![Dioden2](https://upload.wikimedia.org/wikipedia/commons/6/60/Dioden2.jpg)
A diode is a two-
terminal
Terminal may refer to:
Computing Hardware
* Terminal (electronics), a device for joining electrical circuits together
* Terminal (telecommunication), a device communicating over a line
* Computer terminal, a set of primary input and output dev ...
electronic component that conducts
current
Currents, Current or The Current may refer to:
Science and technology
* Current (fluid), the flow of a liquid or a gas
** Air current, a flow of air
** Ocean current, a current in the ocean
*** Rip current, a kind of water current
** Current (stre ...
primarily in one direction (asymmetric
conductance); it has low (ideally zero)
resistance in one direction, and high (ideally infinite)
resistance in the other.
A diode
vacuum tube
A vacuum tube, electron tube, valve (British usage), or tube (North America), is a device that controls electric current flow in a high vacuum between electrodes to which an electric potential difference has been applied.
The type known as ...
or thermionic diode is a vacuum tube with two
electrode
An electrode is an electrical conductor used to make contact with a nonmetallic part of a circuit (e.g. a semiconductor, an electrolyte, a vacuum or air). Electrodes are essential parts of batteries that can consist of a variety of materials d ...
s, a heated
cathode
A cathode is the electrode from which a conventional current leaves a polarized electrical device. This definition can be recalled by using the mnemonic ''CCD'' for ''Cathode Current Departs''. A conventional current describes the direction in wh ...
and a
plate
Plate may refer to:
Cooking
* Plate (dishware), a broad, mainly flat vessel commonly used to serve food
* Plates, tableware, dishes or dishware used for setting a table, serving food and dining
* Plate, the content of such a plate (for example: ...
, in which electrons can flow in only one direction, from cathode to plate.
A semiconductor diode, the most commonly used type today, is a
crystalline
A crystal or crystalline solid is a solid material whose constituents (such as atoms, molecules, or ions) are arranged in a highly ordered microscopic structure, forming a crystal lattice that extends in all directions. In addition, macros ...
piece of
semiconductor
A semiconductor is a material which has an electrical conductivity value falling between that of a conductor, such as copper, and an insulator, such as glass. Its resistivity falls as its temperature rises; metals behave in the opposite way. ...
material with a
p–n junction connected to two electrical terminals. Semiconductor diodes were the first
semiconductor electronic devices. The discovery of asymmetric electrical conduction across the contact between a crystalline mineral and a metal was made by German physicist
Ferdinand Braun
Karl Ferdinand Braun (; 6 June 1850 – 20 April 1918) was a German electrical engineer, inventor, physicist and Nobel laureate in physics. Braun contributed significantly to the development of radio and television technology: he shared the ...
in 1874. Today, most diodes are made of
silicon
Silicon is a chemical element with the symbol Si and atomic number 14. It is a hard, brittle crystalline solid with a blue-grey metallic luster, and is a tetravalent metalloid and semiconductor. It is a member of group 14 in the periodic ta ...
, but other semiconducting materials such as
gallium arsenide and
germanium are also used.
Among many uses, diodes are found in
rectifiers to convert AC power to DC,
demodulation in radio receivers, and can even be used as
temperature sensor Mechanical temperature sensors
* Thermometer
* Therm
Electrical temperature sensors
* Thermistor- Thermistors are thermally sensitive resistors whose prime function is to exhibit a large, predictable and precise change in electrical resistance wh ...
s. A common variant of a diode is a
light emitting diode, which is used as
electric lighting and status indicators on electronic devices. Diodes may be combined with other components to form
logic gates.
Main functions
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). As such, the diode can be viewed as an electronic version of a
check valve. This unidirectional behavior is called
rectification and is used to convert
alternating current
Alternating current (AC) is an electric current which periodically reverses direction and changes its magnitude continuously with time in contrast to direct current (DC) which flows only in one direction. Alternating current is the form in whic ...
(ac) to
direct current
Direct current (DC) is one-directional flow of electric charge. An electrochemical cell is a prime example of DC power. Direct current may flow through a conductor such as a wire, but can also flow through semiconductors, insulators, or eve ...
(dc). As
rectifiers, diodes can be used for such tasks as extracting
modulation from
radio signals in
radio receivers.
However, diodes can have more complicated behavior than this simple on–off action, because of their
nonlinear
In mathematics and science, a nonlinear system is a system in which the change of the output is not proportional to the change of the input. Nonlinear problems are of interest to engineers, biologists, physicists, mathematicians, and many othe ...
current-voltage characteristics.
For instance, a diode's forward-direction 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 sensor Mechanical temperature sensors
* Thermometer
* Therm
Electrical temperature sensors
* Thermistor- Thermistors are thermally sensitive resistors whose prime function is to exhibit a large, predictable and precise change in electrical resistance wh ...
or as a
voltage reference
A voltage reference is an electronic device that ideally produces a fixed (constant) voltage irrespective of the loading on the device, power supply variations, temperature changes, and the passage of time. Voltage references are used in power sup ...
. And its 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
The breakdown voltage of an insulator is the minimum voltage that causes a portion of an insulator to experience electrical breakdown and become electrically conductive.
For diodes, the breakdown voltage is the minimum reverse voltage that mak ...
. Semiconductor diodes in the forward direction also need to surpass a threshold voltage, or cut-in voltage, before being able to conduct electricity.
