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Schottky diodes, or more precisely - with the Schottky barrier diode - are semiconductor devices formed on the contact base of the metal-semiconductor, while the diodes used in conventional semiconductor p-n-transition.
Schottky diode takes its name, and the advent of electronics German physicist inventor Walter Schottky that in 1938 year, the study of the newly opened barrier effect confirmed put forward earlier theory that even though the emission of electrons of the metal and prevents potential barrier, but with increasing applied external electric field this barrier will decrease. Walter Schottky discovered this effect, which was then called the Schottky effect, in honor of the scientist.
Investigating the metal-semiconductor contact it can be seen that if near the semiconductor surface has a region depleted of majority carriers, the contact area of the semiconductor side of the semiconductor metal, a region of the space charge of ionized acceptors and donors, thus implemented locking contact - the same Schottky barrier . In what conditions does this barrier arise? The current of thermionic emission from the surface of a solid body determines the Richardson equation:
Let us create conditions when, upon contact of a semiconductor, for example, an n-type, with a metal, the thermodynamic work of the electron exit from the metal would be greater than the thermodynamic work of the electron exit from the semiconductor. Under such conditions, in accordance with the Richardson equation, the current of thermionic emission from the surface of the semiconductor will be greater than the current of thermionic emission from the surface of the metal:
At the initial time when contacting said materials, the current from the semiconductor into the metal exceeds the reverse current (from the metal to the semiconductor), resulting in the surface areas of semiconductor and metal - will accumulate space charges - positive in the semiconductor and negative - in metal. In the contact area there will be an electric field formed by these charges, and there will be a bending of the energy bands.
Under the influence of the field thermodynamic work function of a semiconductor increases, and the increase would occur as long as the contact area does not become equal thermodynamic work function, and the corresponding currents as applied to the thermionic emission surface.
The transition to the equilibrium state with the formation of a potential barrier for a p-type semiconductor and a metal is analogous to the example with an n-type semiconductor and a metal. The role of external voltage is to adjust the height of the potential barrier and the strength of the electric field in the space charge region of the semiconductor.
The diagram above shows the zone diagrams of various stages of the Schottky barrier formation. The equilibrium conditions in the contact region of thermionic emission currents equalize due to the field effect originated potential barrier, the height of which is equal to the difference of thermodynamic work function: φk = FMe - Fp / n.
Obviously, the current-voltage characteristic for the Schottky barrier is asymmetric. AT forward direction the current grows exponentially along with the increase in the applied voltage. In the reverse direction, the current is independent of the voltage. In both cases, the current is due to electrons as the main charge carriers.
Schottky diodes are therefore very fast, because they eliminate diffuse and recombination processes that require additional time. The dependence of the current on the voltage is connected with the change in the number of carriers, since these carriers participate in the process of charge transfer. The external voltage changes the number of electrons that can pass from one side of the Schottky barrier to the other side of it.
Due to manufacturing technology and based on the described principle of operation, Schottky diodes have a small voltage drop in the forward direction, which is significantly smaller than that of traditional p-n diodes.
Here, even a small initial current through the contact region leads to the release of heat, which then contributes to the appearance of additional current carriers. In this case, there is no injection of minority charge carriers.
The Schottky diodes therefore do not have a diffuse capacitance, since there are no minority carriers, and as a result - the speed is quite high compared with semiconductor diodes. It turns out a semblance of a sharp asymmetric p-n junction.
Thus, first of all, Schottky diodes are microwave diodes for various purposes: detector, mixing, avalanche-span, parametric, pulse, multiplier. Schottky diodes can be used as radiation receivers, strain gauges, nuclear radiation detectors, light modulators, and finally rectifiers for high-frequency current.
Designation of a Schottky diode in circuits
Schottky diodes today
To date, Schottky diodes are widely distributed in electronic devices. On the diagrams, they are depicted in a different way than conventional diodes. Often you can find a double Schottky rectifier diodes, made in a three-lead case inherent in power keys. Such dual structures contain inside two Schottky diodes, united by cathodes or anodes, more often - cathodes.
