlides.pdf Inclusion of Switching Loss in the Averaged Equivalent Circuit Model] {{Webarchive|url=https://web.archive.org/web/20111007214049/http://ecee.colorado.edu/~ecen5797/course_material/SwLossSlides.pdf |date=2011-10-07 }}. ECEN5797. ecee.colorado.edu</ref> However, when the [[slew rate]] of the current is not so severe (e.g. Line frequency) the effect can be safely ignored. For most applications, the effect is also negligible for [[Schottky diode]]s.

The reverse current ceases abruptly when the stored charge is depleted; this abrupt stop is exploited in [[step recovery diode]]s for the generation of extremely short pulses.

===Types of semiconductor diode===
[[File:Forward_and_Reverse_Characteristics_for_diodes-en.svg|thumb|upright=1.4|[[Current–voltage characteristic|Current–voltage curves]] of several types of diodes]]
Normal (p–n) diodes, which operate as described above, are usually made of doped [[silicon]] or [[germanium]]. Before the development of silicon power rectifier diodes, [[cuprous oxide]] and later [[selenium]] was used. Their low efficiency required a much higher forward voltage to be applied (typically 1.4 to 1.7&nbsp;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 (semiconductor)|substrate]]), much larger than the later silicon diode of the same current ratings would require. The vast majority of all diodes are the p–n diodes found in [[CMOS]] [[integrated circuits]],<ref>{{Cite journal|last=Roddick|first=R.G.|title=Tunnel Diode Circuit Analysis|date=1962-10-01|doi=10.2172/4715062|url=https://digital.library.unt.edu/ark:/67531/metadc1033487/}}</ref> which include two diodes per pin and many other internal diodes.

