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Rectifier Diode is a semiconductor device used to convert alternating current to direct current.It has obvious unidirectional conductivity, and can be made of materials such as semiconductor germanium or silicon. This article gives you a brief introduction to rectifier diodes.
Catalog
I Rectifier Diode Selection
Rectifier diodes are generally planar silicon diodes, which are used in various power rectifier circuits.
When we select a rectifier diode, the parameters such as its maximum rectifier current, maximum reverse working current, cut-off frequency and reverse recovery time should be considered.
The rectifier diode used in the series stabilized power supply circuit does not have high requirements for the reverse recovery time of the cut-off frequency. As long as the maximum rectification current and the maximum reverse working current meet the requirements of the circuit, the rectifier diode is selected. For example, 1N series, 2CZ series, RLR series, etc.
Figue 1. 2CZ Rectifier Diode
In the rectifier circuit and pulse rectifier circuit of the switching stabilized power supply, the rectifier diode should have a higher operating frequency and a shorter reverse recovery time (such as RU series, EU series, V series, 1SR series, etc.). Or we can select a fast recovery diode or a Schottky rectifier diode.
II Rectifier Diode Parameters
1. Maximum average rectified current IF: the maximum forward average current allowed to pass through in long-term work.
The current is determined by the junction area and heat dissipation conditions of the PN junction. The average current through the diode can not be greater than this value and should meet the heat dissipation conditions. For example, the IF of a 1N rectified series diode is 1A.
2. Maximum working reverse voltage VR: the maximum allowable reverse voltage applied across the diode. If this value is exceeded, the reverse current (IR) will increase sharply and the unidirectional conductivity of the diode will be destroyed, causing reverse breakdown.
Usually take half of the reverse breakdown voltage (VB) as (VR). For example:
Parameter
1N
1N
1N
1N
1N
1N
1N
VR
50V
100V
200V
400V
600V
800V
V
3. Maximum reverse current IR: the reverse current allowed to flow through the diode under the highest reverse working voltage. This parameter reflects the unidirectional conductivity of the diode. Therefore, the smaller the current value, the better the diode quality.
4. Breakdown voltage VB: the voltage rectifier value at the sharp bending point of the reverse volt-ampere characteristic curve of the diode. When the reverse is a soft characteristic, it refers to the voltage value under a given reverse leakage current.
5. Maximum operating frequency fm: the highest operating frequency of the diode under normal conditions. It is mainly determined by the junction capacitance and diffusion capacitance of the PN junction. If the operating frequency exceeds fm, the unidirectional conductivity of the diode will not be well reflected.
For example, the fm of 1N series diode is 3kHz. Also, fast recovery diodes are used for the rectification of high-frequency alternating currents, such as switching power supplies.
6. Reverse recovery time trr: refers to the reverse recovery time under the specified load, forward current and maximum reverse transient voltage.
7. Zero-bias capacitance CO: the sum of the diffusion capacitance and the junction capacitance when the diode voltage is zero.
Due to the limitation of the manufacturing process, even for the same type of diodes, their parameters have a large dispersion. The parameters given in the manual are often within a range. If the test conditions change, the corresponding parameters will also change.
For example, the IR of the 1N series silicon plastic-sealed rectifier diode at 25°C is less than 10uA, and at 100°C, it becomes less than 500uA.
III Cause of Damage
1. Inadequate lightning protection and over-voltage protection. Even if there are lightning protection and overvoltage protection devices, if the work is unreliable, the rectifier diode is damaged due to lightning strikes or overvoltage.
2. Bad operating conditions. In the indirect operation generator set, because the calculation of the speed ratio is incorrect, or the diameter ratio of the two belt pulleys does not meet the requirements of the speed ratio, the generator runs at a high speed for a long time. Also, the rectifier is working at a higher voltage for a long time, accelerating aging and causing breakdown.
