Editing Diode (section)

==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