; [[Avalanche diode]]s
: These are diodes that conduct in the reverse direction when the reverse bias voltage exceeds the breakdown voltage. These are electrically very similar to Zener diodes (and are often mistakenly called Zener diodes), but break down by a different mechanism: the ''avalanche effect''. This occurs when the reverse electric field applied across the p–n junction causes a wave of ionization, reminiscent of an avalanche, leading to a large current. Avalanche diodes are designed to break down at a well-defined reverse voltage without being destroyed. The difference between the avalanche diode (which has a reverse breakdown above about 6.2&nbsp;V) and the Zener is that the channel length of the former exceeds the mean free path of the electrons, resulting in many collisions between them on the way through the channel. The only practical difference between the two types is they have temperature coefficients of opposite polarities.
; [[Constant-current diode]]s
: These are actually [[JFET]]s<ref>[http://digikey.com/Web%20Export/Supplier%20Content/Vishay_8026/PDF/Vishay_CurrentRegulatorDiodes.pdf Current regulator diodes]. Digikey.com (2009-05-27). Retrieved 2013-12-19.</ref> with the gate shorted to the source, and function like a two-terminal current-limiting analog to the voltage-limiting Zener diode. They allow a current through them to rise to a certain value, and then level off at a specific value. Also called ''CLDs'', ''constant-current diodes'', ''diode-connected transistors'', or ''current-regulating diodes''.
; [[#Point-contact diodes|Crystal rectifiers or crystal diodes]]
: These are point-contact diodes.<ref name="HC" /> The 1N21 series and others are used in mixer and detector applications in radar and microwave receivers.<ref name="SG" /><ref name="AS"/><ref name="MB"/> The 1N34A is another example of a crystal diode.<ref>{{cite web| url = http://www.nteinc.com/specs/original/1N34A.pdf| title = NTE data sheet}}</ref>
; [[Gunn diode]]s
: These are similar to tunnel diodes in that they are made of materials such as GaAs or InP that exhibit a region of [[negative resistance|negative differential resistance]]. With appropriate biasing, dipole domains form and travel across the diode, allowing high frequency [[microwave]] [[electronic oscillator|oscillators]] to be built.
;[[Light-emitting diode]]s (LEDs)
:In a diode formed from a [[Direct bandgap|direct band-gap]] semiconductor, such as [[gallium arsenide]], charge carriers that cross the junction emit [[photon]]s when they recombine with the majority carrier on the other side. Depending on the material, [[wavelength]]s (or colors)<ref>[http://digikey.com/Web%20Export/Supplier%20Content/Vishay_8026/PDF/Vishay_ClassificationOfComponents.pdf Classification of components]. Digikey.com (2009-05-27). Retrieved 2013-12-19.</ref> from the [[infrared]] to the near [[ultraviolet]] may be produced.<ref>{{cite web |url=http://www.element-14.com/community/docs/DOC-22517/l/component-construction--vishay-optoelectronics |title=Component Construction |date=2010-05-25 |access-date=2010-08-06 |archive-url=http://arquivo.pt/wayback/20160516081713/http://www.element-14.com/community/docs/DOC-22517/l/component-construction--vishay-optoelectronics |archive-date=2016-05-16 |url-status=dead }}</ref> The first LEDs were red and yellow, and higher-frequency diodes have been developed over time. All LEDs produce incoherent, narrow-spectrum light; [[Light-emitting diode#White|"white" LEDs]] are actually a blue LED with a yellow [[scintillator]] coating, or combinations of three LEDs of a different color. LEDs can also be used as low-efficiency photodiodes in signal applications. An LED may be paired with a photodiode or phototransistor in the same package, to form an [[opto-isolator]].
; [[Laser diode]]s
: When an LED-like structure is contained in a [[optical cavity|resonant cavity]] formed by polishing the parallel end faces, a [[laser]] can be formed. Laser diodes are commonly used in [[optical storage]] devices and for high speed [[optical communication]].
; [[Thermal diode]]s
: This term is used both for conventional p–n diodes used to monitor temperature because of their varying forward voltage with temperature, and for [[Peltier–Seebeck effect|Peltier heat pumps]] for [[thermoelectric cooling|thermoelectric heating and cooling]]. Peltier heat pumps may be made from semiconductors, though they do not have any rectifying junctions, they use the differing behavior of charge carriers in N and P-type semiconductor to move heat.
; [[Photodiode]]s
: All semiconductors are subject to optical [[charge carrier]] generation. This is typically an undesired effect, so most semiconductors are packaged in light-blocking material. Photodiodes are intended to sense light ([[photodetector]]), so they are packaged in materials that allow light to pass, and are usually PIN (the kind of diode most sensitive to light).<ref>[http://digikey.com/Web%20Export/Supplier%20Content/Vishay_8026/PDF/Vishay_ComponentConstruction.pdf Component Construction]. Digikey.com (2009-05-27). Retrieved 2013-12-19.</ref> A photodiode can be used in [[solar cell]]s, in [[photometry (optics)|photometry]], or in [[optical communication]]s. Multiple photodiodes may be packaged in a single device, either as a linear array or as a two-dimensional array. These arrays should not be confused with [[charge-coupled device]]s.
; [[PIN diode]]s
: A PIN diode has a central un-doped, or ''intrinsic'', layer, forming a p-type/intrinsic/n-type structure.<ref>{{cite web |url=http://www.element-14.com/community/docs/DOC-22516/l/physics-and-technology--vishay-optoelectronics |title=Physics and Technology |date=2010-05-25 |access-date=2010-08-06 |archive-url=http://arquivo.pt/wayback/20160516081725/http://www.element-14.com/community/docs/DOC-22516/l/physics-and-technology--vishay-optoelectronics |archive-date=2016-05-16 |url-status=dead }}</ref> They are used as radio frequency switches and attenuators. They are also used as large-volume, ionizing-radiation detectors and as [[photodetector]]s. PIN diodes are also used in [[power electronics]], as their central layer can withstand high voltages. Furthermore, the PIN structure can be found in many [[power semiconductor device]]s, such as [[IGBT]]s, power [[MOSFET]]s, and [[thyristor]]s.
