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Schottky diode vs. Schottky diodes - device, types, characteristics and use

A Schottky diode is a semiconductor electrical rectifier element where a metal-semiconductor junction is used as a barrier. As a result, useful properties are acquired: high speed and low voltage drop in the forward direction.

From the history of the discovery of Schottky diodes

The rectifying properties of the metal-semiconductor junction were first noticed in 1874 by Ferdinand Braun using sulfides as an example. Passing current in the forward and reverse directions, he noted a difference of 30%, which fundamentally contradicted the well-known Ohm's law. Brown could not explain what was happening, but, continuing his research, he found that the resistance of the section was also proportional to the flowing current. Which also looked odd.

The experiments were repeated by physicists. For example, Werner Siemens noted similar properties of selenium. Brown found that the properties of the structure manifest themselves most clearly with a small size of contacts applied to the sulfide crystal. The researcher used:

spring-loaded wire with a pressure of 1 kg;
mercury contact;
pad plated with copper.

So a point diode was born, which in 1900 prevented our compatriot Popov from taking out a patent for a radio detector. In his own writings, Brown outlines research on manganese ore (psilomelan). By pressing the contacts to the crystal with a clamp and isolating the sponges from the current-carrying part, the scientist obtained excellent results, but at that time there was no application of the effect. Describing the unusual properties of copper sulfide, Ferdinand laid the foundation for solid-state electronics.

For Brown, like-minded people found practical application. On April 27, 1899, Professor Jagdish Chandra Bose announced the creation of the first detector-receiver to work in tandem with a radio transmitter. He used galena (lead oxide) paired with a simple wire and caught the millimeter wave. In 1901 he patented his brainchild. It is possible that under the influence of rumors about Popov. The Bose detector was used in Marconi's first transatlantic radio transmission. A similar kind of device on a silicon crystal was patented in 1906 by Greenleaf Witter Pickard.

In his speech at the Nobel Prize in 1909, Brown noted that he did not understand the principles of the phenomenon he had discovered, but he discovered a number of materials that exhibited new properties. These are the above-mentioned galena, pyrite, pyrolusite, tetrahedrite and a number of others. The listed materials attracted attention for a simple reason: they conducted an electric current, although they were considered compounds of the elements of the periodic table. Previously, such properties were considered the prerogative of simple metals.

Finally, in 1926, the first transistors with a Schottky barrier already appeared, and William Bradford Shockley summed up the theory under the phenomenon in 1939. At the same time, Neville Francis Mot explained the phenomena occurring at the junction of two materials by calculating the diffusion current and drift of the main charge carriers. Walter Schottky supplemented the theory by replacing the linear electric field with a damped one and adding the concept of ion donors located in the near-surface layer of a semiconductor. The volume charge at the interface under the metal layer was named after the scientist.

Davydov made similar attempts to bring the theory under the existing fact in 1939, but incorrectly gave the limiting factors for the current and made other mistakes. The most correct conclusions were drawn by Hans Albrecht Bethe in 1942, who linked the current to the thermionic emission of carriers through a potential barrier at the interface between two materials. Thus, the modern name of the phenomenon and diodes should have been named after the last scientist, Schottky's theory showed flaws.

Theoretical studies run up against the difficulty of measuring the work function of electrons from a material into a vacuum. Even for the chemically inert and stable metal gold, certain readings vary from 4 to 4.92 eV. At a high degree of vacuum, in the absence of mercury from the pump or oil film, values of 5.2 eV are obtained. As technology advances in the future, more accurate values are foreseen. Another solution would be to use information about the electronegativity of materials to correctly predict events at the transition boundary. These values (on the Polling scale) are known to within 0.1 eV. From what has been said, it is clear that today it is not possible to correctly predict the height of the barrier using the indicated methods and, consequently, the rectifying properties of Schottky diodes.