Diodes in the assembly have very close parameters, since each such assembly is manufactured by a single process cycle, and as a result, their operating temperature regime is the same, respectively, higher and reliability. A direct voltage drop of 0.2 to 0.4 volts, along with high speed (units of nanoseconds), are undoubted advantages of Schottky diodes before p-n-counterparts.
The peculiarity of the Schottky barrier in diodes, with reference to a small voltage drop, is manifested with applied voltages up to 60 volts, although the performance remains unshakable. Today Schottky diodes of the 25CTQ045 type (up to 45 volts, up to 30 amperes for each pair of diodes in the assembly) can be found in many impulse sources Power, where they serve as power rectifiers for currents up to several hundred kilohertz.
One can not help but touch on the subject of the shortcomings of Schottky diodes, they certainly are, and there are two. First, a short-term excess of the critical voltage will instantly disable the diode. Secondly, the temperature strongly affects the maximum reverse current. At a very high transition temperature, the diode will simply pierce even when operating at rated voltage.
No radio amateur can do without Schottky diodes in his practice. Here you can note the most popular diodes: 1N5817, 1N5818, 1N5819, 1N5822, SK12, SK13, SK14. These diodes are both in the output version, and in SMD. The main thing for which radio fans appreciate it so much is high speed and low voltage drop at the transition - a maximum of 0.55 volts - at a low price of these components.
A rare printed circuit board is dispensed with without Schottky diodes in one or another designation. Somewhere the Schottky diode serves as a low-power rectifier for the feedback loop, somewhere - as a voltage stabilizer at a level of 0.3 to 0.4 volts, and somewhere is a detector.
In the table below you can see the parameters of the most common today little high-power diodes Schottky.
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Schottky diodes from 1 Ampere
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Quick Schottky diodes
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To a large family of semiconductor diodes named after the names of scientists who have discovered an unusual effect, you can add one more. This is a Schottky diode.
German physicist Walter Schottk discovered and studied the so-called barrier effect that occurs with a certain technology for creating a metal-semiconductor transition.
The main "chip" of the Schottky diode is that, in contrast to conventional diodes based on the p-n junction, a metal-semiconductor junction is used here, which is also called the Schottky barrier. This barrier, as well as the semiconductor p-n junction, has the property of one-sided electrical conductivity and a number of distinctive properties.
Silicon (Si) and gallium arsenide (GaAs), as well as metals such as gold, silver, platinum, palladium and tungsten, are predominantly used as the material for fabricating diodes with a Schottky barrier.
On schematic diagrams Schottky's diode is depicted like this.
As you can see, his image is slightly different from the usual of a semiconductor diode.
In addition to this notation on the diagrams, one can also find the image of a Schottky diode (assembly).
A dual diode is two diodes mounted in one common housing. The conclusions of cathodes or anodes in them are combined. Therefore, such an assembly, as a rule, has three conclusions. In impulse power supplies, assemblies with a common cathode are usually used.
Since the two diodes are housed in one housing and executed in a single technological process, their parameters are very close. Since they are housed in a single housing, their temperature regime is the same. This increases the reliability and life of the element.
The Schottky diodes have two positive qualities: a very small direct voltage drop (0.2-0.4 volts) at the transition and very high speed.
Unfortunately, such a small voltage drop occurs when the applied voltage is not more than 50-60 volts. With its further increase, the Schottky diode behaves like an ordinary silicon rectifying diode. The maximum reverse voltage for Schottky does not usually exceed 250 volts, although samples can be found on the market, calculated at 1.2 kilovolts (VS-10ETS12-M3).
So, the Schottky rectifier diode 60CPQ150 is designed for a maximum reverse voltage of 150V, and each of the assembly diodes is able to pass through direct inclusion 30 amps!
Also you can meet the samples rectified for half-cycle current which can reach 400A maximum! An example is the model VS-400CNQ045.
Very often in conceptual schemes, a complex graphical representation of the cathode is simply omitted and the Schottky diode is depicted as an ordinary diode. And the type of the applied element is specified in the specification.
The disadvantages of diodes with a Schottky barrier include the fact that even with a short-time excess of the reverse voltage, they instantly fail and the main irreversible. While the silicon power valves after the termination of the overvoltage are perfectly self-healing and continue to work. In addition, the reverse current of the diodes depends very strongly on the transition temperature. On a large reverse current, a thermal breakdown occurs.
