Schottky diodes
(1 764)Schottky diodes are a group of commonly used electronic elements which belong to the category of semiconductors. They are mainly characterized by asymmetrical current conduction, i.e. more in one direction than in the other. The nature of operation of Schottky diodes can be described by comparing them to standard rectifier diodes compared to which they are characterized with a much lower (even up to three times) voltage drop in the forward direction. It is a huge asset, which allows them to conduct much greater current, since the power, and in consequence the heat that is also being emitted on such a diode will be lower. This occurs, for example, during the rectification of alternating voltage, e.g. in switched-mode power supplies. It is worth noting that the voltage drop on the Schottky diodes depends on their temperature. What makes them different from the standard rectifying diodes is also the capability of proper functioning at significantly higher frequencies of current changes (of several hundred thousand hertz or even several gigahertz). Thanks to those assets, they are commonly used in power supplies, impulse chargers, inverters and mixers.
Despite unquestionable advantages, Schottky diodes have some drawbacks, too. One of them is the significantly higher reverse current, also called the leakage current, in comparison to the standard rectifying diodes. It is reverse biased and increases along with the junction temperature. For that reason, such diodes are not applied e.g. to measuring instruments. The yet another drawback is the relatively low reverse rated voltage, which rarely exceeds 200V, however, it is possible to encounter Schottky diodes, in which this parameter will even reach the level of several kilovolts. Reverse rated voltage determines the maximum voltage applied to diode terminals in reverse to its forward direction which will not permanently damage it. Unfortunately, along with the increase of the diode’s reverse rated voltage, its forward voltage also increases. Because of the aforementioned drawbacks and assets, Schottky diodes are used wherever the high voltage operations are not required, but where the voltage can alter at a very high frequency. Moreover, wherever the high current conducting without high power losses is necessary, Schottky diodes will also be a good solution.
Schottky diode – construction
The construction of the Schottky diode differs from that of a standard diode. In the latter one, the p-n junction is used, whereas in the Schottky diodes the metal-semiconductor junction is applied, which is often marked with m-s symbol, however, the term “barrier junction” may also be encountered. In this case, the load is carried by majority carriers, which means that e.g. in the case of using the n-type semiconductor for constructing a diode, beside the metal layer that is electrically neutral, these will be electrons. Due to their construction, Schottky diodes have a lower electrical capacitance, which provides them with a low level of inertia, and thus they can be applied in the systems working at the high frequency of current alteration. Certainly, as it is the case in the standard rectifier diodes, Schottky diodes do not have the reverse-biased voltage.
Schottky diodes, as well as the rest of electronical components, can be encountered in a form of surface-mounted (SMD) or through-hole (THT) devices, however there are also screw-on diodes which are referred to as Schottky screw-mounted diodes. For this reason, they are found in many various housings, and they can have a different number of terminals, which is closely related with their purpose, but also with the structure of the semiconductor used to build them. There are switching diodes and rectifying ones, among which there are single, double or triple diodes. The two latter types can be found in the solutions with a common anode or cathode, but can also be integrated in a series and parallel form. There are also double and triple independent diodes, which have two or three single Schottky diodes in one housing that are not connected together.
Schottky diode – selection
When choosing Schottky diodes, attention must be paid to their maximum reverse rated voltage, which is given in volts [V]. Using a diode to rectify a current with a voltage higher than component’s maximum reverse voltage can lead to the avalanche breakdown of and irreparable damage to the diode itself as well as other cooperating components that are located in a given electronic system. It is also worth remembering about the parameter that determines the maximum forward voltage. It determines the maximum voltage drop which can occur at the diode junction.
When selecting a diode, remember to have adequately high forward current, which is given in amperes [A] or in the case of the low-power components, in milliamperes [mA]. If higher amperage is carried through the diode than its rated forward current, it can lead to its irreparable thermic damage caused by too high power discharge at the component’s junction. In the case of Schottky diodes, there is also a parameter referred to as the maximum peak pulse current, which lasts for a very short time and is not the current that constantly flows through the diode.
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