A semiconductor diode's
current–voltage characteristic
A current–voltage characteristic or I–V curve (current–voltage curve) is a relationship, typically represented as a chart or graph, between the electric current through a circuit, device, or material, and the corresponding voltage, or ...
can be tailored by selecting the
semiconductor materials
A semiconductor is a material which has an electrical conductivity value falling between that of a conductor, such as copper, and an insulator, such as glass. Its resistivity falls as its temperature rises; metals behave in the opposite way. ...
and the
doping impurities introduced into the materials during manufacture.
These techniques are used to create special-purpose diodes that perform many different functions.
For example, diodes are used to regulate voltage (
Zener diode
A Zener diode is a special type of diode designed to reliably allow current to flow "backwards" (inverted polarity) when a certain set reverse voltage, known as the ''Zener voltage'', is reached.
Zener diodes are manufactured with a great var ...
s), to protect circuits from high voltage surges (
avalanche diode
In electronics, an avalanche diode is a diode (made from silicon or other semiconductor) that is designed to experience avalanche breakdown at a specified reverse bias voltage. The junction of an avalanche diode is designed to prevent current co ...
s), to electronically tune radio and TV receivers (
varactor diode
In electronics, a varicap diode, varactor diode, variable capacitance diode, variable reactance diode or tuning diode is a type of diode designed to exploit the voltage-dependent capacitance of a reverse-biased p–n junction.
Applications
Vara ...
s), to generate
radio-frequency
Radio frequency (RF) is the oscillation rate of an alternating electric current or voltage or of a magnetic, electric or electromagnetic field or mechanical system in the frequency range from around to around . This is roughly between the upp ...
oscillation
Oscillation is the repetitive or periodic variation, typically in time, of some measure about a central value (often a point of equilibrium) or between two or more different states. Familiar examples of oscillation include a swinging pendulum ...
s (
tunnel diode
A tunnel diode or Esaki diode is a type of semiconductor diode that has effectively " negative resistance" due to the quantum mechanical effect called tunneling. It was invented in August 1957 by Leo Esaki, Yuriko Kurose, and Takashi Suz ...
s,
Gunn diode
A Gunn diode, also known as a transferred electron device (TED), is a form of diode, a two-terminal semiconductor electronic component, with negative resistance, used in high-frequency electronics. It is based on the "Gunn effect" discovered in 1 ...
s,
IMPATT diode An IMPATT diode (impact ionization avalanche transit-time diode) is a form of high-power semiconductor diode used in high-frequency microwave electronics devices. They have negative resistance and are used as oscillators and amplifiers at microw ...
s), and to produce light (
light-emitting diode
A light-emitting diode (LED) is a semiconductor device that emits light when current flows through it. Electrons in the semiconductor recombine with electron holes, releasing energy in the form of photons. The color of the light (co ...
s). Tunnel, Gunn and IMPATT diodes exhibit
negative resistance
In electronics, negative resistance (NR) is a property of some electrical circuits and devices in which an increase in voltage across the device's terminals results in a decrease in electric current through it.
This is in contrast to an ordina ...
, which is useful in
microwave
Microwave is a form of electromagnetic radiation with wavelengths ranging from about one meter to one millimeter corresponding to frequencies between 300 MHz and 300 GHz respectively. Different sources define different frequency ra ...
and switching circuits.
Diodes, both vacuum and semiconductor, can be used as
shot-noise generators.
History
Thermionic (
vacuum-tube
A vacuum tube, electron tube, valve (British usage), or tube (North America), is a device that controls electric current flow in a high vacuum between electrodes to which an electric potential difference has been applied.
The type known as a ...
) diodes and
solid-state
Solid state, or solid matter, is one of the four fundamental states of matter.
Solid state may also refer to:
Electronics
* Solid-state electronics, circuits built of solid materials
* Solid state ionics, study of ionic conductors and their use ...
(semiconductor) diodes were developed separately, at approximately the same time, in the early 1900s, as radio receiver
detectors
A sensor is a device that produces an output signal for the purpose of sensing a physical phenomenon.
In the broadest definition, a sensor is a device, module, machine, or subsystem that detects events or changes in its environment and sends ...
. 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
A selenium rectifier is a type of metal rectifier, invented in 1933. They were used in power supplies for electronic equipment and in high-current battery-charger applications until they were superseded by silicon diode rectifiers in the late 1960 ...
s) that were available at that time.
In 1873,
Frederick Guthrie
Frederick Guthrie FRS FRSE (15 October 1833 – 21 October 1886) was a British physicist and chemist and academic author.
He was the son of Alexander Guthrie, a London tradesman, and the younger brother of mathematician Francis Guthrie. Al ...
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. In 1880, Thomas Edison observed unidirectional current between heated and unheated elements in a bulb, later called
Edison effect
Thermionic emission is the liberation of electrons from an electrode by virtue of its temperature (releasing of energy supplied by heat). This occurs because the thermal energy given to the charge carrier overcomes the work function of the mate ...
, and was granted a patent on application of the phenomenon for use in a
DC voltmeter
A voltmeter is an instrument used for measuring electric potential difference between two points in an electric circuit. It is connected in parallel. It usually has a high resistance so that it takes negligible current from the circuit.
A ...
. About 20 years later,
John Ambrose Fleming
Sir John Ambrose Fleming FRS (29 November 1849 – 18 April 1945) was an English electrical engineer and physicist who invented the first thermionic valve or vacuum tube, designed the radio transmitter with which the first transatlantic ra ...
(scientific adviser to the
Marconi Company
The Marconi Company was a British telecommunications and engineering company that did business under that name from 1963 to 1987. Its roots were in the Wireless Telegraph & Signal Company founded by Italian inventor Guglielmo Marconi in 189 ...
and former Edison employee) realized that the Edison effect could be used as a
radio detector. Fleming patented the first true thermionic diode, the
Fleming valve
The Fleming valve, also called the Fleming oscillation valve, was a thermionic valve or vacuum tube invented in 1904 by English physicist John Ambrose Fleming as a detector for early radio receivers used in electromagnetic wireless telegraphy. ...