3. Poor operation management. Operators are irresponsible and do not understand the changes in external load (especially between midnight and 6 am the next day). Or there is a load fault outside, and the operator did not take measures in time. These will cause overvoltage and the rectifier diode will be broken down and damaged.
4. Bad installation or manufacturing. Because the generator set has been operating under large vibration for a long time, the rectifier diode is also under this interference. Also, the generator set does not operate at an even pace, so the working voltage of the rectifier diode also fluctuates. This greatly accelerates the aging and damage of the rectifier diode.
5. Improper diode specifications and models. If the parameters of the replaced rectifier diode do not meet the requirements, or the wiring is wrong, the rectifier diode will breakdown and damage.
6. The safety margin of the rectifier diode is too small. The over-voltage and over-current safety margin of the rectifier diode is too small, so it can't withstand peak attack in the excitation circuit.
IV What does a Rectifier do?
The rectifier diode has obvious unidirectional conductivity. It can be made of materials such as semiconductor germanium or silicon. The function of the rectifier diode is to use the unidirectional conductivity of the PN junction to convert alternating current into pulsating direct current. So what are the main functions of the rectifier diode? The following is a detailed introduction:
1. Forward Characteristic
The most prominent feature of the rectifier diode is its forward feature. When a forward voltage is applied to the rectifier diode, the initial part of the forward voltage is very small, and it cannot effectively overcome the blocking effect of the electric field in the PN junction.
When the forward current is almost zero, the forward voltage can't conduct the diode, which is called the dead zone voltage.
When the forward voltage is greater than the dead zone voltage, the electric field is effectively overcome, the rectifier diode is turned on, and the current rises rapidly as the voltage increases. In the normal current range, the rectifier diode terminal voltage remains almost unchanged when it is turned on.
Figure 2. Rectifer Forward and Reverse Characteristics
2. Reverse Characteristic
When the reverse voltage applied to the rectifier diode does not exceed a certain range, the reverse current is formed by the drift of the minority carriers. Because the reverse current is very small, the rectifier diode is in an off state.
The reverse saturation current of the rectifier diode is affected by temperature. Generally, the reverse current of silicon rectifier diodes is much smaller than that of germanium rectifier diodes. The reverse saturation current of low-power silicon rectifier diodes is on the order of nA, and that of low-power germanium rectifier diodes is on the order of μA.
When the temperature of the rectifier diode increases, the semiconductor is excited, and the number of minority carriers increases.
3. Reverse breakdown
The reverse breakdown of the rectifier diode is divided into two types: Zener breakdown and avalanche breakdown.
In high doping concentration, due to the small width of the barrier region, the reverse voltage will destroy the covalent bond structure, so the electrons will break away from the covalent bond, and electron holes will be generated. This is called the Zener breakdown.
Another type of breakdown is the avalanche breakdown. As the reverse voltage of the rectifier diode increases, the external electric field will accelerate the electron drift speed, so the valence electrons will collide with each other out of the covalent bond, generating new electron-hole pairs.
Figure 3. Zener Breakdown and Avalanche Breakdown
V What is a Rectifier Circuit?
The rectifier circuit refers to the conversion of alternating current into direct current. Generally, it is composed of a transformer, the main rectifier circuit, and a filter circuit. If you want to get a constant voltage value, you need to add a voltage regulator circuit. Here, we will only talk about the main rectifier circuit.
1. Half-wave Rectifier Circuit
The structure of this half-wave rectifier circuit is very simple. The main component is a diode as shown in the schematic diagram below.
Figure 4. Half-wave Rectifier Circuit Schematic Diagram
The 220V input is a sine wave AC. It passes through a transformer and is reduced after the transformer, but it is still a sinusoidal AC signal in the end.
A typical feature of diodes is unidirectional conductivity. If the diode anode voltage is greater than the diode cathode voltage, the diode will be turned on. In the opposite situation, the diode will be turned off.
