A diode is a two-terminal electronic component that conducts electric current in only one direction.
A diode is an electronic component with two electrodes which a signal can flow between (but thermionic diodes can have one or two more electrodes).
The most common function of a diode is to allow an electric current to flow in one direction and to block it in the opposite direction.
Today, the most common diodes are made from semiconductor materials such as silicon or germanium.
There are many kinds of diode. For example, Schottky Diode, LED(Light Emitting Diode), Photo Diode, Laser Diode, Varactor Diode, Current Regulator Diode, PIN Diode, Tunnel Diode, Step Recovery Diode, IMPATT Diode, etc.
Types of Diodes:
- Silicon diode.
- Germanum diode
- Zener diode
- Photo diode
- Light emitting diode (LED)
- Tunnel diode
The first types of diodes were called Fleming valves. They worked inside a glass tube (much like a light bulb). Inside the glass bulb there was a small metal wire and a large metal plate. The small metal wire would heat up and emit electricity, which was captured by the plate. However, the large metal plate would not heat up enough to emit electricity because it was too big. So, electricity could go in one direction through the tube but not in the reverse direction. Flemming valves are mostly obsolete now, because they have been replaced by semiconductor diodes.
Semiconducting diodes are usually made of two types of semiconducting metals connected to each other. One type of metal has atoms connected together with a few electrons to spare. The other metal has atoms connected together and needs a few electrons to be complete. Because one metal has too many electrons and the other metal has too few, the electricity will flow easily from the metal with too many electrons into the metal with too few. However, electricity will not flow easily in the reverse direction -- from the metal with too few electrons to the metal with too many. Silicon with arsenic dissolved in it makes a good metal with electrons to spare, while silicon with aluminum dissolved in it makes a good metal with too few electrons to be complete. There are actually many types of combinations of metals that will make p-type and n-type semiconductors.
Positive voltage at p-side
If you give positive voltage to the p-side and negative voltage to the n-side, the electrons from the n-side wants to go to the positive voltage at the p-side and the holes of the p-side wants to go to the negative voltage at the n-side. In fact of this, current flow is able to exist. This is called breakdown. The breakdown voltage of a silicon Diode is at about 0.7V. A germanium Diode needs a breakdown voltage at about 0.3V.
Negative voltage at p-side
If you give negative voltage to the p-side and positive voltage to the n-side, the electrons of the n-side go to the negative voltage at the n-side. The holes of the p-side do to the positive voltage at the p-side. So there won´t be a current flow between p- and n-side. If you give to much voltage to the Diode in negative direction, the Diode will be destroyed.
When the temperature increases, the voltage, when the breakdown happens, will go down.
Types of Diodes
The standard rectifier diode is the actual original diode. It has different requirements. It should have high current densities in the forward area, and a high barrier permissible temperature. It should also have a minimum passing-voltage and a high cut-off-frequency. You should also have a high blocking voltage, whereby the blocking flows should remain low. Their applications are the whole modern analog and digital electronics. Especially it becomes straightening of changing and turning tension, and to limit power supply voltage used. The Diode is often needed for measurement and drive.
Z-diode (Zener diode) operates in the direction lock and so the direction of their work area is located in the 3rd Quadrant. In working towards the passage it is like a normal diode. The name comes from Zenereffekt, the man with the name Zener discovered. The term Z-diode is only a shortcut. A Z-diode can vary how high it is doped and then has different properties breakthrough. With a high allocation to the diode it has a low and a small space charge zone. Is it high, it works with Zener \ tunneleffect. At low doping, it has a large breakdown voltage and space charge zone and works with the avalanche effect. For medium-doping is the breakdown voltage 5-8 volts and there are two effects. Z-diodes are best suited to stabilize voltage for circuits with low power consumption. But the limitation of voltage spikes is a possibility to use it. With appropriate Zener voltage they can be used as donors in nominal value of measuring and control technology, or where reference voltages are required. It can also be used as a protective diode.
Top diodes are actually the exact opposite of junction diodes. They have a small barrier layer capacity and are also in high-frequency applications up to several GHz. But they must ensure only at low currents and voltages to operate. As a particular example the gold wire germanium diode is mentioned. Junction diodes have a p-n-transition over a large area and are often made of silicon. They are designed for high currents and voltages. Because they have a pretty big barrier layer capacity, they are not suitable for high frequency applications. A specific example of the application is called the power diode.