;[[Schottky diode]]s
:[[Walter H. Schottky|Schottky]] diodes are constructed from metal to semiconductor contact. They have a lower forward voltage drop than p–n junction diodes. Their forward voltage drop at forward currents of about 1&nbsp;mA is in the range 0.15&nbsp;V to 0.45&nbsp;V, which makes them useful in voltage [[Clamper (electronics)|clamping applications]] and prevention of transistor saturation. They can also be used as low loss [[rectifier]]s, although their reverse leakage current is in general higher than that of other diodes. Schottky diodes are [[majority carrier]] devices and so do not suffer from minority carrier storage problems that slow down many other diodes—so they have a faster reverse recovery than p–n junction diodes. They also tend to have much lower junction capacitance than p–n diodes, which provides for high switching speeds and their use in high-speed circuitry and RF devices such as [[switched-mode power supply]], [[Frequency mixer|mixers]], and [[Detector (radio)|detectors]].
; Super barrier diodes
: Super barrier diodes are rectifier diodes that incorporate the low forward voltage drop of the Schottky diode with the surge-handling capability and low reverse leakage current of a normal p–n junction diode.
;[[Gold]]-doped diodes
: As a dopant, gold (or [[platinum]]) acts as recombination centers, which helps the fast recombination of minority carriers. This allows the diode to operate at higher signal frequencies, at the expense of a higher forward voltage drop. Gold-doped diodes are faster than other p–n diodes (but not as fast as Schottky diodes). They also have less reverse-current leakage than Schottky diodes (but not as good as other p–n diodes).<ref>[http://www.ixyspower.com/images/technical_support/Application%20Notes%20By%20Topic/FREDs,%20Schottky%20and%20GaAS%20Diodes/IXAN0044.pdf Fast Recovery Epitaxial Diodes (FRED) Characteristics – Applications – Examples] {{Webarchive|url=https://web.archive.org/web/20090326113147/http://www.ixyspower.com/images/technical_support/Application%20Notes%20By%20Topic/FREDs,%20Schottky%20and%20GaAS%20Diodes/IXAN0044.pdf |date=2009-03-26 }}. (PDF). Retrieved 2013-12-19.</ref><ref>Sze, S. M. (1998) ''Modern Semiconductor Device Physics'', Wiley Interscience, {{ISBN|0-471-15237-4}}</ref> A typical example is the 1N914.
; Snap-off or [[step recovery diode]]s
: The term ''step recovery'' relates to the form of the reverse recovery characteristic of these devices. After a forward current has been passing in an [[Step recovery diode|SRD]] and the current is interrupted or reversed, the reverse conduction will cease very abruptly (as in a step waveform). SRDs can, therefore, provide very fast voltage transitions by the very sudden disappearance of the charge carriers.
; [[Stabistor]]s or ''forward reference diodes''
: The term ''stabistor'' refers to a special type of diodes featuring extremely stable [[p–n junction#Forward bias|forward voltage]] characteristics. These devices are specially designed for low-voltage stabilization applications requiring a guaranteed voltage over a wide current range and highly stable over temperature.
;[[Transient voltage suppression diode]] (TVS)
: These are avalanche diodes designed specifically to protect other semiconductor devices from high-voltage [[Transient (oscillation)|transients]].<ref>[http://digikey.com/Web%20Export/Supplier%20Content/Vishay_8026/PDF/Vishay_ProtectingLowCurrentLoads.pdf Protecting Low Current Loads in Harsh Electrical Environments]. Digikey.com (2009-05-27). Retrieved 2013-12-19.</ref> Their p–n junctions have a much larger cross-sectional area than those of a normal diode, allowing them to conduct large currents to ground without sustaining damage.
; [[Tunnel diode]]s or [[Leo Esaki|Esaki diodes]]
: These have a region of operation showing [[negative resistance]] caused by [[quantum tunneling]],<ref>{{cite journal|author=Jonscher, A. K. |doi=10.1088/0508-3443/12/12/304|title=The physics of the tunnel diode|year=1961|journal=British Journal of Applied Physics|volume=12|issue=12|page=654|bibcode = 1961BJAP...12..654J }}</ref> allowing amplification of signals and very simple bistable circuits. Because of the high carrier concentration, tunnel diodes are very fast, may be used at low (mK) temperatures, high magnetic fields, and in high radiation environments.<ref>{{cite journal|author1=Dowdey, J. E. |author2=Travis, C. M. |doi= 10.1109/TNS2.1964.4315475|title=An Analysis of Steady-State Nuclear Radiation Damage of Tunnel Diodes|year=1964|journal=IEEE Transactions on Nuclear Science|volume=11|issue=5|page=55|bibcode = 1964ITNS...11...55D }}</ref> Because of these properties, they are often used in spacecraft.
; [[Varicap]] or varactor diodes
: These are used as voltage-controlled [[capacitors]]. These are important in PLL ([[phase-locked loop]]) and FLL ([[frequency-locked loop]]) circuits, allowing tuning circuits, such as those in television receivers, to lock quickly on to the frequency. They also enabled tunable oscillators in the early discrete tuning of radios, where a cheap and stable, but fixed-frequency, crystal oscillator provided the reference frequency for a [[voltage-controlled oscillator]].
; [[Zener diode]]s
: These can be made to conduct in reverse bias (backward), and are correctly termed reverse breakdown diodes. This effect called [[Zener breakdown]], occurs at a precisely defined voltage, allowing the diode to be used as a precision voltage reference. The term Zener diodes is colloquially applied to several types of breakdown diodes, but strictly speaking, Zener diodes have a breakdown voltage of below 5 volts, whilst avalanche diodes are used for breakdown voltages above that value. In practical voltage reference circuits, Zener and switching diodes are connected in series and opposite directions to balance the temperature coefficient response of the diodes to near-zero. Some devices labeled as high-voltage Zener diodes are actually avalanche diodes (see above). Two (equivalent) Zeners in series and in reverse order, in the same package, constitute a transient absorber (or [[Transorb]], a registered trademark).