The best ways to determine the height of the Schottky barrier

The height can be determined by the well-known formula (see fig.). Where C is a coefficient weakly dependent on temperature. The dependence on the applied voltage Va, despite the complex shape, is considered to be almost linear. The slope of the graph is q/kT. The barrier height is determined from the plot of lnJ versus 1/T at a fixed voltage. The calculation is based on the angle of inclination.

An alternative method is to irradiate the metal-semiconductor junction with light. Methods are used:

Light passes through the thickness of the semiconductor.
The light falls directly on the sensitive area of the photocell.

If the photon energy falls within the energy gap between the band gap of the semiconductor and the barrier height, electron emission from the metal is observed. When the parameter is higher than both of these values, the output current increases sharply, which is easily seen on the experimental setup. This method allows you to establish that the work function for the same semiconductor, with different types of conduction types (n and p), in total give the band gap of the material.

A new method for determining the height of the Schottky barrier is to measure the junction capacitance as a function of the applied reverse voltage. The graph shows the form of a straight line that intersects the abscissa axis at a point characterizing the desired value. The result of experiments strongly depends on the quality of surface preparation. The study of technological methods of processing shows that etching in hydrofluoric acid leaves a layer of oxide film with a thickness of 10-20 angstroms on a silicon sample.

The effect of aging is invariably noted. Less typical for Schottky diodes formed by chipping a crystal. The barrier heights differ for a particular material, in some cases they strongly depend on the electronegativity of the metals. For gallium arsenide, the factor almost does not manifest itself; in the case of zinc sulfide, it plays a decisive role. However, in the latter case, the quality of surface preparation has a weak effect; for GaAs, this is extremely important. Cadmium sulfide is in an intermediate position relative to these materials.

In the study, it turned out that most semiconductors behave like GaAs, including silicon. Mead explained this by the fact that a number of formations are formed on the surface of the material, where the energy of electrons lies in the region of a third of the forbidden band from the valence band. As a result, upon contact with a metal, the Fermi level in the latter tends to occupy a similar position. History repeats itself with any conductor. At the same time, the barrier height becomes the difference between the Fermi level and the edge of the conduction band in the semiconductor.

A strong effect of metal electronegativity is observed in materials with pronounced ionic bonds. These are primarily tetravalent silicon oxide and zinc sulfide. This fact is explained by the absence of formations that affect the Fermi level in the metal. In conclusion, we add that an exhaustive theory regarding the issue under consideration has not been created today.

Advantages of Schottky diodes

It's no secret that Schottky diodes serve as rectifiers at the output of switching power supplies. Manufacturers rest on the fact that power losses and heating in this case are much lower. It has been established that the voltage drop during direct connection on the Schottky diode is 1.5 - 2 times less than in any type of rectifiers. Let's try to explain the reason.

Consider the operation of a conventional p-n junction. When materials with two different types of conduction come into contact, the majority carriers begin to diffuse beyond the contact boundary, where they are no longer the majority. In physics, this is called a barrier layer. If a positive potential is applied to the n-region, the majority electron carriers will be instantly attracted to the terminal. Then the blocking layer will expand, the current will not flow. In direct switching, the main carriers, on the contrary, step on the barrier layer, where they actively recombine with it. The junction opens and current flows.

It turns out that neither opening nor closing a simple diode will instantly work. There are processes of formation and elimination of the barrier layer, which require time. The Schottky diode behaves a little differently. The applied forward voltage opens the junction, but there is practically no injection of holes into the n-semiconductor, the barrier for them is high, and there are few such carriers in the metal. When reversed, a tunneling current can flow in heavily doped semiconductors.

Readers familiar with the topic of LED Lighting will already be aware that Henry Joseph Round originally made a discovery in 1907 with a crystal detector. This is a Schottky diode in the first approximation: the border of metal and silicon carbide. The difference is that n-type semiconductor and aluminum are used today.