To the positive qualities of Schottky diodes, in addition to high speed, and therefore, small recovery time, it is possible to include a small capacitance of the junction (barrier), which allows to increase the operating frequency. This allows them to be used in impulse rectifiers at frequencies of hundreds of kilohertz. A lot of Schottky diodes find their application in integrated microelectronics. Schottky diodes made using nano technology are part of integrated circuits, where they shunt the transitions of transistors to improve performance.
In radio amateur practice, the Schottky diodes of the series 1N581x (1N5817, 1N5818, 1N5819) took root. All of them are designed for maximum direct current ( I F (AV)) - 1 amp and reverse voltage ( V RRM) from 20 to 40 volts. Voltage drop ( V F) on the transition is from 0.45 to 0.55 volts. As already mentioned, the direct voltage drop ( Forward voltage drop) for diodes with a Schottky barrier is very small.
Also a fairly well-known element is 1N5822. It is designed for direct current of 3 amps and is made in the case of DO-201AD.
Also on printed circuit boards you can find diodes of SK12-SK16 series for surface mount . They are quite small. Despite this, the SK12-SK16 can withstand a forward current of up to 1 ampere at a reverse voltage of 20 to 60 volts. The direct voltage drop is 0.55 volts (for SK12, SK13, SK14) and 0.7 volts (for SK15, SK16). Also in practice, you can find diodes series SK32 - SK310, for example, SK36, which is designed for a direct current of 3 amperes.
Application of Schottky diodes in power supplies.
Schottky diodes are actively used in computer power supplies and pulse voltage regulators . Among the low-voltage supply voltages the most high-current (tens of amperes) are voltages +3.3 volts and +5.0 volts. It is in these secondary power supplies that diodes with a Schottky barrier are used. The most commonly used are three-lead assemblies with a common cathode. It is the application of assemblies that can be considered a sign of a high-quality and technological power unit.
The failure of Schottky diodes is one of the most common faults in switching power supplies. It can have two "dead" states: a clean electrical breakdown and a leak. If one of these conditions exists, the computer power supply is blocked, as protection is activated. But this can happen in different ways. 
In the first case, all secondary stresses are absent. Protection blocked the power supply. In the second case, the fan "jiggles" and at the output of the power sources periodically, then voltage pulsations appear, then disappear.
That is, the protection circuit periodically triggers, but there is no complete blockage of the power supply. Schottky diodes are guaranteed to fail if the radiator on which they are installed is heated very strongly before the appearance of an unpleasant odor. And the last version of diagnostics connected with the leak: when the load on the central processor increases in multiprogram mode, the power supply unit switches off spontaneously.
It should be borne in mind that when the power supply is professionally repaired after replacement of the secondary diodes, especially with suspected leakage, all power transistors performing the function of the keys should be checked and vice versa: after the replacement of the key transistors, the secondary diodes check is mandatory. Always it is necessary to be guided by a principle: trouble one does not come.
Checking the Schottky diodes with a multimeter.
You can check the Schottky diode using an ordinary multimeter. The procedure is the same as for verification of a conventional semiconductor diode from p-n transition. But even here there are pitfalls. It is especially difficult to check the diode with a leak. First of all, the element must be dropped from the circuit for more precise testing. It is easy enough to determine a fully pierced diode. On all limits of resistance measurement, the faulty element will have an infinitely small resistance, both in forward and reverse switching. This is tantamount to a short circuit.
It is more difficult to check the diode with suspicion of "leakage". If we carry out a test with the DT-830 multimeter in the diode mode, then we will see a perfectly working element. You can try to measure in the ohmmeter mode its reverse resistance. At the limit of "20 kΩ", the reverse resistance is defined as infinitely large. If the device shows at least some resistance, say 3 kOhm, then this diode should be regarded as suspicious and changed to a known good. A 100% guarantee can be given by a complete replacement of the Schottky diodes via the + 3.3V and + 5.0V power rails.
