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Schottky Diode

Schottky Diode

Introduction:

A Schottky diode, also known as a hot carrier diode, is a semiconductor diode which has a low forward voltage drop and a very fast switching action. There is a small voltage drop across the diode terminals when current flows through a diode. A normal diode will have a voltage drop between 0.6 to 1.7 volts, while a Schottky diode voltage drop is usually between 0.15 and 0.45 volts. This lower voltage drop provides better system efficiency and higher switching speed. In a Schottky diode, a semiconductor–metal junction is formed between a semiconductor and a metal, thus creating a Schottky barrier. The N-type semiconductor acts as the cathode and the metal side acts as the anode of the diode. This Schottky barrier results in both a low forward voltage drops and very fast switching.


A Schottky diode, also known as a hot carrier diode, is a semiconductor diode which has a low forward voltage drop and a very fast switching action. There is a small voltage drop across the diode terminals when current flows through a diode.


In a Schottky diode, a semiconductor–metal junction is formed between a semiconductor and a metal, thus creating a Schottky barrier. The N-type semiconductor acts as the cathode and the metal side acts as the anode of the diode. This Schottky barrier results in both a low forward voltage drop and very fast switching.

2.2. Types of Schottky Diodes

There are many different kinds of Schottky diodes and at Future Electronics we stock many of the most common types categorized by maximum average rectified current, maximum reverse voltage, maximum reverse current, forward voltage, packaging type and maximum peak current. The parametric filters on our website can help refine your search results depending on the required specifications.


The most common sizes for maximum average rectified current are 70 mA, 100 mA, 200 mA and 1 A. We also carry Schottky diodes with maximum average rectified current as high as 300 A. Forward voltage can range from 280 mV to 430 V, with the most common Schottky diode semiconductor chips having a forward voltage of 800 mV or 1 V.

2.3. Circuit symbol

The Schottky circuit symbol used in many circuit schematic diagrams may be that of an ordinary diode symbol. However, it is often necessary to use a specific Schottky diode symbol to signify that a Schottky diode rather than another one must be used because it is essential to the operation of the circuit. Accordingly, a specific Schottky diode symbol has been accepted for use. The circuit symbol is shown below:

Schottky diode symbol

It can be seen from the circuit symbol that it is based on the normal diode one, but with additional elements to the bar across the triangle shape.

2.6. Schottky diode 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 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.

2.7. IV characteristic:

· The IV characteristic is generally that shown below. In the forward direction the current rises exponentially, having a knee or turn on voltage of around 0.2 V. In the reverse direction, there is a greater level of reverse current than that experienced using a more conventional PN junction diode.

The use of a guard ring in the fabrication of the diode has an effect on its performance in both forward and reverse directions. Both forward and reverse characteristics show a better level of performance.


· However the main advantage of incorporating a guard ring into the structure is to improve the reverse breakdown characteristic. There is around a 4 : 1 difference in breakdown voltage between the two - the guard ring providing a distinct improvement in reverse breakdown. Some small signal diodes without a guard ring may have a reverse breakdown of only 5 to 10 V.

2.8. Limitations:

· The most evident limitations of Schottky diodes are their relatively low reverse voltage ratings, and their relatively high reverse leakage current. For silicon-metal Schottky diodes, the reverse voltage is typically 50 V or less. Some higher-voltage designs are available (200 V is considered a high reverse voltage). Reverse leakage current, since it increases with temperature, leads to a thermal instability issue. This often limits the useful reverse voltage to well below the actual rating.


· However that does not mean the Schottky diode is not to be used. The reverse bias breakdown voltage is in fact made to be relatively constant and specific and the diodes are sold and used based on the reverse bias breakdown voltage, for the purpose of being used as a conductor ( a voltage limiter) in reverse bias breakdown mode.


· While higher reverse voltages are achievable, they would present a higher forward breakdown voltage, comparable to other types of standard diodes. Such Schottky diodes would have no advantage unless great switching speed is required.

2.9. Applications of Schottky Diode

Despite the fact that Schottky barrier diodes have many applications in electronics scene, it is actually one of the oldest semiconductor devices in existence. As a metal-semiconductor device, its applications can be tracked back to before 1900 where crystal detectors, cat’s whisker detectors and the like were all effectively Schottky barrier diodes.


The Schottky diodes are widely used in the electronic 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.


Some of its applications are:


2.9.1. RF mixer and Detector Diode


The Schottky diode has come into its own 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 their low turn on voltage and high frequency capability and low capacitance make them ideal as RF detectors.


2.9.2. Power Rectifier


The Schottky barrier diodes 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.


2.9.3. 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 of the Schottky diode is particularly useful, and as a result they are the favoured form of diode in this application.


2.9.4. 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. They were used in this role in the 74LS (low power Schottky) and 74S (Schottky) families of logic circuits. In these chips the 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 this 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.