Other diodes: Diode types
The Schottky diode or Schottky Barrier diode is used in a variety of circuits.
Although it was ine of the first types of diode ever made, the Schottky diode is widely sued because it is able to provide a very low forward voltage drop.
As a result the Schottky barrier diode is used in a varity of applications from RF design to power rectification and many more.
Although the name used most widely for this type of diode is Schottky diode, it has also been given a number of other names that may be used from time to time. These names include surface barrier diode, Schottky barrier diode, hot carrier or even hot electron diode.
Schottky diode circuit symbol
The circuit symbol for the Schottky diode is based around the basic diode circuit symbol. The Schottky symbol is differentiated from other types of diode by the addition of the two extra legs on the bar on the symbol.
Schottky diode advantages
Schottky diodes are used in many places where other types of diode do not perform as well. They offer a number of advantages which can be utilised:
- Low turn on voltage: The turn on voltage for the diode is between 0.2 and 0.3 volts for a silicon Schottky diode whereas a standard silicon diode has a turn on voltage of between 0.6 to 0.7 volts. This reduces resistive losses when used as a power rectifier, and enables lower signals to be detected when used as an RF detector.
- Low junction capacitance: In view of the very small active area of the Schottky diode, the capacitance levels are very small.
- Fast recovery time: The fast recovery time because of the small amount of stored charge means that it can be used for high speed switching applications.
The advantages of the Schottky diode, mean that its performance can far exceed that of other diodes in many areas.
Schottky diode applications
The Schottky barrier diodes are widely used in the electronics industry finding many uses as diode rectifier. Its unique properties enable it to be used in a number of applications where other diodes would not be able to provide the same level of performance. In particular it is used in areas including:
- RF mixer and detector diode: The Schottky diode is a very useful component for radio frequency applications because of its high switching speed and high frequency capability. In view of this Schottky barrier diodes are used in many high performance diode ring mixers. In addition to this the low turn on voltage in addition to the low junction capacitance makes this type of diode ideal for use in RF detectors.
- Power rectifier: Schottky diodes are also used as high power rectifiers. Their high current density and low forward voltage drop mean that less power is wasted than if ordinary PN junction diodes were used. This increase in efficiency means that less heat has to be dissipated, and smaller heat sinks can be used, thereby saving weight and cost.
- Power OR circuits: Schottky diodes can be used in applications where a load is driven by two separate power supplies. One example may be a mains power supply and a battery supply. In these instances it is necessary that the power from one supply does not enter the other. This can be achieved using diodes. However it is important that any voltage drop across the diodes is minimised to ensure maximum efficiency. As in many other applications, this diode is ideal for this in view of its low forward voltage drop.
Schottky diodes tend to have a high reverse leakage current. This can lead to problems with any sensing circuits that may be in use. Leakage paths into high impedance circuits can give rise to false readings. This must therefore be accommodated in the circuit design.
- Solar cell applications: Solar cells are typically connected to rechargeable batteries, often lead acid batteries because power may be required 24 hours a day and the Sun is not always available. Solar cells do not like the reverse charge applied and therefore a diode is required in series with the solar cells. Any voltage drop will result in a reduction in efficiency and therefore a low voltage drop diode is needed. As in other applications, the low voltage drop of the Schottky diode is particularly useful, and as a result they are the favoured form of diode in this application.
- Clamp diode: Schottky barrier diodes may also be used as a clamp diode in a transistor circuit to speed the operation when used as a switch. Years ago they found widespread use in this application, forming a key element in the 74LS (low power Schottky) and 74S (Schottky) families of logic circuits. When used in this manner the Schottky diodes are inserted between the collector and base of the driver transistor to act as a clamp. To produce a low or logic "0" output the transistor is driven hard on, and in this situation the base collector junction in the diode is forward biased. When the Schottky diode is present this takes most of the current and allows the turn off time of the transistor to be greatly reduced, thereby improving the speed of the circuit.
The Schottky diode or Schottky barrier diode is used in many applications. It is unusual in that it is used for both very low power signal detection and also for high power rectification. The properties of the Schottky diode make it idea for use at both ends of the spectrum.
The Schottky diode is also used within a number of other devices from photodiodes to MESFETs. In this way, not only does this form of diode find uses in many circuits in its discrete format, but it is also an essential part of many other components and technologies as well.
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