The capacity diode is a semiconductor diode barrier in the direction of running, so does the barrier layer or space charge zone on pn-transition as a capacity. If the voltage on the diode is changing, than the capacity of the barrier will change, too. Then you look at the capacity diode you see the barrier layer capacity is especially great. Because of the capacity variation can be set 3 different p-n-transitions. With a 1:3 ratio, it is a linear transition, in an abrupt 1:6 and 1:30 a hyper-abrupt p-n-transition. They are used as a substitute for rotary capacitors for the swing vote in the district of radios and televisions, and they will also find use in circuits for the generation of frequency modulation.
Step-Recovery-Diode: It is especially used in circuits with high frequencies up to GHz. The idea is, to work with the current that flows after the polarity was reversed. It has also an intrinsic layer between the p- and the n-layer. After the polarization has changed, there develops a layer without carriers. That means, there is a layer that is almost not conducting. So you can achieve a high slew rate. This diode is used with high gigahertz-frequencies.
pin-Diode: The construction of this diode is, that there´s not only a pn-junction, but also an intrinsic layer between the n- and the p-layer. This means, this layer is nearly nonconducting. If it´s forward biased, especially at lower frequencies, it has almost the same characteristics as a usual standard diode. But if it runs in reverse direction, there develops two space charge regions with different extensions. Because of this broad space charge region in the i-zone, the i-zone becomes very conductive. The pin-diode is useful for a high block voltage. It´s a quite fast diode and also used at very high frequencies.
Schottky-Diode: It consists of a pn-junction with a metal layer, which is oxidized on the n-doped silicon. This metal can be e.g. aluminum or nickel. In forward direction, the threshold voltage is about 0.3 volt. This is about half of the threshold voltage of a usual diode. The function of this diode is that no minority carriers are injected. So, there´s no diffusion capacitance, because there are no carriers, that could diffuse. This explains why this diode is a very fast one. So it is always used, when speed plays a role. The advantage of this diode is, that it is faster, but has otherwise no restrictions and works like a usual diode. The only disadvantage is, it isn´t appropriate in reverse direction.
A tunnel diode consists of a high doped pn-junction. That means both, the n- and the p-layer are high doped. Because of this high doping, there is only a very narrow gap, where the electrons are able to pass through. This so called tunnel-effect appears in both directions. After a certain amount of electrons have passed, the current through the gap decreases, until the normal current through the diode at the threshold voltage begins. This causes an area of a negative differential resistance. This diods are used to deal with really high frequencies ( 100 GHz) and are of course mostly used in the area of the negative differential resistance.
Backwarddiode: This diode has a construction that is similar to the tunnel diode, but the n- and the p-layer aren´t doped as high. It works with small negative voltages, because it has no threshold voltage in the third quadrant. The current increases immediately, there. From this reason, this diode works in most times in this area.
Topics of Interest
In electronics, a diode is a two-terminal electronic component that conducts electric current in only one direction. The term usually refers to a semiconductor diode, the most common type today, which is a crystal of semiconductor connected to two electrical terminals, a P-N junction. A vacuum tube diode, now little used, is a vacuum tube with two electrodes; a plate and a cathode.
The most common function of a diode is to allow an electric current in one direction (called the diode's forward direction) while blocking current in the opposite direction (the reverse direction). Thus, the diode can be thought of as an electronic version of a check valve. This unidirectional behavior is called rectification, and is used to convert alternating current to direct current, and extract modulation from radio signals in radio receivers.
However, diodes can have more complicated behavior than this simple on-off action, due to their complex non-linear electrical characteristics, which can be tailored by varying the construction of their P-N junction. These are exploited in special purpose diodes that perform many different functions. Diodes are used to regulate voltage (Zener diodes), electronically tune radio and TV receivers (varactor diodes), generate radio frequency oscillations (tunnel diodes), and produce light (light emitting diodes).
Diodes were the first semiconductor electronic devices. The discovery of crystals' rectifying abilities was made by German physicist Ferdinand Braun in 1874. The first semiconductor diodes, called cat's whisker diodes were made of crystals of minerals such as galena. Today most diodes are made of silicon, but other semiconductors such as germanium are sometimes used.
Although the crystal semiconductor diode was popular before the thermionic diode (also known as vacuum tubes, tubes, or valves), thermionic and solid state diodes were developed in parallel.