, in Britain on 16 November 1904 (followed by 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
A selenium rectifier is a type of metal rectifier, invented in 1933. They were used in power supplies for electronic equipment and in high-current battery-charger applications until they were superseded by silicon diode rectifiers in the late 1960 ...
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.
In 1874, German scientist
Karl Ferdinand Braun
Karl Ferdinand Braun (; 6 June 1850 – 20 April 1918) was a German electrical engineer, inventor, physicist and Nobel laureate in physics. Braun contributed significantly to the development of radio and television technology: he shared the ...
discovered the "unilateral conduction" across a contact between a metal and a
mineral
In geology and mineralogy, a mineral or mineral species is, broadly speaking, a solid chemical compound with a fairly well-defined chemical composition and a specific crystal structure that occurs naturally in pure form.John P. Rafferty, ed. (2 ...
. Indian scientist
Jagadish Chandra Bose was the first to use a crystal for detecting radio waves in 1894.
The
crystal detector
A crystal detector is an obsolete electronic component used in some early 20th century radio receivers that consists of a piece of crystalline mineral which rectifies the alternating current radio signal. It was employed as a detector ( de ...
was developed into a practical device for
wireless telegraphy
Wireless telegraphy or radiotelegraphy is transmission of text messages by radio waves, analogous to electrical telegraphy using cables. Before about 1910, the term ''wireless telegraphy'' was also used for other experimental technologies for ...
by
Greenleaf Whittier Pickard
Greenleaf Whittier Pickard (February 14, 1877, Portland, Maine
– January 8, 1956, Newton, Massachusetts) was a United States radio pioneer. Pickard was a researcher in the early days of wireless. While not the earliest discoverer of the rectifyi ...
, who invented a
silicon
Silicon is a chemical element with the symbol Si and atomic number 14. It is a hard, brittle crystalline solid with a blue-grey metallic luster, and is a tetravalent metalloid and semiconductor. It is a member of group 14 in the periodic ta ...
crystal detector in 1903 and received a patent for it on 20 November 1906. 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.
[Scaff, J. H., Ohl, R. S]
"Development of Silicon Crystal Rectifiers for Microwave Radar Receivers"
''The Bell System Technical Journal'', Vol. 24, No. 1, January 1947. pp. 1 - 30 Researchers at
Bell Labs
Nokia Bell Labs, originally named Bell Telephone Laboratories (1925–1984),
then AT&T Bell Laboratories (1984–1996)
and Bell Labs Innovations (1996–2007),
is an American industrial Research and development, research and scientific developm ...
,
Western Electric,
MIT
The Massachusetts Institute of Technology (MIT) is a private land-grant research university in Cambridge, Massachusetts. Established in 1861, MIT has played a key role in the development of modern technology and science, and is one of the m ...
,
Purdue and in the
UK intensively developed point-contact diodes (''crystal rectifiers'' or ''crystal diodes'') during World War II for application in radar.
After World War II, AT&T used these in their 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. During the early 1950s, junction diodes were developed.
In 2022, the first
superconducting diode effect without an external
magnetic field was realized.
Etymology
At the time of their invention, asymmetrical conduction devices were known as
rectifiers. In 1919, the year
tetrode
A tetrode is a vacuum tube (called ''valve'' in British English) having four active electrodes. The four electrodes in order from the centre are: a thermionic cathode, first and second grids and a plate (called ''anode'' in British English). ...
s were invented,
William Henry Eccles
William Henry Eccles FRS (23 August 1875 – 29 April 1966) was a British physicist and a pioneer in the development of radio communication.
He was born in Barrow-in-Furness, Lancashire, England. Following graduation from the Royal College ...
coined the term ''diode'' from the
Greek roots ''di'' (from ''δί''), meaning 'two', and ''ode'' (from ''οδός''), meaning 'path'. The word ''diode'', however, as well as ''
triode
A triode is an electronic amplifying vacuum tube (or ''valve'' in British English) consisting of three electrodes inside an evacuated glass envelope: a heated filament or cathode, a grid, and a plate (anode). Developed from Lee De Forest's ...
,
tetrode
A tetrode is a vacuum tube (called ''valve'' in British English) having four active electrodes. The four electrodes in order from the centre are: a thermionic cathode, first and second grids and a plate (called ''anode'' in British English). ...
,
pentode,
hexode
The pentagrid converter is a type of radio receiving valve (vacuum tube) with five grids used as the frequency mixer stage of a superheterodyne radio receiver.
The pentagrid was part of a line of development of valves that were able to take an ...
'', were already in use as terms of
multiplex
Multiplex may refer to:
* Multiplex (automobile), a former American car make
* Multiplex (comics), a DC comic book supervillain
* Multiplex (company), a global contracting and development company
* Multiplex (assay), a biological assay which measu ...
telegraphy
Telegraphy is the long-distance transmission of messages where the sender uses symbolic codes, known to the recipient, rather than a physical exchange of an object bearing the message. Thus flag semaphore is a method of telegraphy, whereas ...
.
Although all diodes ''rectify'', the term ''rectifier'' is usually applied to diodes intended for
power supply
A power supply is an electrical device that supplies electric power to an electrical load. The main purpose of a power supply is to convert electric current from a source to the correct voltage, current, and frequency to power the load. As a ...
application in order to differentiate them from diodes intended for
small signal circuits.
