The most common function of a diode is to allow an electric current to pass in one direction (called the diode's
forward direction), while
blocking current in the opposite direction (the reverse direction). Thus, the diode can be viewed as an electronic version of a check valve. This unidirectional behavior is called rectification, and is used to
convert alternating current to direct current, including extraction of modulation from radio signals in radio receivers—these diodes are forms of rectifiers.
However, diodes can have more complicated behavior than this simple on–off action. Semiconductor diodes begin conducting electricity only if a
certain threshold voltage or cut-in voltage is present in the forward direction (a state in which the diode is said to be
forward-biased). The voltage drop across a
forward-biased diode varies only a little with the current, and is a function of temperature; this effect can be used as a temperature sensor or voltage reference.
Thermionic (vacuum tube) diodes and solid state (semiconductor) diodes were developed separately, at
approximately the same time, in the early 1900s, as radio receiver detectors.
Until the 1950s vacuum tube diodes were more often used in radios because the
early point-contact type semiconductor diodes (cat's-whisker
detectors) were less stable, and because most receiving sets had vacuum tubes for amplification that could easily have diodes included in the tube
(for example the 12SQ7double-diode triode), and vacuum tube
rectifiers and gas-filled rectifiers handled some high voltage/high current rectification tasks beyond the capabilities of semiconductor diodes (such as selenium rectifiers) available at the time.
In 1873, Frederick Guthrie discovered the basic principle of
operation of thermionic diodes.
Guthrie discovered that a positively charged electroscope could be
discharged by bringing a grounded piece of white-hot metal
close to it (but not actually touching it). The same did not apply to a negatively charged electroscope, indicating that the current flow was only
possible in one direction.
Thomas Edison independently rediscovered the principle on February 13,
1880. At the time, Edison was investigating why the filaments of his carbon-filament light bulbs nearly always burned out at the positive-connected end.
He had a special bulb made with a metal plate sealed into the glass envelope. Using this device, he confirmed that an invisible current flowed from the
glowing filament through the vacuum to the metal plate, but only when the plate was
connected to the positive supply.
Edison devised a circuit where his modified light bulb effectively replaced the resistor in a DCvoltmeter. Edison was
awarded a patent for this invention in 1884.
Since there was no apparent practical use for such a device at the time, the patent application was most likely simply a precaution in case someone else
did find a use for the so-called Edison effect.
Indian scientist Jagadish Chandra Bose was the first to
use a crystal for detecting radio waves in 1894.  The crystal detector was developed into a practical device for wireless telegraphy by Greenleaf Whittier Pickard, who invented a silicon crystal detector in 1903 and received a patent for it on November 20, 1906. Other experimenters tried a variety
of other substances, of which the most widely used was the mineral galena (lead sulfide). Other substances offered slightly better performance, but galena was most
widely used because it had the advantage of being cheap and easy to obtain. The crystal detector in these early crystal radio sets consisted of an adjustable wire point-contact (the so-called "cat's
whisker"), which could be manually moved over the face of the crystal in order to obtain optimum signal. This troublesome device was superseded by
thermionic diodes by the 1920s, but after high purity semiconductor materials became available, the crystal detector returned to dominant use with the
advent of inexpensive fixed-germanium diodes in the 1950s. Bell Labs also developed a germanium diode for microwave reception, and AT&T used these in their
microwave towers that criss-crossed the nation starting in the late 1940s, carrying telephone and network television signals. Bell Labs did not develop a satisfactory thermionic diode for microwave reception.
In operation, a separate current through the filament (heater), a high resistance wire made of nichrome, heats the cathode red hot (800-1000° C), causing it to release electrons into the vacuum, a process called thermionic emission. The cathode is coated with oxides of alkaline
earth metals such as barium and strontiumoxides, which have a low work function, to increase the number of electrons emitted. (Some valves use
heating, in which a tungsten filament acts as both heater and cathode.) The alternating voltage to be rectified is applied between the cathode and the
concentric plate electrode. When the plate has a positive voltage with respect to the cathode, it electrostatically attracts the electrons from the cathode, so a current of electrons flows through
the tube from cathode to plate. However when the polarity is reversed and the plate has a negative voltage, no current flows, because the cathode
electrons are not attracted to it. The unheated plate does not emit any electrons itself. So current can only flow through the tube in one direction, from
cathode to plate.