The properties of the transition depend on the materials used and on the geometric dimensions. The space charge in this case is less than when two semiconductors of different types are in contact, which means that the switching time is significantly reduced. In a typical case, it falls within the range from hundreds of ps to tens of ns. For ordinary diodes, at least an order of magnitude higher. In theory, this looks like the absence of an increase in the level of the barrier when the reverse voltage is applied. It is easy to explain the small voltage drop by the fact that part of the junction is composed of a pure conductor. Actual for devices designed for relatively low voltages of tens of volts.

According to the properties of Schottky diodes, they are widely used in switching power supplies for household appliances. This allows you to reduce losses, improve the thermal mode of operation of rectifiers. The small junction area results in low breakdown voltages, which is slightly offset by an increase in the metallization area on the chip, covering a part of the region isolated by silicon oxide. This area, resembling a capacitor, when the diode is turned back on, depletes the adjacent layers of the main charge carriers, significantly improving performance.

Due to their speed, Schottky diodes are actively used in integrated circuits aimed at using high frequencies - operating and synchronization frequencies.

The development of electronics requires ever higher standards from radio components. To operate at high frequencies, a Schottky diode is used, which is superior in its parameters to silicon counterparts. Sometimes you can find the name Schottky barrier diode, which basically means the same thing.

Design
Miniaturization
Use in practice

Design

The Schottky diode differs from ordinary diodes in its design, which uses a semiconductor metal, and not a p-n junction. It is clear that the properties here are different, which means that the characteristics should also be different.

Indeed, a semiconductor metal has the following parameters:

The leakage current is of great importance;
Low voltage drop across the junction with direct connection;
Restores the charge very quickly, as it has a low value.

The Schottky diode is made from materials such as gallium arsenide, silicon; much less often, but can also be used - germanium. The choice of material depends on the properties that need to be obtained, however, in any case, the maximum reverse voltage for which these semiconductors can be manufactured is not higher than 1200 volts - these are the highest voltage rectifiers. In practice, they are much more often used at a lower voltage - 3, 5, 10 volts.

On the circuit diagram, the Schottky diode is designated as follows:

But sometimes you can see this designation:

This means a dual element: two diodes in one package with a common anode or cathode, so the element has three terminals. Power supplies use such designs with a common cathode, they are convenient to use in rectifier circuits. Often, the markings of a conventional diode are drawn on the diagrams, but the description indicates that this is Schottky, so you need to be careful.

Diode assemblies with a Schottky barrier are available in three types:

type 1 - with a common cathode;

type 2 - with a common anode;

Type 3 - according to the doubling scheme.

Such a connection helps to increase the reliability of the element: after all, being in the same housing, they have the same temperature regime, which is important if you need powerful rectifiers, for example, 10 amperes.

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But there are also disadvantages. The thing is that a small voltage drop (0.2–0.4 V) for such diodes appears at low voltages, usually 50–60 volts. At a higher value, they behave like ordinary diodes. But in terms of current, this circuit shows very good results, because it is often necessary - especially in power circuits, power modules - for the operating current of semiconductors to be at least 10A.

Another major drawback: for these devices, the reverse current cannot be exceeded even for a moment. They immediately fail, while silicon diodes, if their temperature has not been exceeded, restore their properties.

But there are more positives. In addition to the low voltage drop, the Schottky diode has a low junction capacitance value. As you know: lower capacitance - higher frequency. Such a diode has found application in switching power supplies, rectifiers and other circuits, with frequencies of several hundred kilohertz.

The CVC of such a diode has an asymmetric form. When a forward voltage is applied, it can be seen that the current grows exponentially, and when the reverse voltage is applied, the current does not depend on the voltage.

All this is explained if you know that the principle of operation of this semiconductor is based on the movement of the main carriers - electrons. For the same reason, these devices are so fast: they do not have the recombination processes inherent in devices with p-n junctions. For all devices with a barrier structure, the asymmetry of the CVC is characteristic, because it is the number of electric charge carriers that determines the dependence of the current on the voltage.