===Graphic symbols===
{{Main article|Electronic symbol}}

The symbol used to represent a particular type of diode in a [[circuit diagram]] conveys the general electrical function to the reader. There are alternative symbols for some types of diodes, though the differences are minor. The triangle in the symbols points to the forward direction, i.e. in the direction of [[conventional current]] flow.

<gallery>
File:Diode symbol.svg|Diode
File:LED symbol.svg|[[Light-emitting diode]] (LED)
File:Photodiode symbol.svg|[[Photodiode]]
File:Schottky diode symbol.svg|[[Schottky diode]]
File:Transient voltage suppression diode symbol.svg|[[Transient-voltage-suppression diode]] (TVS)
File:Tunnel diode symbol.svg|[[Tunnel diode]]
File:Varicap symbol.svg|[[Varicap]]
File:Zener diode symbol.svg|[[Zener diode]]
File:Diode pinout en fr.svg|Typical diode packages in same alignment as diode symbol. The thin bar depicts the [[cathode]].
</gallery>

===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 [[Electronic Industries Alliance|EIA]]/[[JEDEC]] standard and the European [[Pro Electron]] standard:

====EIA/JEDEC====
The standardized 1N-series numbering ''[[JEDEC#Origins|EIA370]]'' system was introduced in the US by EIA/JEDEC (Joint Electron Device Engineering Council) about 1960. Most diodes have a 1-prefix designation (e.g., 1N4003). Among the most popular in this series were: 1N34A/1N270 (germanium signal), 1N914/[[1N4148 signal diode|1N4148]] (silicon signal), [[1N400x general-purpose diodes|1N400x]] (silicon 1A power rectifier), and [[1N58xx schottky diodes|1N580x]] (silicon 3A power rectifier).<ref>{{cite web|url=http://www.jedec.org/Home/about_jedec.cfm |title=About JEDEC |publisher=Jedec.org |access-date=2008-09-22}}</ref><ref>{{cite web|url=http://news.elektroda.net/introduction-dates-of-common-transistors-and-diodes-t94332.html |title=Introduction dates of common transistors and diodes? |publisher=EDAboard.com |date=2010-06-10 |access-date=2010-08-06 |url-status=dead |archive-url=https://web.archive.org/web/20071011133032/http://news.elektroda.net/introduction-dates-of-common-transistors-and-diodes-t94332.html |archive-date=October 11, 2007 }}</ref><ref>{{cite web|url=http://semiconductormuseum.com/Museum_Index.htm |title=Transistor Museum Construction Projects Point Contact Germanium Western Electric Vintage Historic Semiconductors Photos Alloy Junction Oral History |publisher=Semiconductormuseum.com |author=I.D.E.A |access-date=2008-09-22}}</ref>

====JIS====
The [[JIS semiconductor designation]] system has all semiconductor diode designations starting with "1S".

====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:
* 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)

Other common numbering/coding systems (generally manufacturer-driven) include:
* GD-series germanium diodes (e.g., GD9){{spaced ndash}}this is a very old coding system
* OA-series germanium diodes (e.g., OA47){{spaced ndash}}a [[Mullard–Philips tube designation|coding sequence]] developed by [[Mullard]], a UK company

==Related devices==
* [[Rectifier]]
* [[Transistor]]
* [[Thyristor]] or silicon controlled rectifier (SCR)
* [[TRIAC]]
* [[DIAC]]
* [[Varistor]]
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. It uses a [[Faraday rotator]] as the main component.<ref>{{cite book |doi=10.1016/B978-0-12-805345-4.00006-8 |chapter=Passive optical components |title=Introduction to Fiber-Optic Communications |date=2020 |last1=Hui |first1=Rongqing |pages=209–297 |isbn=978-0-12-805345-4 }}</ref>