In a mercury-arc valve, an arc forms between a refractory
conductive anode and a pool of liquid mercury acting as cathode. Such units were made with ratings up to hundreds of kilowatts, and were important in the
development of HVDC power transmission. Some
types of smaller thermionic rectifiers sometimes had mercury vapor fill to reduce their forward voltage drop and to increase current rating over
thermionic hard-vacuum devices.
Throughout the vacuum tube era, valve diodes were used in analog signal applications and as rectifiers in DC power supplies in consumer electronics
such as radios, televisions, and sound systems. They were replaced in power supplies beginning in the 1940s by selenium rectifiers and then by semiconductor diodes by the 1960s. Today they are still used
in a few high power applications where their ability to withstand transients and their robustness gives them an advantage over semiconductor devices. The
recent (2012) resurgence of interest among audiophiles and recording studios in
old valve audio gear such as guitar amplifiers and home audio
systems has provided a market for the legacy consumer diode valves.
Figure 7: Typical diode packages in
same alignment as diode symbol.
Thin bar depicts the cathode.
A point-contact diode works the same as the junction diodes described below, but their construction is simpler. A block of n-type semiconductor is
built, and a conducting sharp-point contact made with some group-3 metal is placed in contact with the semiconductor. Some metal migrates into the
semiconductor to make a small region of p-type semiconductor near the contact. The long-popular 1N34 germanium version is still used in radio receivers as
a detector and occasionally in specialized analog electronics.
Most diodes today are silicon junction diodes. A junction is formed between the p and n regions which is also called a depletion region.
A p–n junction diode is made of a crystal of semiconductor.
Impurities are added to it to create a region on one side that contains negative charge carriers (electrons), called n-type semiconductor, and a region on the other side that contains positive charge carriers
(holes), called p-type semiconductor. When two materials i.e. n-type and p-type are attached together, a
momentary flow of electrons occur from n to p side resulting in a third region where no charge carriers are present. It is called Depletion region due to
the absence of charge carriers (electrons and holes in this case). The diode's terminals are attached to each of these regions. The boundary between these
two regions, called a p–n junction, is where the action
of the diode takes place. The crystal allows electrons to flow from the N-type side (called the cathode) to the P-type side (called the anode),
but not in the opposite direction.
A semiconductor diode's behavior in a circuit is given by its current–voltage characteristic, 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 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-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 that recombines, a positively charged dopant ion is left behind in the N-doped region, and a negatively charged dopant ion is
left behind in the P-doped region. As recombination proceeds 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.
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 pairs are actively being created in the junction by, for instance,
light. see photodiode). This is the
reverse bias phenomenon. However, if the polarity of the external voltage
opposes the built-in potential, recombination can once again proceed, resulting in 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 current is passed through the diode, about 0.7 V will be developed across the diode such that the
P-doped region is positive with respect to the N-doped region and the diode is said to be "turned on" as it has a
Figure 5: I-V characteristics of a p-n junction diode.
At very large reverse bias, beyond the peak
inverse voltage or PIV, a process called reverse 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 the avalanche region. 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
voltage), and avalanche does not occur. Both devices, however, do have a limit to the maximum current and power in the clamped reverse-voltage region.
Also, following the end of forward 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.
The second region, at reverse biases more positive than the PIV, has only a very small reverse saturation current. In the reverse bias region for a
normal P–N rectifier diode, the current through the device is very low (in the µA range). However, this is temperature dependent, and at
sufficiently high temperatures, a substantial amount of reverse current can be observed (mA or more).
The third region is forward but small bias, where only a small forward current is conducted.