Miniaturization

With the development of microelectronics, special microcircuits, single-chip microprocessors, began to be widely used. All this does not exclude the use of hinged elements. However, if radioelements of ordinary sizes are used for this purpose, then this will nullify the whole idea of miniaturization as a whole. Therefore, unpackaged elements were developed - smd components, which are 10 or more times smaller than conventional parts. The I–V characteristics of such components are no different from the I–V characteristics of conventional devices, and their reduced dimensions allow the use of such spare parts in various micro assemblies.

smd components come in several sizes. For manual soldering, smd size 1206 is suitable. They have a size of 3.2 by 1.6 mm, which allows them to be soldered on their own. Other smd elements are more miniature, they are assembled at the factory with special equipment, and it is impossible to solder them yourself at home.

The principle of operation of the smd component also does not differ from its large counterpart, and if, for example, we consider the CVC of a diode, then it will be equally suitable for semiconductors of any size. By current, they are made from 1 to 10 amperes. The marking on the body often consists of a digital code, the decoding of which is given in special tables. They can be tested for suitability by a tester, as well as large analogues.

Use in practice

Schottky rectifiers are used in switching power supplies, voltage stabilizers, pulse rectifiers. The most demanding current - 10A or more - is a voltage of 3.3 and 5 volts. It is in such secondary power circuits that Schottky devices are used most often. To amplify the current values, they are connected together according to the scheme with a common anode or cathode. If each of the dual diodes is 10 amps, then a significant margin of safety will be obtained.

One of the most common malfunctions of switching power modules is the failure of these same diodes. As a rule, they either completely break through or leak. In both cases, the faulty diode must be replaced, then check the power transistors with a multimeter, and also measure the supply voltage.

Testing and Interchangeability

Schottky rectifiers can be checked in the same way as conventional semiconductors, since they have similar characteristics. With a multimeter, you need to ring it in both directions - it should show itself in the same way as a conventional diode: anode-cathode, while there should be no leaks. If it shows even a slight resistance - 2-10 kilo-ohms, this is already a reason for suspicion.

A diode with a common anode or cathode can be tested like two ordinary semiconductors connected together. For example, if the anode is common, then it will be one leg out of three. We put one tester probe on the anode, the other legs are different diodes, another probe is placed on them.

Can it be replaced with another type? In some cases, Schottky diodes are changed to ordinary germanium ones. For example, D305 at a current of 10 amperes gave a drop of only 0.3 volts, and at currents of 2-3 amperes they can generally be installed without radiators. But the main goal of the Schottky installation is not a small drop, but a low capacitance, so it will not always be possible to replace.

As you can see, electronics does not stand still, and further options for the use of high-speed devices will only increase, making it possible to develop new, more complex systems.

Hi all!

In this article, we will look at Schottky diode. This topic of the article will be very useful for beginner radio mechanics (TV technicians).

As you may have noticed, the term "Schottky diode" is quite common in modern radio circuits, but not everyone knows what it is and what it is.

So, the Schottky diode is a semiconductor device, more precisely a diode, which is made on the basis of a metal-semiconductor contact. This diode is named after the German physicist Walter Hermann Schottky.

Schematically Schottky diode similar to a regular diode, but with some minor differences. On the diagrams Schottky diode is denoted So:

Schottky diode differs from the fact that instead of a p-n junction, a semiconductor metal is used as a barrier. The potential barrier that appears in the region of this transition is called the Schottky barrier. If you change the height of the Schottky barrier, this will lead to a change in the current flow through this device. Feature of this diode in that it has a low forward voltage drop after the transition, and no reverse recovery charge. To put it simply, taking the Schottky barrier as a basis, ultrafast and high-speed diodes are made, which serve as microwave diodes and have various purposes.

The structure of the Schottky diode is shown in the figure below:

1 – semiconductor substrate; 2 – epitaxial film; 3 – metal-semiconductor contact; 4 - metal film; 5 - external contact.