Vacuum tube diodes
A thermionic diode is a
thermionic-valve device consisting of a sealed, evacuated glass or metal envelope containing two
electrode
An electrode is an electrical conductor used to make contact with a nonmetallic part of a circuit (e.g. a semiconductor, an electrolyte, a vacuum or air). Electrodes are essential parts of batteries that can consist of a variety of materials d ...
s: a
cathode
A cathode is the electrode from which a conventional current leaves a polarized electrical device. This definition can be recalled by using the mnemonic ''CCD'' for ''Cathode Current Departs''. A conventional current describes the direction in wh ...
and a
plate
Plate may refer to:
Cooking
* Plate (dishware), a broad, mainly flat vessel commonly used to serve food
* Plates, tableware, dishes or dishware used for setting a table, serving food and dining
* Plate, the content of such a plate (for example: ...
. 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
The practice of using colours to determine the temperature of a piece of (usually) ferrous metal comes from blacksmithing. Long before thermometers were widely available it was necessary to know what state the metal was in for heat treating it an ...
, around . 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.
![5U4GB](https://upload.wikimedia.org/wikipedia/commons/d/da/5U4GB.agr.jpg)
The operating temperature of the cathode causes it to release
electron
The electron ( or ) is a subatomic particle with a negative one elementary electric charge. Electrons belong to the first generation of the lepton particle family,
and are generally thought to be elementary particles because they have no ...
s into the vacuum, a process called
thermionic emission
Thermionic emission is the liberation of electrons from an electrode by virtue of its temperature (releasing of energy supplied by heat). This occurs because the thermal energy given to the charge carrier overcomes the work function of the mater ...
. The cathode is coated with
oxides of
alkaline earth metal
The alkaline earth metals are six chemical elements in group 2 of the periodic table. They are beryllium (Be), magnesium (Mg), calcium (Ca), strontium (Sr), barium (Ba), and radium (Ra).. The elements have very similar properties: they are all ...
s, such as
barium and
strontium oxides. These have a low
work function
In solid-state physics, the work function (sometimes spelt workfunction) is the minimum thermodynamic work (i.e., energy) needed to remove an electron from a solid to a point in the vacuum immediately outside the solid surface. Here "immediately" ...
, 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
Electrostatics is a branch of physics that studies electric charges at rest (static electricity).
Since classical times, it has been known that some materials, such as amber, attract lightweight particles after rubbing. The Greek word for amber ...
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
Point-contact diodes
Point-contact diodes were developed starting in the 1930s, out of the early
crystal detector
A crystal detector is an obsolete electronic component used in some early 20th century radio receivers that consists of a piece of crystalline mineral which rectifies the alternating current radio signal. It was employed as a detector ( de ...
technology, and are now generally used in the 3 to 30 gigahertz range.
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.
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. Point contact diodes generally exhibit lower capacitance, higher forward resistance and greater reverse leakage than junction diodes.
Junction diodes
p–n junction diode
A p–n junction diode is made of a crystal of
semiconductor
A semiconductor is a material which has an electrical conductivity value falling between that of a conductor, such as copper, and an insulator, such as glass. Its resistivity falls as its temperature rises; metals behave in the opposite way. ...
, 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 carriers (electrons), called an
n-type semiconductor, and a region on the other side that contains positive charge carriers (
holes
A hole is an opening in or through a particular medium, usually a solid body. Holes occur through natural and artificial processes, and may be useful for various purposes, or may represent a problem needing to be addressed in many fields of en ...
), 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
In semiconductor physics, the depletion region, also called depletion layer, depletion zone, junction region, space charge region or space charge layer, is an insulating region within a conductive, doped semiconductor material where the mobile ...
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
electrical potential
The electric potential (also called the ''electric field potential'', potential drop, the electrostatic potential) is defined as the amount of work energy needed to move a unit of electric charge from a reference point to the specific point in ...
is applied to the P side (the
anode
An anode is an electrode of a polarized electrical device through which conventional current enters the device. This contrasts with a cathode, an electrode of the device through which conventional current leaves the device. A common mnemonic ...
) than to the N side (the
cathode
A cathode is the electrode from which a conventional current leaves a polarized electrical device. This definition can be recalled by using the mnemonic ''CCD'' for ''Cathode Current Departs''. A conventional current describes the direction in wh ...
), 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
Another type of junction diode, the
Schottky diode
The Schottky diode (named after the German physicist Walter H. Schottky), also known as Schottky barrier diode or hot-carrier diode, is a semiconductor diode formed by the junction of a semiconductor with a metal. It has a low forward voltag ...
, is formed from a
metal–semiconductor junction
In solid-state physics, a metal–semiconductor (M–S) junction is a type of electrical junction in which a metal comes in close contact with a semiconductor material. It is the oldest practical semiconductor device. M–S junctions can either ...
rather than a p–n junction, which reduces capacitance and increases switching speed.
Current–voltage characteristic
![Forward and Reverse Characteristics for diodes-en](https://upload.wikimedia.org/wikipedia/commons/6/62/Forward_and_Reverse_Characteristics_for_diodes-en.svg)
A semiconductor diode's behavior in a circuit is given by its
current–voltage characteristic
A current–voltage characteristic or I–V curve (current–voltage curve) is a relationship, typically represented as a chart or graph, between the electric current through a circuit, device, or material, and the corresponding voltage, or ...
, or I–V graph (see graph below). The shape of the curve is determined by the transport of charge carriers through the so-called ''
depletion layer
In semiconductor physics, the depletion region, also called depletion layer, depletion zone, junction region, space charge region or space charge layer, is an insulating region within a conductive, doped semiconductor material where the mobile ...