As the potential difference is increased above an arbitrarily defined "cut-in voltage" or "on-voltage" or "diode forward
voltage drop (Vd)", the diode current becomes appreciable (the level of current considered "appreciable" and the value of cut-in
voltage depends on the application), and the diode presents a very low resistance. The current–voltage curve is exponential. In a normal silicon diode at rated currents, the arbitrary cut-in voltage is
defined as 0.6 to 0.7 volts. The value is different for other diode types—Schottky diodes can be rated as low as 0.2 V, Germanium diodes 0.25 to 0.3
V, and red or blue light-emitting diodes (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 transistor
co-inventor William Bradford Shockley) gives the I–V
characteristic of an ideal diode in either forward or reverse bias (or no bias). The
Shockley ideal diode equation is below, where n, the ideality
factor, is equal to 1 :
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 in many cases
is assumed to be approximately equal to 1 (thus the notation n is omitted). The ideality factor does not form part of the
diode equation, and was added to account for imperfect junctions as observed in real transistors. By setting n = 1 above, the equation reduces to
the Shockley ideal diode equation.
The thermal voltage
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:
The reverse saturation current, IS, is not constant for a given device, but varies with temperature; usually more significantly than
so that VD 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 doesn't account for the processes involved in reverse breakdown and photon-assisted R–G. Additionally, it doesn't 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 (see Figure 5) the exponential in the diode equation is negligible, and the current is a constant (negative) reverse
current value of −IS. The reverse breakdown region is not modeled by the Shockley diode equation.
For even rather small forward bias voltages (see Figure 5) the exponential is very large because the thermal voltage is very small, so the
subtracted '1' in the diode equation is negligible and the forward diode current is often approximated as
The use of the diode equation in circuit problems is illustrated in the article on diode modeling.
For circuit design, a small-signal model of the diode behavior often proves useful. A specific example of diode modeling is discussed in the article on
Following the end of forward conduction in a p–n type diode, a reverse current flows 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" tr
(on the order of tens of nanoseconds to a few microseconds) may be required to remove the reverse recovery charge Qr from the diode. During
this recovery time, the diode can actually conduct in the reverse direction. In certain real-world cases it can be important to consider the losses
incurred by this non-ideal diode effect.
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 diodes.
The reverse current ceases abruptly when the stored charge is depleted; this abrupt stop is exploited in step recovery diodes for generation of extremely short pulses.
Types of semiconductor diode
Figure 8: Several types of diodes. The scale is centimeters.
Typical datasheet drawing showing the dimensions of a DO-41 diode package.
There are several types of p–n junction diodes, which
emphasize either a different physical aspect of a diode often by geometric scaling, doping level, choosing the right electrodes, are just an application
of a diode in a special circuit, or are really different devices like the Gunn and laser diode and the MOSFET:
Normal (p–n) diodes, which operate as described above, are usually made of doped silicon
or, more rarely, germanium. Before the development of silicon power rectifier
diodes, cuprous oxide and later selenium was used; its low efficiency gave it a much higher forward voltage drop (typically 1.4 to 1.7 V
per "cell", with multiple cells stacked to increase the peak inverse voltage rating in high voltage rectifiers), and required a large heat sink
(often an extension of the diode's metal substrate), much larger than a silicon diode of the same current ratings would require.
The vast majority of all diodes are the p–n diodes found in CMOSintegrated circuits, which include two diodes
per pin and many other internal diodes.
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 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, so there are collisions between them on the way out. The only practical difference is that the two types have temperature coefficients of
These are a type of point-contact diode. The cat's whisker diode consists of a thin or sharpened metal wire pressed against a semiconducting
crystal, typically galena or a piece of coal. The wire forms the anode and the crystal forms the cathode. Cat's whisker diodes were also called
crystal diodes and found application in crystal radio receivers. Cat's whisker diodes are generally obsolete, but may be
available from a few manufacturers.[citation
These are actually JFETs with the gate shorted to the source, and function
like a two-terminal current-limiter analog to the Zener diode, which is limiting voltage. 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
These have a region of operation showing negative resistance
caused by quantum tunneling, allowing
amplification of signals and very simple bistable circuits. Due to 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
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. With appropriate biasing, dipole domains form and
travel across the diode, allowing high frequency microwaveoscillators to be built.