Where else in electronics are Schottky diodes used? They can be found in rather exotic devices, such as the receivers of alpha and beta radiation, neutron radiation detectors, and recently on the Schottky barrier junctions panels solar panels. So, they feed electricity and space vehicles.
During the assembly of power supplies and voltage converters for car amplifiers, there is often a problem with rectifying current from the transformer. To get a powerful pulse diodes is quite a serious problem, so I decided to print an article that provides a full list and parameters of Schottky's powerful diodes. Some time ago I personally had a problem with the converter rectifier for an auto amplifier. The converter is quite powerful (500-600 watts), the frequency of the output voltage is 60kHz, any common diode that can be found in old trash will immediately burn as a match. The only affordable option at that time were domestic KD213A. The diodes are quite good, they hold up to 10 amperes, the operating frequency is within 100kHz, but they also overheated under the load.
In fact, powerful diodes can be found almost everyone. A computer BS is one that feeds a whole computer. As a rule, they are made with a power from 200 watts to 1 kW or more, and since the computer is powered by direct current, then there must be a rectifier in the power supply. In modern power supply units for voltage rectification, Schottky's powerful diode assemblies are used - they have the minimum voltage drop at the transition and the ability to work in impulse circuits, where the operating frequency is much higher than the 50 Hz network. Recently on a freebie brought several power supplies, from which the diodes were removed for this small survey. AT computer blocks power supply you can find a variety of diode assemblies, there are almost no single diodes - in one case there are two powerful diode, often (almost always) with a common cathode. Here are some of them:
D83-004 (ESAD83-004) - Powerful assembly of Schottky diodes, reverse voltage of 40 volts, allowable current 30A, in the pulsed mode up to 250A is perhaps one of the most powerful diodes that can be found in computer power supplies.
STPS3045CW - Dual Schottky diode, rectified current 15A, forward voltage 570mV, reverse leakage current 200mA, reverse voltage constant 45V.
The basic Schottky diodes that occur in power supplies
Schottky TO-220 SBL2040CT 10A x 2 = 20A 40V Vf = 0.6V at 10A
Schottky TO-247 S30D40 15A x 2 = 30A 40V Vf = 0.55V at 15A
Ultrastest TO-220 SF1004G 5A x 2 = 10A 200V Vf = 0.97V at 5A
Ultrastest TO-220 F16C20C 8A x 2 = 16A 200V Vf = 1.3V at 8A
Ultrafast SR504 5A 40V Vf = 0.57
Schottky TO-247 40CPQ060 20A x 2 = 40A 60V Vf = 0.49V at 20A
Schottky TO-247 STPS40L45C 20A x 2 = 40A 45V Vf = 0.49V
Ultrastest TO-247 SBL4040PT 20A x 2 = 40A 45V Vf = 0.58V at 20A
Schottky TO-220 63CTQ100 30A x 2 = 60A 100 Vf = 0.69V at 30A
Schottky TO-220 MBR2545CT 15A x 2 = 30A 45V Vf = 0.65V at 15A
Schottky TO-247 S60D40 30A x 2 = 60A 40-60V Vf = 0.65V at 30A
Schottky TO-247 30CPQ150 15A x 2 = 30A 150V Vf = 1V at 15A
Schottky TO-220 MBRP3045N 15A x 2 = 30A 45V Vf = 0.65V at 15A
Schottky TO-220 S20C60 10A x 2 = 20A 30-60V Vf = 0.55V at 10A
Schottky TO-247 SBL3040PT 15A x 2 = 30A 30-40V Vf = 0.55V at 15A
Schottky TO-247 SBL4040PT 20A x 2 = 40A 30-40V Vf = 0.58V at 20A
Ultrastest TO-220 U20C20C 10A x 2 = 20A 50-200V Vf = 0.97V at 10A
There are also modern domestic diode assemblies for high current. Here is their marking and internal scheme:
Also produced , which can be used, for example, in power supplies of tube amplifiers and other equipment with increased power. The list is given below:
High-voltage power diodes Schottky with voltage up to 1200 V
Although more preferable is the use of Schottky diodes in low-voltage high-power rectifiers with output voltages of a couple of tens of volts, at high switching frequencies.