The basic principle of operation of thermionic diodes was discovered by Frederick Guthrie in 1873. 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.
The principle was independently rediscovered by Thomas Edison on February 13, 1880. At the time Edison was carrying out research into 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, and he was able to confirm that an invisible current could be drawn 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 more or less replaced the resistor in a DC voltmeter and on this basis was awarded a patent for it in 1883. There was no apparent practical use for such device at the time, and the patent application was most likely simply a precaution in case someone else did find a use for the so-called “Edison Effect”.
About 20 years later, John Ambrose Fleming (scientific adviser to the Marconi Company and former Edison employee) realized that the Edison effect could be used as a precision radio detector. Fleming patented the first true thermionic diode in Britain on November 16, 1904 (followed by U.S. Patent 803,684 in November 1905).
The principle of operation of crystal diodes was discovered in 1874 by the German scientist Karl Ferdinand Braun. Braun patented the crystal rectifier in 1899. Braun’s discovery was further developed by Jagdish Chandra Bose into a useful device for radio detection.
The first actual radio receiver using a crystal diode was built by Greenleaf Whittier Pickard. Pickard received a patent for a silicon crystal detector on November 20, 1906.
Other experimenters tried a variety of minerals and other substances, although by far the most popular was the lead sulfide mineral Galena. Although other substances offered slightly better performance, galena had the advantage of being cheap and easy to obtain, and was used almost exclusively in home-built “crystal sets”, until the advent of inexpensive fixed-germanium diodes in the 1950s.
At the time of their invention, such devices were known as rectifiers. In 1919, William Henry Eccles coined the term diode from the Greek roots dia, meaning “through”, and ode, meaning “path”.
Thermionic and gaseous state (vacuum tube) diodes
Thermionic diodes are thermionic-valve devices (also known as vacuum tubes, tubes, or valves), which are arrangements of electrodes surrounded by a vacuum within a glass envelope. Early examples were fairly similar in appearance to incandescent light bulbs.
In thermionic valve diodes, a current through the heater filament indirectly heats the cathode, another internal electrode treated with a mixture of barium and strontium oxides, which are oxides of alkaline earth metals; these substances are chosen because they have a small work function. (Some valves use direct heating, in which a tungsten filament acts as both heater and cathode.) The heat causes thermionic emission of electrons into the vacuum. In forward operation, a surrounding metal electrode called the anode is positively charged so that it electrostatically attracts the emitted electrons. However, electrons are not easily released from the unheated anode surface when the voltage polarity is reversed. Hence, any reverse flow is negligible.
For much of the 20th century, thermionic valve diodes were used in analog signal applications, and as rectifiers in many power supplies. Today, valve diodes are only used in niche applications such as rectifiers in electric guitar and high-end audio amplifiers as well as specialized high-voltage equipment.
A modern semiconductor diode is made of a crystal of semiconductor like silicon that has impurities 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. The diode's terminals are attached to each of these regions. The boundary within the crystal between these two regions, called a PN junction, is where the action of the diode takes place. The crystal conducts conventional current in a direction from the p-type side (called the anode) to the n-type side (called the cathode), but not in the opposite direction.
Another type of semiconductor diode, the Schottky diode, is formed from the contact between a metal and a semiconductor rather than by a p-n junction.
I–V characteristics of a P-N junction diode (not to scale).
A semiconductor diode’s behavior in a circuit is given by its current–voltage characteristic, or I–V graph (see graph at right). 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 which 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 0.6 V. Thus, if an external current is passed through the diode, about 0.6 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 forward bias.
A diode’s 'I–V characteristic' can be approximated by four regions of operation (see the figure at right).
At very large reverse bias, beyond the peak inverse voltage or PIV, a process called reverse breakdown occurs which causes a large increase in current (i.e. a large number of electrons and holes are created at, and move away from the pn 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 zener 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 suffiently 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 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.
Types of semiconductor diode
There are several types of junction diodes, which either emphasizes
a different physical aspects 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 JFET:
- Normal (p-n) diodes
- which operate as described above. Usually made of doped silicon or, more rarely, germanium. Before the development of modern silicon power rectifier diodes, cuprous oxide and later selenium
was used; its low efficiency gave it a much higher forward voltage drop
(typically 1.4–1.7 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 CMOS integrated circuits, which include 2 diodes per pin and many other internal diodes.