The following picture shows this process. Picture a is the AC output from the transformer. When the output voltage is in the positive half cycle, the voltage at point a is higher than the voltage at point b, and the diode will be turned on. And the voltage across the load RL is about the output voltage of the transformer.
When the output voltage is in the negative half cycle, the voltage at point b is higher than the voltage at point a, then the diode will be cut off. The corresponding current cannot flow to the load, so half of the cycle is missing in Figure b.
Figure 5. Half-wave Rectifier Circuit Waveform before and after Filtering
2. Full-wave Rectifier Circuit
Since half cycle is lost in half-wave rectification, the efficiency is limited. A full-wave bridge rectifer can solve the problem.
Compared with half-wave rectification, full-wave rectification uses one more diode. However, the transformer here is with a central axis, which uses the unidirectional conductivity of the diode.
Figure 6. Full-wave Rectifier Circuit Schematic Diagram
Let's analyze this principle. If the AC is in the positive half cycle, the voltage at point a is higher than the voltage at point b, then diode D1 will be turned on and diode D2 will be cut off. So the current will only flow from point a, through the diode D1 and the resistor RL, and finally to the central axis of the transformer.
If the AC is in the negative half cycle, the voltage at point b is higher than the voltage at point a, diode D2 will be turned on and diode D1 will be cut off. So the current will only flow from point b, and through diode D2 and resistor RL, finally to the central axis of the transformer.
Repeating these cycles achieves the filtering. The following picture shows the waveform before and after filtering.
Figure 7. Full-wave Rectifier Circuit Waveform before and after Filtering
3. Bridge Rectifier Circuit
The bridge rectifier circuit is more complicated than the previous two. The schematic diagram is as follows. The simple bridge rectifier circuit consists of a transformer and the main rectifier bridge, and load.
Figure 8. Bridge Rectifier Diagram-1
If the AC signal output is in the positive half cycle, under normal circumstances, the current flows to point A, facing diode 2 and diode 1.
Figure 9. Bridge Rectifier Circuit Schematic Diagram-2
However, due to the high voltage at point A, diode 1 is in the off state and diode 2 is in the on the state. So the current will flow through diode 2, then flow out from point B and then reach point D through the load.
Figure 10. Bridge Rectifier Circuit Schematic Diagram-3
At first glance, both diode 1 and diode 4 can be turned on, but the current is flowing from point A into the rectifier bridge and then through the load. The voltage will decrease after the current flows through the load, so the voltage at point D is far lower than the voltage at point A, and diode 4 is turned on and diode 1 is turned off. Finally, the current flows into the lower end of the transformer.
Figure 11. Bridge Rectifier Circuit Schematic Diagram-4
When the voltage at the lower end is higher than the voltage at the higher end, the current reaches point C.
Figure 12. Bridge Rectifier Circuit Schematic Diagram-5
Also, because the voltage at point C is high, diode 4 is in the off state and diode 3 is in the on state. The current will flow through diode 3 from point B, and then reach point D through the load.
Figure 13. Bridge Rectifier Circuit Schematic Diagram-6
Similar to the positive half-cycle, at first glance, diode 1 and diode 4 can be turned on. But since the current is flowing from point C into the rectifier bridge then through the load, the voltage at point D is far lower than point C, so diode 1 is turned on and diode 4 is turned off. Finally, the current flows into the upper part of the transformer.
Figure 14. Bridge Rectifier Circuit Schematic Diagram-7
Advantages of Bridge Rectification
Compared with full-wave rectification, bridge rectification has many advantages.
Full-wave rectification requires a transformer with a central axis, while bridge rectification does not have this requirement.
When the diode is in the off state, the voltage on the two ends of the bridge rectifier diode is less than half that of full-wave rectification. So the performance requirements of the bridge rectifier diode are not so high.
VI Rectifier Diode Replacement and Inspection
1. Replacement
After the rectifier diode is damaged, it can be replaced with a rectifier diode of the same model or another model with the same parameters.