==Applications==

===Radio demodulation===
[[File:Simple envelope detector.svg|thumb|300x300px|A simple [[envelope detector|envelope demodulator]] circuit.]]
The first use for the diode was the demodulation of [[amplitude modulation|amplitude modulated]] (AM) radio broadcasts. The history of this discovery is treated in depth in the [[crystal detector]] article. In summary, an AM signal consists of alternating positive and negative peaks of a radio carrier wave, whose [[amplitude]] or envelope is proportional to the original audio signal. The diode [[rectifier|rectifies]] the AM radio frequency signal, leaving only the positive peaks of the carrier wave. The audio is then extracted from the rectified carrier wave using a simple [[electronic filter|filter]] and fed into an audio amplifier or [[transducer]], which generates sound waves via [[Loudspeaker|audio speaker]].

In microwave and millimeter wave technology, beginning in the 1930s, researchers improved and miniaturized the crystal detector. [[#Point-contact diodes|Point contact diodes]] (''crystal diodes'') and [[#Junction diodes|Schottky diodes]] are used in radar, microwave and millimeter wave detectors.<ref name="skyworks_01"/>

===Power conversion===
{{Main article|Rectifier}}
[[File:ACtoDCpowersupply.png|250px|thumb|Schematic of basic AC-to-DC power supply]]

[[Rectifier]]s are constructed from diodes, where they are used to convert [[alternating current]] (AC) electricity into [[direct current]] (DC). Automotive [[alternator (auto)|alternators]] are a common example, where the diode, which rectifies the AC into DC, provides better performance than the [[Commutator (electric)|commutator]] or earlier, [[electrical generator|dynamo]]. Similarly, diodes are also used in ''[[Cockcroft-Walton generator|Cockcroft–Walton]] [[voltage multiplier]]s'' to convert AC into higher DC voltages.

===Reverse-voltage protection===
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. This concept is known by multiple naming variations that mean the same thing: reverse voltage protection, reverse polarity protection, and reverse battery protection.

===Over-voltage protection===
Diodes are frequently used to conduct damaging high voltages away from sensitive electronic devices. They are usually reverse-biased (non-conducting) under normal circumstances. When the voltage rises above the normal range, the diodes become forward-biased (conducting). For example, diodes are used in ([[stepper motor]] and [[H-bridge]]) [[motor controller]] and [[relay]] circuits to de-energize coils rapidly without the damaging [[voltage spike]]s that would otherwise occur. (A diode used in such an application is called a [[flyback diode]]). Many [[integrated circuits]] also incorporate diodes on the connection pins to prevent external voltages from damaging their sensitive [[transistors]]. Specialized diodes are used to protect from over-voltages at higher power (see [[#Types of semiconductor diode|Diode types]] above).

===Logic gates===
{{Main|Diode logic}}
[[Diode logic|Diode–resistor logic]] constructs [[logical conjunction|AND]] and [[logical disjunction|OR]] [[logic gate]]s. [[Functional completeness]] can be achieved by adding an active device to provide [[Inverter (logic gate)|inversion]] (as done with [[diode–transistor logic]]).

===Ionizing radiation detectors===
In addition to light, mentioned above, [[semiconductor]] diodes are sensitive to more [[energy|energetic]] radiation. In [[electronics]], [[cosmic ray]]s and other sources of ionizing radiation cause [[noise]] [[pulse]]s and single and multiple bit errors. This effect is sometimes exploited by [[particle detector]]s to detect radiation. A single particle of radiation, with thousands or millions of [[electron volt]], s of energy, generates many charge carrier pairs, as its energy is deposited in the semiconductor material. If the depletion layer is large enough to catch the whole shower or to stop a heavy particle, a fairly accurate measurement of the particle's energy can be made, simply by measuring the charge conducted and without the complexity of a magnetic spectrometer, etc. These semiconductor radiation detectors need efficient and uniform charge collection and low leakage current. They are often cooled by [[liquid nitrogen]]. For longer-range (about a centimeter) particles, they need a very large depletion depth and large area. For short-range particles, they need any contact or un-depleted semiconductor on at least one surface to be very thin. The back-bias voltages are near breakdown (around a thousand volts per centimeter). Germanium and silicon are common materials. Some of these detectors sense position as well as energy. They have a finite life, especially when detecting heavy particles, because of radiation damage. Silicon and germanium are quite different in their ability to convert [[gamma ray]]s to electron showers.