'' or ''
depletion region
In semiconductor physics, the depletion region, also called depletion layer, depletion zone, junction region, space charge region or space charge layer, is an insulating region within a conductive, doped semiconductor material where the mobile ...
'' 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-
doped region diffuse into the P-
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 carriers and thus behaves as an
insulator.
However, the width of the depletion region (called the
depletion width) cannot grow without limit. For each
electron–hole pair
In the solid-state physics of semiconductors, carrier generation and carrier recombination are processes by which mobile charge carriers (electrons and electron holes) are created and eliminated. Carrier generation and recombination processes are ...
recombination made, a positively charged
dopant
A dopant, also called a doping agent, is a trace of impurity element that is introduced into a chemical material to alter its original electrical or optical properties. The amount of dopant necessary to cause changes is typically very low. When ...
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.
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
In the solid-state physics of semiconductors, carrier generation and carrier recombination are processes by which mobile charge carriers (electrons and electron holes) are created and eliminated. Carrier generation and recombination processes are ...
s are actively being created in the junction by, for instance, light; see
photodiode). This is called the ''
reverse bias
Reverse or reversing may refer to:
Arts and media
* ''Reverse'' (Eldritch album), 2001
* ''Reverse'' (2009 film), a Polish comedy-drama film
* ''Reverse'' (2019 film), an Iranian crime-drama film
* ''Reverse'' (Morandi album), 2005
* ''Reverse'' ...
'' phenomenon.
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). For silicon diodes, the built-in potential is approximately 0.7 V (0.3 V for germanium and 0.2 V for Schottky). Thus, if an external voltage greater than and opposite to the built-in voltage is applied, a current will flow and the diode is said to be "turned on" as it has been given an external ''
forward bias
Forward is a relative direction, the opposite of backward.
Forward may also refer to:
People
*Forward (surname)
Sports
* Forward (association football)
* Forward (basketball), including:
** Point forward
** Power forward (basketball)
** Smal ...
''. The diode is commonly said to have a forward "threshold" voltage, above which it conducts and below which conduction stops. However, this is only an approximation as the forward characteristic is smooth (see I-V graph above).
A diode's
current–voltage characteristic
A current–voltage characteristic or I–V curve (current–voltage curve) is a relationship, typically represented as a chart or graph, between the electric current through a circuit, device, or material, and the corresponding voltage, or ...
can be approximated by four regions of operation:
# At very large reverse bias, beyond the
peak inverse voltage The peak inverse voltage is either the specified maximum voltage that a diode rectifier can block, or, alternatively, the maximum voltage that a rectifier needs to block in a given circuit. The peak inverse voltage increases with an increase in te ...
or PIV, a process called reverse
breakdown
Breakdown may refer to:
Breaking down
*Breakdown (vehicle), failure of a motor vehicle in such a way that it cannot be operated
*Chemical decomposition, also called chemical breakdown, the breakdown of a substance into simpler components
*Decompo ...
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
In electronics, an avalanche diode is a diode (made from silicon or other semiconductor) that is designed to experience avalanche breakdown at a specified reverse bias voltage. The junction of an avalanche diode is designed to prevent current co ...
is deliberately designed for use in that manner. In the
Zener diode
A Zener diode is a special type of diode designed to reliably allow current to flow "backwards" (inverted polarity) when a certain set reverse voltage, known as the ''Zener voltage'', is reached.
Zener diodes are manufactured with a great var ...
, 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.
# For a bias less than the PIV, the reverse current is very small. For a normal P–N rectifier diode, the reverse current through the device in the micro-ampere (μA) range is very low. 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.
# With a small forward bias, where only a small forward current is conducted, the current-voltage curve is
exponential
Exponential may refer to any of several mathematical topics related to exponentiation, including:
*Exponential function, also:
**Matrix exponential, the matrix analogue to the above
*Exponential decay, decrease at a rate proportional to value
*Expo ...
in accordance with the ideal diode equation. There is a definite forward voltage at which the diode starts to conduct significantly. This is called the ''knee voltage'' or ''cut-in voltage'' and is equal to the
barrier potential of the p-n junction. This is a feature of the exponential curve and appears sharper on a current scale more compressed than in the diagram shown here.
# 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. The diode can be modeled as an ideal diode in series with a fixed resistor.
In a small silicon diode operating at its rated currents, the voltage drop is about 0.6 to 0.7
volt
The volt (symbol: V) is the unit of electric potential, electric potential difference (voltage), and electromotive force in the International System of Units (SI). It is named after the Italian physicist Alessandro Volta (1745–1827).
Defin ...
s. The value is different for other diode types—
Schottky diode
The Schottky diode (named after the German physicist Walter H. Schottky), also known as Schottky barrier diode or hot-carrier diode, is a semiconductor diode formed by the junction of a semiconductor with a metal. It has a low forward voltag ...
s can be rated as low as 0.2 V, germanium diodes 0.25 to 0.3 V, and red or blue
light-emitting diode
A light-emitting diode (LED) is a semiconductor device that emits light when current flows through it. Electrons in the semiconductor recombine with electron holes, releasing energy in the form of photons. The color of the light (co ...
s (LEDs) can have values of 1.4 V and 4.0 V respectively.
At higher currents, the forward voltage drop of the diode increases. A drop of 1 V to 1.5 V is typical at full rated current for power diodes.