Generally, rectifier diodes with high withstand voltage (reverse voltage) can replace rectifier diodes with low withstand voltage. And rectifier diodes with low withstand voltage cannot replace one with high withstand voltage.
A diode with a high rectification current can replace one with a low rectification current, while a diode with a low rectification current value cannot replace a diode with a high rectification current value.
2. How to Test a Bridge Rectifier
(1) Remove all the rectifier diodes in the rectifier.
(2) Use the 100×R or ×R ohm range of a multimeter to measure the two lead wires of the rectifier diode. Then swop the head and the tail and test again.
(3) If the resistance value measured twice has a great difference, it indicates that the diode is good (except for diodes with soft breakdown).
If the resistance value measured twice is small and almost the same, it means the diode has been broken down and cannot be used.
If the resistance value measured twice is infinite, it means that the diode has been internally disconnected and cannot be used.
Recommended Articles:
How does a Photodiode Work?
What are Avalanche Diodes?
What are Laser Diodes?
Close-up view of a silicon diode. The anode is on the right side; the cathode is on the left side (where it is marked with a black band). The square silicon crystal can be seen between the two leads.
TypePassivePin configuration
Anode and cathodeElectronic symbol Various semiconductor diodes. Bottom: A bridge rectifier. In most diodes, a white or black painted band identifies the cathode into which electrons will flow when the diode is conducting. Electron flow is the reverse of conventional current flow.[1][2][3] Structure of a vacuum tube diode. The filament itself may be the cathode, or more commonly (as shown here) used to heat a separate metal tube which serves as the cathode.A diode is a two-terminal electronic component that conducts current primarily in one direction (asymmetric conductance). It has low (ideally zero) resistance in one direction and high (ideally infinite) resistance in the other.
A semiconductor diode, the most commonly used type today, is a crystalline piece of semiconductor material with a p&#;n junction connected to two electrical terminals.[4] It has an exponential current&#;voltage characteristic. 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 in . Today, most diodes are made of silicon, but other semiconducting materials such as gallium arsenide and germanium are also used.[5]
The obsolete thermionic diode is a vacuum tube with two electrodes, a heated cathode and a plate, in which electrons can flow in only one direction, from cathode to plate.
Among many uses, diodes are found in rectifiers to convert alternating current (AC) power to direct current (DC), demodulation in radio receivers, and can even be used for logic or as temperature sensors. A common variant of a diode is a light-emitting diode, which is used as electric lighting and status indicators on electronic devices.
Main functions
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Unidirectional current flow
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The most common function of a diode is to allow an electric current to pass in one direction (called the diode's forward direction), while blocking it in the opposite direction (the reverse direction). Its hydraulic analogy is a check valve. This unidirectional behavior can convert alternating current (AC) to direct current (DC), a process called rectification. As rectifiers, diodes can be used for such tasks as extracting modulation from radio signals in radio receivers.
Threshold voltage
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A semiconductor diode's exponential current&#;voltage characteristic results in more complicated behavior than a simple on&#;off action.[6] Since exponential functions can be viewed as having a "knee" voltage, for simplicity, a diode is commonly said to have a forward threshold voltage, above which there is significant current and below which there is almost no current. However, this is only an approximation as the forward characteristic is gradual in its current&#;voltage curve.
Forward threshold voltage for various semiconductors Type Forward threshold voltage Silicon diodes 0.6 V to 0.7 V Germanium diodes 0.25 V to 0.3 V Schottky diodes 0.15 V to 0.45 V Light-emitting diodes (LEDs) 1.6 V (red) to 4 V (violet). Light-emitting diode physics § Materials has a complete list.Since 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 or as a somewhat imprecise voltage reference.
Reverse breakdown
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A diode's high resistance to current flowing in the reverse direction suddenly drops to a low resistance when the reverse voltage across the diode reaches a value called the breakdown voltage. This effect is used to regulate voltage (Zener diodes) or to protect circuits from high voltage surges (avalanche diodes).