[[Semiconductor detector]]s for high-energy particles are used in large numbers. Because of energy loss fluctuations, accurate measurement of the energy deposited is of less use.

===Temperature measurements===
A diode can be used as a [[temperature]] measuring device, since the forward voltage drop across the diode depends on temperature, as in a [[silicon bandgap temperature sensor]]. From the Shockley ideal diode equation given above, it might ''appear'' that the voltage has a ''positive'' temperature coefficient (at a constant current), but usually the variation of the [[Saturation current|reverse saturation current]] term is more significant than the variation in the thermal voltage term. Most diodes therefore have a ''negative'' temperature coefficient, typically −2 mV/°C for silicon diodes. The temperature coefficient is approximately constant for temperatures above about 20 [[kelvin]]. Some graphs are given for 1N400x series,<ref>{{cite web |url=http://www.cliftonlaboratories.com/1n400x_diode_family_forward_voltage.htm |title=1N400x Diode Family Forward Voltage |website=cliftonlaboratories.com |access-date=2013-12-19 |archive-url=https://web.archive.org/web/20130524153406/http://www.cliftonlaboratories.com/1n400x_diode_family_forward_voltage.htm |archive-date=2013-05-24}}</ref> and CY7 cryogenic temperature sensor.<ref>[http://www.omega.com/Temperature/pdf/CY7.pdf Cryogenic Temperature Sensors]. omega.com</ref>

===Current steering===
Diodes will prevent currents in unintended directions. To supply power to an electrical circuit during a power failure, the circuit can draw current from a [[Battery (electricity)|battery]]. An [[uninterruptible power supply]] may use diodes in this way to ensure that the current is only drawn from the battery when necessary. Likewise, small boats typically have two circuits each with their own battery/batteries: one used for engine starting; one used for domestics. Normally, both are charged from a single alternator, and a heavy-duty split-charge diode is used to prevent the higher-charge battery (typically the engine battery) from discharging through the lower-charge battery when the alternator is not running.

Diodes are also used in [[electronic keyboards|electronic musical keyboards]]. To reduce the amount of wiring needed in electronic musical keyboards, these instruments often use [[keyboard matrix circuit]]s. The keyboard controller scans the rows and columns to determine which note the player has pressed. The problem with matrix circuits is that, when several notes are pressed at once, the current can flow backward through the circuit and trigger "[[Keyboard (computing)#Control processor|phantom keys]]" that cause "ghost" notes to play. To avoid triggering unwanted notes, most keyboard matrix circuits have diodes soldered with the switch under each key of the [[musical keyboard]]. The same principle is also used for the switch matrix in solid-state [[pinball machine]]s.

===Waveform clipper===
{{Main article|Clipper (electronics)}}
Diodes can be used to limit the positive or negative excursion of a signal to a prescribed voltage.

===Clamper===
{{Main article|Clamper (electronics)}}
[[File:DiodeClamp.png|150px|thumb|This simple diode clamp will clamp the negative peaks of the incoming waveform to the common rail voltage]]
A diode [[Clamper (electronics)|clamp circuit]] can take a periodic alternating current signal that oscillates between positive and negative values, and vertically displace it such that either the positive or the negative peaks occur at a prescribed level. The clamper does not restrict the peak-to-peak excursion of the signal, it moves the whole signal up or down so as to place the peaks at the reference level.

===Computing exponentials and logarithms===
The diode's exponential current–voltage relationship is exploited to evaluate [[exponentiation]] and its [[inverse function]] the [[logarithm]] using analog voltage signals (see {{Slink|Operational amplifier applications|Exponential output|Logarithmic output}}).