The Schottky diode has a very low level of high frequency noise. This advantage allows the use of this diode in digital equipment and switching power supplies.

These diodes are widely used in solar batteries as radiation receivers and light modulators.

These are all advantages, but there are also disadvantages. Since these devices are highly sensitive to reverse voltage and current values, they often fail. The allowable reverse voltage of these diodes has a limit of 250 V. The temperature regime of these devices varies from -65 to +160 degrees. Celsius. Also, these diodes are available in SMD packages in glass, plastic and metal versions.

Many malfunctions in system power supplies arise due to malfunctions in the secondary circuits that work in conjunction with power supplies. If earlier power transistor switches very often failed, now the main problem is the breakdown of secondary rectifiers, which are based on the Schottky diode. It uses the principle of transition from metal to semiconductor. As a rule, most of these diodes are used in low voltage circuits.

Positive qualities of the Schottky diode

If in conventional diodes the value of the forward voltage drop is approximately from 0.6 to 0.7 volts, then the use of Schottky diodes can reduce this figure from 0.2 to 0.4 volts. In this case, the maximum reverse voltage can be up to several tens of volts. This indicator limits the use of Schottky diodes and assumes their use only in low-voltage circuits.

With a small electrical capacitance of the junction, it becomes possible to produce a significant increase in the operating frequency. Due to this property, the diode has found a fairly wide application for integrated circuits. In power electrical appliances, low capacitance junctions have a short recovery period, which allows rectifiers to operate at high frequencies.

Improved characteristics compared to conventional rectifiers allow them to be effectively used for switching power supplies and digital equipment.

Flaws

In the event that the maximum reverse voltage exceeds the permissible level for a short time, the Schottky diode completely fails. This is an irreversible process, after which it becomes impossible to restore the original properties.

In addition, increased reverse currents are observed, which increase with an increase in the temperature of the crystal itself. In case of poor heat dissipation, the action of positive thermal feedback can lead to emergency overheating of the diode.

In power supplies, the Schottky diode is effectively used for rectifying currents in channels. Given the high value of the output current, there is a need for a fast action of the rectifiers in order to reduce their energy losses. This factor leads to a significant increase in the efficiency of power supplies. In addition, reliable operation of the power units installed in the first part of the power supplies is ensured.

Thus, Schottky diodes are used in cases where it is necessary to reduce switching dynamic losses, as well as to eliminate short circuits during switching. This device is an efficient rectifying element.

Electrical engineering and radio electronics are full of many concepts, one of which is the Schottky diode, which is used in numerous circuit diagrams. Many people ask questions about what a Schottky diode is, how it is indicated on the diagrams, and also what is the principle of operation of a Schottky diode.

General information and principle of operation

A Schottky diode is a diode semiconductor product that, when connected in a straight line in a circuit, produces a small voltage reduction. This element consists of a metal and a semiconductor. The diode is named after the famous German test physicist W. Schottky, who invented it in the 38th year of the 20th century.

In industry, such a diode with a limited reverse voltage is used - up to 250 V, but in practice, for domestic purposes, to prevent the movement of current in the opposite direction, mainly low-voltage options are used - 3-10V.

Schottky diodes can be divided into 3 classes according to power characteristics:

high power;
medium power;
low power.

A Schottky barrier diode (more accurate product name) consists of a conductor, for contact with which metal is used, protection rings and glass passivation.

At the moment when a current passes through the circuit, negative and positive charges collect in different parts of the case throughout the entire area of the semiconductor barrier and on the protective ring, which leads to the appearance of an electric field and the release of thermal energy - this is a big plus of the diode for many physical experiments.

Diode assemblies of this type can be produced in several variations:

Schottky diodes with a common anode;
diode products having an output from a common cathode;
diodes assembled according to the doubling scheme.