Other functions
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A semiconductor diode's current&#;voltage characteristic can be tailored by selecting the semiconductor materials and the doping impurities introduced into the materials during manufacture.[6] These techniques are used to create special-purpose diodes that perform many different functions.[6] For example, to electronically tune radio and TV receivers (varactor diodes), to generate radio-frequency oscillations (tunnel diodes, Gunn diodes, IMPATT diodes), and to produce light (light-emitting diodes). Tunnel, Gunn and IMPATT diodes exhibit negative resistance, which is useful in microwave and switching circuits.
Diodes, both vacuum and semiconductor, can be used as shot-noise generators.
History
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Thermionic (vacuum-tube) diodes and solid-state (semiconductor) diodes were developed separately, at approximately the same time, in the early s, as radio receiver detectors.[7] Until the s, 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 rectifiers) that were available at that time.
In , Frederick Guthrie observed that a grounded, white-hot metal ball brought in close proximity to an electroscope would discharge a positively charged electroscope, but not a negatively charged electroscope.[8][9] In , Thomas Edison observed unidirectional current between heated and unheated elements in a bulb, later called Edison effect, and was granted a patent on application of the phenomenon for use in a DC voltmeter.[10][11] 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 radio detector. Fleming patented the first true thermionic diode, the Fleming valve, in Britain on 16 November [12] (followed by U.S. patent 803,684 in November ). 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 s due to selenium rectifier technology and then to semiconductor diodes during the s. 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 , German scientist Karl Ferdinand Braun discovered the "unilateral conduction" across a contact between a metal and a mineral.[13][14] Indian scientist Jagadish Chandra Bose was the first to use a crystal for detecting radio waves in .[15] The crystal detector was developed into a practical device for wireless telegraphy by Greenleaf Whittier Pickard, who invented a silicon crystal detector in and received a patent for it on 20 November .[16] Other experimenters tried a variety of other minerals as detectors. Semiconductor principles were unknown to the developers of these early rectifiers. During the s understanding of physics advanced and in the mid-s researchers at Bell Laboratories recognized the potential of the crystal detector for application in microwave technology.[17] Researchers at Bell Labs, Western Electric, MIT, Purdue and in the UK intensively developed point-contact diodes (crystal rectifiers or crystal diodes) during World War II for application in radar.[17] After World War II, AT&T used these in its microwave towers that criss-crossed the United States, and many radar sets use them even in the 21st century. In , Sylvania began offering the 1N34 crystal diode.[18][19][20] During the early s, junction diodes were developed.
In , the first superconducting diode effect without an external magnetic field was realized.[21]
Etymology
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At the time of their invention, asymmetrical conduction devices were known as rectifiers. In , the year tetrodes were invented, William Henry Eccles coined the term diode from the Greek roots di (from δί), meaning 'two', and ode (from οδός), meaning 'path'. The word diode however was already in use, as were triode, tetrode, pentode, hexode, as terms of multiplex telegraphy.[22]
Although all diodes rectify, "rectifier" usually applies to diodes used for power supply, to differentiate them from diodes intended for small signal circuits.
Vacuum tube diodes
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Thermionic diodeA high power vacuum diode used in radio equipment as a rectifier
TypeThermionicPin configuration
Plate and Cathode, heater (if indirectly heated)Electronic symbolA thermionic diode is a thermionic-valve device consisting of a sealed, evacuated glass or metal envelope containing two electrodes: a cathode and a plate. The cathode is either indirectly heated or directly heated. If indirect heating is employed, a heater is included in the envelope.
In operation, the cathode is heated to red heat, around 800&#;1,000 °C (1,470&#;1,830 °F). 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.
A vacuum tube containing two power diodesThe operating temperature of the cathode causes 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 strontium oxides. These have a low work function, meaning that they more readily emit electrons than would the uncoated cathode.