In a diode formed from a direct band-gap
semiconductor, such as gallium arsenide, carriers that cross the
junction emit photons when they recombine with the majority carrier on the other
side. Depending on the material, wavelengths (or colors) from the infrared to the near ultraviolet may be produced. The forward potential of these diodes depends on the
wavelength of the emitted photons: 2.1 V corresponds to red, 4.0 V to violet. 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 combinations of three
LEDs of a different color, or a blue LED with a yellow scintillator
coating. 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.
This term is used both for conventional p–n diodes used to monitor temperature due to their varying forward voltage with temperature, and for
Peltier heat pumps
for thermoelectric heating and cooling. Peltier heat
pumps may be made from semiconductor, though they do not have any rectifying junctions, they use the differing behaviour of charge carriers in N and P
type semiconductor to move heat.
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, 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.
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 diodes are constructed from a 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 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, mixers, and 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.
As a dopant, gold (or platinum) acts as recombination centers, which helps a
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.
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 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.
The term stabistor refers to a special type of diodes featuring extremely stable 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.
These are avalanche diodes designed specifically to protect other semiconductor devices from high-voltage 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.
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. They also enabled tunable oscillators in early discrete tuning of radios, where a
cheap and stable, but fixed-frequency, crystal oscillator provided the reference frequency for a voltage-controlled oscillator.
These can be made to conduct 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 diode is colloquially applied to several types of breakdown diodes, but strictly
speaking Zener diodes have a breakdown voltage of below 5 volts, whilst those above that value are usually avalanche diodes. In practical voltage
reference circuits, Zener and switching diodes are connected in series and opposite directions to balance the temperature coefficient 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). The Zener diode is named for Dr. Clarence Melvin Zener of Carnegie Mellon University, inventor of the device.
There are a number of common, standard and manufacturer-driven numbering and coding schemes for diodes; the two most common being the EIA/JEDEC standard and the European Pro
The standardized 1N-series numbering EIA370 system was introduced in the
US by EIA/JEDEC (Joint Electron Device Engineering Council) about 1960. Among the most popular in this series were: 1N34A/1N270 (Germanium signal),
1N914/1N4148 (Silicon signal), 1N4001-1N4007 (Silicon 1A power
rectifier) and 1N54xx (Silicon 3A power rectifier)
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:
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.
The first use for the diode was the demodulation of amplitude
modulated (AM) radio broadcasts. The history of this discovery is treated in depth in the radio
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 (originally a crystal diode) 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 filter and fed into an audio amplifier or transducer, which generates sound waves.
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. (Any diode used in such an
application is called a flyback diode). Many integrated circuits also incorporate diodes
on the connection pins to prevent external voltages from damaging their sensitive transistors. Specialized diodes are used to protect from over-voltages at higher power (see Diode types above).
In addition to light, mentioned above, semiconductor diodes are
sensitive to more energetic radiation. In electronics, cosmic rays
and other sources of ionizing radiation cause noisepulses 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 volts
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 centimetre) 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 centimetre).
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.
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 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 at room temperature. This is approximately linear for temperatures above about 20 kelvins.
Some graphs are given for: 1N400x
series, and CY7 cryogenic temperature sensor.
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. An uninterruptible power supply may use diodes in this way to ensure that 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
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 backwards through the circuit and trigger "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.
Diodes are usually referred to as D for diode on PCBs.
Sometimes the abbreviation CR for crystal rectifier is used.
Two-terminal nonlinear devices
Many other two-terminal nonlinear devices exist, for example a neon lamp has
two terminals in a glass envelope and has interesting and useful nonlinear properties. Lamps including arc-discharge lamps, incandescent lamps, fluorescent lamps and mercury vapor lamps have two terminals and display nonlinear current–voltage