===Oscillator===
It is possible to modify a regular semiconductor diode like 1N4148 to give it a negative differential resistance by injection of calibrated current pulses ,the diode being reversely biased  near its avalanche zone .After this treatment the diode associated with an L/C circuit can oscillate , the frequency set by the L/C circuit .The maximum frequency depends on the diode used .With a 1N4148 oscillation can go up to 100 Mhz  (see https://www.researchgate.net/publication/384043395_Another_way_to_create_negative_differential_resistance_Author)

==Abbreviations==
Diodes are usually referred to as ''D'' for diode on [[printed circuit board|PCBs]]. Sometimes the abbreviation ''CR'' for ''crystal rectifier'' is used.<ref>{{cite book|author=John Ambrose Fleming|year=1919|url=https://archive.org/details/principleselect01flemgoog|title=The Principles of Electric Wave Telegraphy and Telephony|place=London|publisher=Longmans, Green|page=[https://archive.org/details/principleselect01flemgoog/page/n588 550]}}</ref>

==See also==
{{Portal|Electronics}}
* [[Active rectification]]
* [[Diode-connected transistor]]
* [[Diode modelling]]
* [[fast diode|Fast/ultrafast diode]]
* [[Flame rectification]]
* [[Lambda diode]]
* [[Lr-diode]]
* [[p–n junction]]
* [[Small-signal model]]

==References==
{{Reflist}}

==Further reading==
;Historical circuit books
* ''50 Simple LED Circuits''; 1st Ed; R.N. Soar; Babani Press; 62 pages; 1977; {{ISBN|978-0859340434}}. <small>[https://www.americanradiohistory.com/Archive-Bookshelf/Author-Groups/Babani/42-Soar-50-Simple-LED-circuits.pdf (archive)]</small>
* ''38 Practical Tested Diode Circuits For the Home Constructor''; 1st Ed; Bernard Babani; Krisson Printing; 48 pages; 1972. <small>[https://www.americanradiohistory.com/Archive-Bookshelf/Author-Groups/Bernards%20Radio%20Manuals/09-38-Practical-Tested-Diode-Circuits.pdf (archive)]</small>
* ''Diode Circuits Handbook''; 1st Ed; Rufus Turner; Howard Sams & Co; 128 pages; 1963; LCCN 63-13904. <small>[https://www.americanradiohistory.com/Archive-Bookshelf/Author-Groups/Rufus-Turner/Diode-Circuits-Handbook-Rufus-Turner.pdf (archive)]</small>
* ''40 Uses for Germanium Diodes''; 2nd Ed; Sylvania Electric Products; 47 pages; 1949. <small>[http://n4trb.com/AmateurRadio/SemiconductorHistory/40%20Uses%20for%20Germanium%20Diodes.pdf (archive)]</small>

;Historical periodicals
* ''Rectifier Applications Handbook''; On Semiconductor; 270 pages; 2001. <small>[https://web.archive.org/web/20090407093106/http://www.ieeta.pt/~alex/docs/ApplicationNotes/Rectifier%20Applications%20Handbook.pdf (archive)]</small>
* ''Silicon Rectifier Handbook''; 1st Ed; Bob Dale; Motorola; 213 pages; 1966. <small>[https://archive.org/details/RectifiersSCRsTriacs-MotorolaSiliconRectifierHandbook1966OCR/ (archive)]</small>
* ''Electronic Rectification''; F.G. Spreadbury; D. Van Nostrand Co; 1962.
* ''Zener Diode Handbook''; International Rectifier; 96 pages; 1960.
* ''F.T. Selenium Rectifier Handbook''; 2nd Ed; Federal Telephone and Radio; 80 pages; 1953. <small>[https://www.americanradiohistory.com/Archive-Bookshelf/Technology/Federal-Selenium-Rectifier-2nd-1953.pdf (archive)]</small>
* ''S.T. Selenium Rectifier Handbook''; 1st Ed; Sarkes Tarzian; 80 pages; 1950. <small>[https://archive.org/details/Selenium_Rectifier_Handbook_Sarkes-Tarzian_Company/ (archive)]</small>

;Historical databooks
* [https://archive.org/details/bitsavers_nationaldaDiscreteSemiconductorProducts_42478563 Discrete Databook]; 1989; National Semiconductor (now Texas Instruments)
* [https://archive.org/details/bitsavers_fairchilddldDiscreteDataBook_35122751 Discrete Databook]; 1985; Fairchild (now ON Semiconductor)
* [https://archive.org/details/bitsavers_sgsdataBooDevices5ed_46325378 Discrete Databook]; 1982; SGS (now STMicroelectronics)
* [https://archive.org/details/1965MotorolaSemiconductorDataManual Semiconductor Databook]; 1965; Motorola (now ON Semiconductor)

==External links==
{{Commons category|Diodes}}

{{Electronic component}}
{{Authority control}}

[[Category:1904 introductions]]
[[Category:Diodes| ]]
[[Category:Semiconductor devices]]