The plate, not being heated, does not emit electrons; but is able to absorb them.
The alternating voltage to be rectified is applied between the cathode and the plate. When the plate voltage is positive with respect to the cathode, the plate electrostatically attracts the electrons from the cathode, so a current of electrons flows through the tube from cathode to plate. When the plate voltage is negative with respect to the cathode, no electrons are emitted by the plate, so no current can pass from the plate to the cathode.
Semiconductor diodes
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Close-up of an EFD108 germanium point-contact diode in DO7 glass package, showing the sharp metal wire (cat whisker) that forms the semiconductor junction.Point-contact diodes were developed starting in the s, out of the early crystal detector technology, and are now generally used in the 3 to 30 gigahertz range.[17][23][24][25] 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.[26] 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.[27][28] Point contact diodes generally exhibit lower capacitance, higher forward resistance and greater reverse leakage than junction diodes.
Junction diodes
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p&#;n junction diode
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A p&#;n junction diode is made of a crystal of semiconductor, usually silicon, but germanium and gallium arsenide are also used. Impurities are added to it to create a region on one side that contains negative charge carriers (electrons), called an n-type semiconductor, and a region on the other side that contains positive charge carriers (holes), called a p-type semiconductor. When the n-type and p-type materials are attached together, a momentary flow of electrons occurs from the n to the p side resulting in a third region between the two where no charge carriers are present. This region is called the depletion region because there are no charge carriers (neither electrons nor holes) in it. The diode's terminals are attached to the n-type and p-type regions. The boundary between these two regions, called a p&#;n junction, is where the action of the diode takes place. When a sufficiently higher electrical potential is applied to the P side (the anode) than to the N side (the cathode), it allows electrons to flow through the depletion region from the N-type side to the P-type side. The junction does not allow the flow of electrons in the opposite direction when the potential is applied in reverse, creating, in a sense, an electrical check valve.
Schottky diode
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Another type of junction diode, the Schottky diode, is formed from a metal&#;semiconductor junction rather than a p&#;n junction, which reduces capacitance and increases switching speed.[29][30]
Current&#;voltage characteristic
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A semiconductor diode's behavior in a circuit is given by its current&#;voltage characteristic. The shape of the curve is determined by the transport of charge carriers through the so-called depletion 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 recombination made, a positively charged dopant ion is left behind in the N-doped region, and a negatively charged dopant ion is created in the P-doped region. As recombination proceeds and more ions are created, an increasing electric field develops through the depletion zone that acts to slow and then finally stop recombination. At this point, there is a "built-in" potential across the depletion zone.
A PN junction diode in low forward bias mode. The depletion width decreases as voltage increases. Both p and n junctions are doped at a 1e15/cm3 doping level, leading to built-in potential of ~0.59V. Observe the different quasi Fermi levels for conduction band and valence band in n and p regions (red curves).Reverse bias
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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 called the reverse bias phenomenon.
Forward bias
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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). 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.
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. (See also: Rectifier § Rectifier voltage drop)
Operating regions
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Current&#;voltage characteristic of a p&#;n junction diode showing three regions: breakdown, reverse biased, forward biased. The exponential's "knee" is at Vd. The leveling off region which occurs at larger forward currents is not shown.A diode's current&#;voltage characteristic can be approximated by four operating regions. From lower to higher bias voltages, these are:
Shockley diode equation
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The Shockley ideal diode equation or the diode law (named after the bipolar junction transistor co-inventor William Bradford Shockley) models the exponential current&#;voltage (I&#;V) relationship of diodes in moderate forward or reverse bias. The article Shockley diode equation provides details.
Small-signal behavior
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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 I = I S e V D / ( n V T ) {\displaystyle I=I_{\text{S}}e^{V_{\text{D}}/(nV_{\text{T}})}} as explained in the Shockley diode equation article.