Shockley diode equation
The ''Shockley ideal diode equation'' or the ''diode law'' (named after the
bipolar junction transistor
A bipolar junction transistor (BJT) is a type of transistor that uses both electrons and electron holes as charge carriers. In contrast, a unipolar transistor, such as a field-effect transistor, uses only one kind of charge carrier. A bipola ...
co-inventor
William Bradford Shockley) gives the I–V characteristic of an ideal diode in either forward or reverse bias (or no bias). The following equation is called the ''Shockley ideal diode equation'' when ''n'', the ideality factor, is set equal to 1 :
:
where
:''I'' is the diode current,
:''I''
S is the reverse bias
saturation current The saturation current (or scale current), more accurately the reverse saturation current, is the part of the reverse current in a semiconductor diode caused by diffusion of minority carriers from the neutral regions to the depletion region. This ...
(or scale current),
:''V''
D is the voltage across the diode,
:''V''
T is the
thermal voltage, and
:''n'' is the ''ideality factor'', also known as the ''quality factor'' or sometimes ''emission coefficient''. The ideality factor ''n'' typically varies from 1 to 2 (though can in some cases be higher), depending on the fabrication process and semiconductor material and is set equal to 1 for the case of an "ideal" diode (thus the n is sometimes omitted). The ideality factor was added to account for imperfect junctions as observed in real transistors. The factor mainly accounts for
carrier recombination as the charge carriers cross the
depletion region
In semiconductor physics, the depletion region, also called depletion layer, depletion zone, junction region, space charge region or space charge layer, is an insulating region within a conductive, doped semiconductor material where the mobile ...
.
The
thermal voltage ''V''
T is approximately 25.85 mV at 300 K, a temperature close to "room temperature" commonly used in device simulation software. At any temperature it is a known constant defined by:
:
where ''k'' is the
Boltzmann constant
The Boltzmann constant ( or ) is the proportionality factor that relates the average relative kinetic energy of particles in a gas with the thermodynamic temperature of the gas. It occurs in the definitions of the kelvin and the gas constant, ...
, ''T'' is the absolute temperature of the p–n junction, and ''q'' is the magnitude of charge of an
electron
The electron ( or ) is a subatomic particle with a negative one elementary electric charge. Electrons belong to the first generation of the lepton particle family,
and are generally thought to be elementary particles because they have no ...
(the
elementary charge).
The reverse saturation current, ''I''
S, is not constant for a given device, but varies with temperature; usually more significantly than ''V''
T, so that ''V''
D typically decreases as ''T'' increases.
The ''Shockley ideal diode equation'' or the ''diode law'' is derived with the assumption that the only processes giving rise to the current in the diode are drift (due to electrical field), diffusion, and thermal
recombination–generation (R–G) (this equation is derived by setting n = 1 above). It also assumes that the R–G current in the depletion region is insignificant. This means that the ''Shockley ideal diode equation'' does not account for the processes involved in the reverse breakdown and photon-assisted R–G. Additionally, it does not describe the "leveling off" of the I–V curve at high forward bias due to internal resistance. Introducing the ideality factor, n, accounts for recombination and generation of carriers.
Under ''reverse bias'' voltages the exponential in the diode equation is negligible, and the current is a constant (negative) reverse current value of −''I
S''. The reverse ''breakdown region'' is not modeled by the Shockley diode equation.
For even rather small ''forward bias'' voltages the exponential is very large since the thermal voltage is very small in comparison. The subtracted '1' in the diode equation is then negligible and the forward diode current can be approximated by
:
The use of the diode equation in circuit problems is illustrated in the article on
diode modeling.
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
as mentioned in the previous section.
In detector and mixer applications, the current can be estimated by a Taylor's series.
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.
The desired current component is approximately proportional to the square of the input voltage, so the response is called ''
square law'' in this region.
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. A certain amount of "reverse recovery time"
r (on the order of tens of nanoseconds to a few microseconds) may be required to remove the reverse recovery charge
r 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. In certain real-world cases it is important to consider the losses that are incurred by this non-ideal diode effect. However, when the
slew rate
In electronics, slew rate is defined as the change of voltage or current, or any other electrical quantity, per unit of time. Expressed in SI units, the unit of measurement is volts/second or amperes/second, but is usually expressed in terms of m ...
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
The Schottky diode (named after the German physicist Walter H. Schottky), also known as Schottky barrier diode or hot-carrier diode, is a semiconductor diode formed by the junction of a semiconductor with a metal. It has a low forward voltag ...
s.
The reverse current ceases abruptly when the stored charge is depleted; this abrupt stop is exploited in
step recovery diodes for the generation of extremely short pulses.
Types of semiconductor diode
Normal (p–n) diodes, which operate as described above, are usually made of doped
silicon
Silicon is a chemical element with the symbol Si and atomic number 14. It is a hard, brittle crystalline solid with a blue-grey metallic luster, and is a tetravalent metalloid and semiconductor. It is a member of group 14 in the periodic ta ...
or
germanium. Before the development of silicon power rectifier diodes,
cuprous oxide
Copper(I) oxide or cuprous oxide is the inorganic compound with the formula Cu2O. It is one of the principal oxides of copper, the other being or copper(II) oxide or cupric oxide (CuO). This red-coloured solid is a component of some antifouling p ...
and later
selenium
Selenium is a chemical element with the symbol Se and atomic number 34. It is a nonmetal (more rarely considered a metalloid) with properties that are intermediate between the elements above and below in the periodic table, sulfur and tellurium, ...
was used. Their low efficiency required a much higher forward voltage to be applied (typically 1.4 to 1.7 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), 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
An integrated circuit or monolithic integrated circuit (also referred to as an IC, a chip, or a microchip) is a set of electronic circuits on one small flat piece (or "chip") of semiconductor material, usually silicon. Large numbers of tiny ...
