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Schottky diode

- an overview of the Schottky diode (Schottky barrier diode) used in many applications where low forward voltage drops are needed.

The Schottky diode or Schottky Barrier diode is used in a variety of circuits. It has the advantage over the ordinary silicon PN junction diode that it has a much lower forward voltage drop. The ordinary silicon PN junction diode will drop at least 0.6 volts, whereas a silicon Schottky diode will drop between 0.1 and 0.4 volts dependent upon the current. In addition to this the Schottky diode is very fast, and can react more quickly than many other types of diode. With these advantages, the Schottky diode has found many uses in a variety of areas of electronics.

Although the name used moist 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, hot carrier or even hot electron diode.


Schottky structure

The basis of the Schottky diode is a metal semiconductor interface and this can be created in a number of ways. This simplest is a point contact diode where a metal wire is pressed against a clean N-type semiconductor surface. This method of manufacture is still used occasionally today as it is cheap, but not particularly reliable and repeatable. In fact the diode that is formed may either be a Schottky barrier diode or a standard PN junction dependent upon the way in which the wire and semiconductor meet and the way the resulting forming process occurs.

Point contact Schottky diode

Point contact Schottky diode

The early Cat's Whisker wireless detectors were made in this way, although using naturally occurring mineral crystals, but it was found with these detectors that the wire had to be carefully positioned for the best results and after a while the performance would fall away and a new position for the whisker was required.

Today, when Shottky diodes are manufactured, the metal is deposited directly onto the silicon, although the process is not quite this straightforward and a few other elements are needed in the design. Nevertheless the modern day Schottky diodes provide a far superior and more reliable level of performance.


Characteristics

The Schottky diode is what is called a majority carrier device. This gives it tremendous advantages in terms of speed because it does not rely on holes or electrons recombining when they enter the opposite type of region as in the case of a conventional diode. By making the devices small the normal RC type time constants can be reduced, making these diodes an order of magnitude faster than the conventional PN diodes. This factor is the prime reason why they are so popular in radio frequency applications.

The diode also has a much higher current density than an ordinary PN junction. This means that forward voltage drops are lower making the diode ideal for use in power rectification applications.

Its main drawback is found in the level of its reverse current which is relatively high. For many uses this may not be a problem, but it is a factor which is worth watching when using it in more exacting applications.

The overall I-V characteristic is shown below. It can be seen that the Schottky diode has the typical forward semiconductor diode characteristic, but with a much lower turn on voltage. At high current levels it levels off and is limited by the series resistance or the maximum level of current injection. In the reverse direction breakdown occurs above a certain level. The mechanism is similar to the impact ionisation breakdown in a PN junction.


Applications

The Schottky diode is widely used in the electronics industry finding many uses as a general purpose rectifier. However it has come into its own for radio frequency applications because of its high switching speed and high frequency capability. In view of this it is used in many high performance diode ring mixers. In addition to this their low turn on voltage and high frequency capability and low capacitance make them ideal as RF detectors.

Schottky didoes are also used in high power applications, as 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 may be able to be incorporated in the design.

The Schottky diode may also be used as a clamp diode in a transistor circuit to speed the operation when used as a switch. They were used in this role in the 74LS (low power Schottky) and 74S (Schottky) families of logic circuits. The diode is 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.

An NPN transistor with Schottky diode clamp

An NPN transistor with Schottky diode clamp

The diode is also used as a fundamental building block in a number of other devices from photodiodes to MESFETs. Not only does it find widespread use in many applications in its own right, but it an essential part of many other components as well.

 


Practical Electronics Handbook

Ian Sinclair, John Dunton


Practical Electronics Handbook

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