In detector and mixer applications, the current can be estimated by a Taylor's series.[32] 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.[32] The desired current component is approximately proportional to the square of the input voltage, so the response is called square law in this region.[26]:&#;p. 3&#;
Reverse-recovery effect
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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.[33] 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. 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.[34] In certain real-world cases it is important to consider the losses that are incurred by this non-ideal diode effect.[35] 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 the generation of extremely short pulses.
Types of semiconductor diode
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Current&#;voltage curves of several types of diodesNormal (p&#;n) diodes, which operate as described above, are usually made of doped silicon or germanium. Before the development of silicon power rectifier diodes, cuprous oxide and later selenium was used. Their low efficiency required a much higher forward voltage to be applied (typically 1.4 to 1.7 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,[36] which include two diodes per pin and many other internal diodes.
Graphic symbols
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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.
Numbering and coding schemes
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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 Electron standard:
The standardized 1N-series numbering EIA370 system was introduced in the US by EIA/JEDEC (Joint Electron Device Engineering Council) about . Most diodes have a 1-prefix designation (e.g., 1N). Among the most popular in this series were: 1N34A/1N270 (germanium signal), 1N914/1N (silicon signal), 1N400x (silicon 1A power rectifier), and 1N580x (silicon 3A power rectifier).[48][49][50]
JIS
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The JIS semiconductor designation system has all semiconductor diode designations starting with "1S".
Pro Electron
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The European Pro Electron coding system for active components was introduced in 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:
Other common numbering/coding systems (generally manufacturer-driven) include:
Related devices
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In optics, an equivalent device for the diode but with laser light would be the optical isolator, also known as an optical diode,[51] that allows light to only pass in one direction. It uses a Faraday rotator as the main component.
Applications
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Radio demodulation
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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 crystal detector article. In summary, an AM signal consists of alternating positive and negative peaks of a radio carrier wave, whose amplitude or envelope is proportional to the original audio signal. The diode 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 via audio speaker.
In microwave and millimeter wave technology, beginning in the s, researchers improved and miniaturized the crystal detector. Point contact diodes (crystal diodes) and Schottky diodes are used in radar, microwave and millimeter wave detectors.[29]
Power conversion
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Schematic of basic ac-to-dc power supplyRectifiers 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, which rectifies the AC into DC, provides better performance than the commutator or earlier, dynamo. Similarly, diodes are also used in Cockcroft&#;Walton voltage multipliers to convert AC into higher DC voltages.
Reverse-voltage protection
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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
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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 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).
Logic gates
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Diode-resistor logic constructs AND and OR logic gates. Functional completeness can be achieved by adding an active device to provide inversion (as done with diode-transistor logic).
Ionizing radiation detectors
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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 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
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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. The temperature coefficient is approximately constant for temperatures above about 20 kelvin. Some graphs are given for 1N400x series,[52] and CY7 cryogenic temperature sensor.[53]
Current steering
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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 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 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 "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
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Diodes can be used to limit the positive or negative excursion of a signal to a prescribed voltage.
Clamper
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This simple diode clamp will clamp the negative peaks of the incoming waveform to the common rail voltageA diode clamp circuit can take a periodic alternating current signal that oscillates between positive and negative values, and vertically displace it such that either the positive or the negative peaks occur at a prescribed level. The clamper does not restrict the peak-to-peak excursion of the signal, it moves the whole signal up or down so as to place the peaks at the reference level.
Computing exponentials & logarithms
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The diode's exponential current&#;voltage relationship is exploited to evaluate exponentiation and its inverse function the logarithm using analog voltage signals (see Operational amplifier applications §§ Exponential output&#; and Logarithmic output).
Abbreviations
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Diodes are usually referred to as D for diode on PCBs. Sometimes the abbreviation CR for crystal rectifier is used.[54]
See also
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References
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Further reading
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If you want to learn more, please visit our website general rectifier diode.