Technical characteristics of popular modifications of Schottky diodes

Name	Limit reverse peak voltage	Limiting rectifying electric current		Peak direct current	Limit reverse electric current	Limit forward voltage
Unit measurements	IN	A	OS	A	µA	IN
1N5817	20	1	90	25	1	0,45
1N5818	30	1	90	25	1	0,55
1N5819	40	1	90	25	1	0,6
1N5821	30	3	95	80	2	0,5
1N5822	40	3	95	80	2	0.525

Differences from other semiconductors

Schottky diodes differ from other diode products in that they have an obstacle in the form of a transition - a semiconductor-metal, characterized by one-sided electrical conductivity. Silicon, gallium arsenide can act as metal in them, compounds of germanium, tungsten, gold, platinum and others can be used less often.

The operation of this electronic component will depend entirely on the chosen metal. Most often, silicon is found in such designs, as it is more reliable and has excellent performance at high powers. Compounds of gallium and arsenic, germanium can also be used. The production technology of this electronic product is simple, resulting in low cost.

The Schottky product is characterized by more stable operation when an electric current is applied than other types of semiconductor diodes. This is achieved due to the fact that special crystalline formations are introduced into its body.

Advantages and disadvantages

The above diodes have some advantages, which are as follows:

electric current is perfectly retained in the circuit;
small capacitance of the Schottky barrier increases the service life of the product;
low voltage drop;
speed in the electrical circuit.

The most significant drawback of the component is the huge reverse current, which even with a jump in this indicator by several units leads to the failure of the diode.

Note! During the operation of the Schottky electric element in circuits with a powerful electric current, under adverse conditions of heat exchange, a heat breakdown occurs.

Schottky diode: designation and marking

The Schottky diode on electrical circuits is designated almost exactly the same as conventional semiconductors, but with some features.

It is worth noting that dual versions of the Schottky diode can also be found on the diagrams. This design is two connected diodes in a common housing, having soldered cathodes or anodes, which leads to the formation of three conclusions.

The marking of such elements is affixed on the side in the form of letters and symbols. Each manufacturer carries out labeling of its products in its own way, but following certain international standards.

Important! If the alphanumeric designation on the diode case is not clear, then it is recommended to look at the decoding in the radio technical reference.

Scope of application

The use of diode designs with a Schottky barrier can be found in many devices and electrical structures. Most often they are used on electrical circuits in the following technique:

household appliances and computers;
power supplies of various types and voltage stabilizers;
TV, - and radio equipment;
transistors and batteries powered by solar energy;
other electronics.

Such a wide range of applications is due to the fact that such an electrical element greatly increases the efficiency and performance of the final product, restores the reverse resistance of the electric current, stores it in the mains, reduces the number of losses in the dynamics of the electric voltage, and also absorbs quite a lot of different types of radiation.

Diagnosis of Schottky diodes

It is not difficult to check the health of the Schottky electric element, but this will take some time. To diagnose problems, do the following:

From the electrical circuit or diode bridge, it is required to initially unsolder the element of interest;
Conduct a visual inspection for possible mechanical damage, the presence of traces of chemical and other reactions;
Check the diode with a tester or multimeter;
If the test is carried out with a multimeter, then after turning it on, it is necessary to bring the probes to the ends of the cathode and anode, as a result, the device will give the real voltage of the diode assembly.

Important! When carrying out test measures with a multimeter, you should take into account the electric current, which is usually indicated on the side of the product.

The result of these simple actions will be the establishment of the technical condition of the semiconductor. The diode can become faulty for the following reasons:

When holes occur, the Schottky element ceases to hold electric current, respectively, from a semiconductor it turns into a conductor;
When a break occurs in the diode bridge or the diode element itself, the flow of electric current stops altogether.

It is worth noting that in such incidents, neither smoke nor the smell of burning will be visible, respectively, all diodes will need to be checked, and it is best to contact specialized workshops.

The Schottky diode is a simple and unpretentious, but at the same time an essential element in modern electronics, since it is thanks to it that it is possible to ensure the uninterrupted operation of many devices and technical products.