, which include two diodes per pin and many other internal diodes.
;
Avalanche diode
In electronics, an avalanche diode is a diode (made from silicon or other semiconductor) that is designed to experience avalanche breakdown at a specified reverse bias voltage. The junction of an avalanche diode is designed to prevent current co ...
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 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
A constant-current diode is an electronic device that limits current to a maximal specified value for the device. It is known as a current-limiting diode (CLD) or current-regulating diode (CRD).
It consists of an n-channel JFET with the gate sho ...
s
:These are actually
JFET
The junction-gate field-effect transistor (JFET) is one of the simplest types of field-effect transistor. JFETs are three-terminal semiconductor devices that can be used as electronically controlled switches or resistors, or to build amplifier ...
s 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''.
;
Crystal rectifiers or crystal diodes
:These are point-contact diodes.
The 1N21 series and others are used in mixer and detector applications in radar and microwave receivers.
The 1N34A is another example of a crystal diode.
;
Gunn diode
A Gunn diode, also known as a transferred electron device (TED), is a form of diode, a two-terminal semiconductor electronic component, with negative resistance, used in high-frequency electronics. It is based on the "Gunn effect" discovered in 1 ...
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 differential resistance
In electronics, negative resistance (NR) is a property of some electrical circuits and devices in which an increase in voltage across the device's terminals results in a decrease in electric current through it.
This is in contrast to an ordina ...
. With appropriate biasing, dipole domains form and travel across the diode, allowing high frequency
microwave
Microwave is a form of electromagnetic radiation with wavelengths ranging from about one meter to one millimeter corresponding to frequencies between 300 MHz and 300 GHz respectively. Different sources define different frequency ra ...
oscillators to be built.
;
Light-emitting diode
A light-emitting diode (LED) is a semiconductor device that emits light when current flows through it. Electrons in the semiconductor recombine with electron holes, releasing energy in the form of photons. The color of the light (co ...
s (LEDs)
:In a diode formed from a
direct band-gap semiconductor, such as
gallium arsenide, charge carriers that cross the junction emit
photon
A photon () is an elementary particle that is a quantum of the electromagnetic field, including electromagnetic radiation such as light and radio waves, and the force carrier for the electromagnetic force. Photons are massless, so they a ...
s when they recombine with the majority carrier on the other side. Depending on the material,
wavelength
In physics, the wavelength is the spatial period of a periodic wave—the distance over which the wave's shape repeats.
It is the distance between consecutive corresponding points of the same phase on the wave, such as two adjacent crests, t ...
s (or colors) from the
infrared
Infrared (IR), sometimes called infrared light, is electromagnetic radiation (EMR) with wavelengths longer than those of visible light. It is therefore invisible to the human eye. IR is generally understood to encompass wavelengths from around ...
to the near
ultraviolet
Ultraviolet (UV) is a form of electromagnetic radiation with wavelength from 10 nm (with a corresponding frequency around 30 PHz) to 400 nm (750 THz), shorter than that of visible light, but longer than X-rays. UV radiation ...
may be produced. The first LEDs were red and yellow, and higher-frequency diodes have been developed over time. All LEDs produce incoherent, narrow-spectrum light;
"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
The laser diode chip removed and placed on the eye of a needle for scale
A laser diode (LD, also injection laser diode or ILD, or diode laser) is a semiconductor device similar to a light-emitting diode in which a diode pumped directly with e ...
s
: When an LED-like structure is contained in a
resonant cavity formed by polishing the parallel end faces, a
laser
A laser is a device that emits light through a process of optical amplification based on the stimulated emission of electromagnetic radiation. The word "laser" is an acronym for "light amplification by stimulated emission of radiation". The fi ...
can be formed. Laser diodes are commonly used in
optical storage
IBM defines optical storage as "any storage method that uses a laser to store and retrieve data from optical media." '' Britannica'' notes that it "uses low-power laser beams to record and retrieve digital (binary) data." Compact disc (CD) an ...
devices and for high speed optical communication.
;Thermal diodes
: 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.
;Photodiodes
: 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). A photodiode can be used in solar cells, in photometry (optics), photometry, or in optical communications. 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 devices.
;PIN diodes
:A PIN diode has a central un-doped, or ''intrinsic'', layer, forming a p-type/intrinsic/n-type structure. They are used as radio frequency switches and attenuators. They are also used as large-volume, ionizing-radiation detectors and as photodetectors. 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 devices, such as IGBTs, power MOSFETs, and thyristors.
;
Schottky diode
The Schottky diode (named after the German physicist Walter H. Schottky), also known as Schottky barrier diode or hot-carrier diode, is a semiconductor diode formed by the junction of a semiconductor with a metal. It has a low forward voltag ...
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 mA is in the range 0.15 V to 0.45 V, which makes them useful in voltage Clamper (electronics), clamping applications and prevention of transistor saturation. They can also be used as low loss
rectifiers, 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 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). A typical example is the 1N914.
; Snap-off or
step recovery diodes
: 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.
;Stabistors 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. 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 diodes or Leo Esaki, Esaki diodes
:These have a region of operation showing
negative resistance
In electronics, negative resistance (NR) is a property of some electrical circuits and devices in which an increase in voltage across the device's terminals results in a decrease in electric current through it.
This is in contrast to an ordina ...
caused by quantum tunneling, 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. 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
A Zener diode is a special type of diode designed to reliably allow current to flow "backwards" (inverted polarity) when a certain set reverse voltage, known as the ''Zener voltage'', is reached.
Zener diodes are manufactured with a great var ...