- Switching diodes
- Switching diodes, sometimes also called small signal diodes, are a
single p-n diode in a discrete package. A switching diode provides
essentially the same function as a switch. Below the specified applied
voltage it has high resistance similar to an open switch, while above
that voltage it suddenly changes to the low resistance of a closed
switch. They are used in devices such as ring modulation.
- Schottky diodes
diodes are constructed from a metal to semiconductor contact. They have
a lower forward voltage drop than any p-n junction diode. 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 generally much higher than non Schottky rectifiers. Schottky diodes are majority carrier
devices and so do not suffer from minority carrier storage problems
that slow down most normal diodes — so they have a faster “reverse
recovery” than any p-n junction diode. They also tend to have much
lower junction capacitance than PN diodes and this contributes towards
their high switching speed and their suitability in high speed circuits
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
- “Gold-doped” diodes
- As a dopant, gold (or platinum)
acts as recombination centers, which help 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.
- 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 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
- Point-contact diodes
- These work the same as the junction semiconductor diodes described
above, but its 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
- Cat’s whisker or crystal diodes
- 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 obsolete.
- 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.
- 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, replacing older designs that took a long
time to warm up and lock. A PLL is faster than a FLL, but prone to
integer harmonic locking (if one attempts to lock to a broadband
signal). 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.
- Zener diodes
- Diodes that can be made to conduct backwards. This effect, called
Zener breakdown, occurs at a precisely defined voltage, allowing the
diode to be used as a precision voltage reference. 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 below). Two (equivalent) Zeners in series and in
reverse order, in the same package, constitute a transient absorber (or
Transorb, a registered trademark). They are named for Dr. Clarence Melvin Zener of Southern Illinois University, inventor of the device.
- Avalanche diodes
- 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 opposite polarities.
- Transient voltage suppression diode (TVS)
- 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.
- 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.
- Light-emitting diodes (LEDs)
- 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: 1.2 V corresponds to red, 2.4 to violet. The
first LEDs were red and yellow, and higher-frequency diodes have been
developed over time. All LEDs are monochromatic; “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.
- Laser diodes
- When an LED-like structure is contained in a resonant cavity formed by polishing the parallel end faces, a laser can be formed. Laser diodes are commonly used in optical storage devices and for high speed optical communication.
- Esaki or tunnel diodes
- these have a region of operation showing negative resistance caused by quantum tunneling,
thus allowing amplification of signals and very simple bistable
circuits. These diodes are also the type most resistant to nuclear
- Gunn diodes
- 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 microwave oscillators to be built.
- Peltier diodes
- are used as sensors, heat engines for thermoelectric cooling. Charge carriers absorb and emit their band gap energies as heat.
- Current-limiting field-effect diodes
- These are actually a JFET
with the gate shorted to the source, and function like a two-terminal
current-limiting analog to the 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.,
Other uses for semiconductor diodes include sensing temperature, and computing analog logarithms.
A standardized 1N-series numbering 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),
IN914/1N4148 (Silicon signal) and 1N4001-1N4007 (Silicon 1A power
- Thyristor or silicon controlled rectifier (SCR)
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 voltage, whose amplitude
or “envelope” is proportional to the original audio signal, but whose
average value is zero. The diode (originally a crystal diode) rectifies
the AM signal, leaving a signal whose average amplitude is the desired
audio signal. The average value is extracted using a simple filter and fed into an audio transducer, which generates sound.
Rectifiers are constructed from diodes, where they are used to convert alternating current (AC) electricity into direct current (DC). Automotive alternators are a common example, where the diode provides better performance than the commutator of earlier dynamo. Similarly, diodes are also used in Cockcroft–Walton voltage multipliers to convert AC into higher DC voltages.
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 above).
Diodes can be combined with other components to construct AND and OR logic gates. This is referred to as diode logic.
Ionising radiation detectors
In addition to light, mentioned above, semiconductor diodes are sensitive to more energetic radiation. In electronics, cosmic rays and other sources of ionising 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 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 or 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. From the Shockley ideal diode equation given
above, it appears the voltage has a positive temperature coefficient
(at a constant current)but depends on doping concentration and
operating temperature. The temperature coefficient can be
negative as in typical thermistors or positive for temperature sense
diodes down to about 20 degrees Kelvin.
Diodes will prevent currents from flowing 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
built in this may use diodes to ensure that current is only drawn from
the battery when necessary. Similarly, 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 charged battery when the alternator is
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