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).
Other uses for semiconductor diodes include the sensing of temperature, and computing analog logarithms (see Operational amplifier applications#Logarithmic output).
Graphic symbols
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.
File:Diode symbol.svg, Diode
File:LED symbol.svg, Light-emitting diode
A light-emitting diode (LED) is a semiconductor device that emits light when current flows through it. Electrons in the semiconductor recombine with electron holes, releasing energy in the form of photons. The color of the light (co ...
(LED)
File:Photodiode symbol.svg, w:Photodiode, Photodiode
File:Schottky diode symbol.svg, w:Schottky diode, 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, w:Varicap, Varicap
File:Zener diode symbol.svg, Zener diode
A Zener diode is a special type of diode designed to reliably allow current to flow "backwards" (inverted polarity) when a certain set reverse voltage, known as the ''Zener voltage'', is reached.
Zener diodes are manufactured with a great var ...
File:Diode pinout en fr.svg, Typical diode packages in same alignment as diode symbol. Thin bar depicts the cathode
A cathode is the electrode from which a conventional current leaves a polarized electrical device. This definition can be recalled by using the mnemonic ''CCD'' for ''Cathode Current Departs''. A conventional current describes the direction in wh ...
.
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).
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)this is a very old coding system
* OA-series germanium diodes (e.g., OA47)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.
Applications
Radio demodulation
![Simple envelope detector](https://upload.wikimedia.org/wikipedia/commons/6/6c/Simple_envelope_detector.svg)
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
A crystal detector is an obsolete electronic component used in some early 20th century radio receivers that consists of a piece of crystalline mineral which rectifies the alternating current radio signal. It was employed as a detector ( de ...
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.
Power conversion
![ACtoDCpowersupply](https://upload.wikimedia.org/wikipedia/commons/4/4e/ACtoDCpowersupply.png)
Rectifiers are constructed from diodes, where they are used to convert
alternating current
Alternating current (AC) is an electric current which periodically reverses direction and changes its magnitude continuously with time in contrast to direct current (DC) which flows only in one direction. Alternating current is the form in whic ...
(AC) electricity into
direct current
Direct current (DC) is one-directional flow of electric charge. An electrochemical cell is a prime example of DC power. Direct current may flow through a conductor such as a wire, but can also flow through semiconductors, insulators, or eve ...
(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 multipliers'' 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 spikes that would otherwise occur. (A diode used in such an application is called a flyback diode). Many
integrated circuits
An integrated circuit or monolithic integrated circuit (also referred to as an IC, a chip, or a microchip) is a set of electronic circuits on one small flat piece (or "chip") of semiconductor material, usually silicon. Large numbers of tiny ...
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
Diodes can be combined with other components to construct logical conjunction, AND and logical disjunction, OR
logic gates. This is referred to as diode logic.
Ionizing radiation detectors
In addition to light, mentioned above,
semiconductor
A semiconductor is a material which has an electrical conductivity value falling between that of a conductor, such as copper, and an insulator, such as glass. Its resistivity falls as its temperature rises; metals behave in the opposite way. ...
diodes are sensitive to more energy, energetic radiation. In electronics, cosmic rays and other sources of ionizing radiation cause noise pulses and single and multiple bit errors.
This effect is sometimes exploited by particle detectors 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 rays to electron showers.
Semiconductor detectors 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, and CY7 cryogenic temperature sensor.
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 circuits. 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 machines.
Waveform clipper
Diodes can be used to limit the positive or negative excursion of a signal to a prescribed voltage.
Clamper
![DiodeClamp](https://upload.wikimedia.org/wikipedia/commons/2/2d/DiodeClamp.png)
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.
Abbreviations
Diodes are usually referred to as ''D'' for diode on printed circuit board, PCBs. Sometimes the abbreviation ''CR'' for ''crystal rectifier'' is used.
See also
* Active rectification
* Diode modelling
* Lambda diode
*
p–n junction
* Small-signal model
* Flame rectification
* fast diode, Fast/ultrafast diode
References
Further reading
;Periodicals
* ''Solid-State Diodes''; ages; 2001.
(archive)
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* ''Silicon Rectifier Handbook''; 1st Ed; Bob Dale; Motorola; 213 pages; 1966. (archive)
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* ''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. (archive)
/small>
* ''S.T. Selenium Rectifier Handbook''; 1st Ed; Sarkes Tarzian; 80 pages; 1950. (archive)
/small>
; Circuit books
* ''50 Simple LED Circuits''; 1st Ed; R.N. Soar; Babani Press; 62 pages; 1977; . (archive)
/small>
* ''38 Practical Tested Diode Circuits For the Home Constructor''; 1st Ed; Bernard Babani; Krisson Printing; 48 pages; 1972. (archive)
/small>
* ''Diode Circuits Handbook''; 1st Ed; Rufus Turner; Howard Sams & Co; 128 pages; 1963; LCCN 63-13904. (archive)
/small>
* ''40 Uses for Germanium Diodes''; 2nd Ed; Sylvania Electric Products; 47 pages; 1949. (archive)
/small>
;Databooks
Discrete Databook
1985; Fairchild (now ON Semiconductor)
Discrete Databook
1982; SGS (now STMicroelectronics)
Discrete Databook
1978; National Semiconductor (now Texas Instruments)
Semiconductor Databook
1965; Motorola (now ON Semiconductor)
External links
Structure and Functional Behavior of PIN Diodes
– PowerGuru
; Interactive and animations
University of Cambridge
Schottky Diode Flash Tutorial Animation
{{Authority control
1904 introductions
Diodes,
Semiconductor devices