Other diodes: Diode types
Although the Schottky diode shares many of its characteristics with the more traditional forms of diode, it still has some significant differences.
Understanding the Schottky diode specs and parameters helps to use them in the most effective manner in any circuits in which they may be used.
Basic Schottky diode characteristics
The Schottky diode is a majority carrier device, i.e. electrons in N-type material.
This gives it a significant advantage 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.
Also 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 as well as many other power applications where switching speed is important, e.g. within switching power supplies.
The Schottky diode also has a much higher current density than an ordinary PN junction. This means that forward voltage drops are much lower. This makes the diode ideal for use in power rectification applications.
The main drawback of the Schottky diode is the level of its reverse breakdown which is much lower than that of a PN diode.
Another disadvantage is the level of 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.
Schottky diode I-V characteristic
The I-V 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.
Additionally the reverse breakdown voltage is also typically lower than an equivalent silicon PN junction diode.
The use of a guard ring included in the structure of some Schottky diodes improves its performance in both forward and reverse directions.
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. Some small signal diodes without a guard ring may have a reverse breakdown of only 5 to 10 V. Whilst this may be acceptable for some low signal applications, it is not ideal for most situations.
Key Schottky diode specifications & parameters
There are several key Schottky diode specifications that need to be understood when using these diodes – they are quite different from those of a normal PN junction diode.
- Forward voltage drop: In view of the low forward voltage drop across the diode, this is a parameter that is of particular concern. As can be seen from the Schottky diode I-V characteristic, the voltage across the diode varies according to the current being carried. Accordingly any specification given provides the forward voltage drop for a given current. Typically the turn-on voltage is assumed to be around 0.2 V.
- Reverse breakdown: Schottky diodes do not have a high breakdown voltage. Figures relating to this include the maximum Peak Reverse Voltage, maximum Blocking DC Voltage and other similar parameter names. If these figures are exceeded then there is a possibility the diode will enter reverse breakdown. It should be noted that the RMS value for any voltage will be 1/√2 times the constant value. The upper limit for reverse breakdown is not high when compared to normal PN junction diodes. Maximum figures, even for rectifier diodes only reach around 100 V. Schottky diode rectifiers seldom exceed this value because devices that would operate above this value even by moderate amounts would exhibit forward voltages equal to or greater than equivalent PN junction rectifiers.
- Capacitance: The capacitance parameter is one of great importance for small signal RF applications. Normally the junctions areas of Schottky diodes are small and therefore the capacitance is small. Typical values of a few picofarads are normal. As the capacitance is dependent upon any depletion areas, etc, the capacitance must be specified at a given voltage.
- Reverse recovery time: This parameter is important when a diode is used in a switching application. It is the time taken to switch the diode from its forward conducting or 'ON' state to the reverse 'OFF' state. The charge that flows within this time is referred to as the 'reverse recovery charge'. The time for this parameter for a Schottky diode is normally measured in nanoseconds, ns. Some exhibit times of 100 ps. In fact what little recovery time is required mainly arises from the capacitance rather than the majority carrier recombination. As a result there is very little reverse current overshoot when switching from the forward conducting state to the reverse blocking state.
- Reverse leakage current: The reverse leakage parameter can be an issue with Schottky diodes. It is found that increasing temperature significantly increases the reverse leakage current parameter. Typically for every 25°C increase in the diode junction temperature there is an increase in reverse current of an order of magnitude for the same level of reverse bias.
- Working temperature: The maximum working temperature of the junction, Tj is normally limited to between 125 to 175°C. This is less than that which can be used with ordinary silicon diodes. Care should be taken to ensure heatsinking of power diodes does not allow this figure to be exceeded.
Schottky diode characteristics summary
The Schottky diode is used in many applications as a result of its characteristics that differ appreciable from several aspects of the more widely used standard PN junction diode.
|Schottky Diode / PN Diode Comparison|
|Characteristic||Schottky Diode||PN Junction Diode|
|Forward current mechanism||Majority carrier transport.||Due to diffusion currents, i.e. minority carrier transport.|
|Reverse current||Results from majority carriers that overcome the barrier. This is less temperature dependent than for standard PN junction.||Results from the minority carriers diffusing through the depletion layer. It has a strong temperature dependence.|
|Turn on voltage||Small - around 0.2 V.||Comparatively large - around 0.7 V.|
|Switching speed||Fast - as a result of the use of majority carriers because no recombination is required.||Limited by the recombination time of the injected minority carriers.|
Example Schottky diode datasheet specifications
To give some idea of the characteristics to be expected from Schottky diodes a couple of real examples are provided below. These summarise the main datasheet specifications to give an idea of their performance.
1N5828 Schottky barrier power rectifier diode
This diode is described as a Schottky diode, stud type, i.e. for power rectification. It gives an indication of how a power Schottky diode performs.
|Typical 1N5258 Shottky Diode Characteristics / Specifications|
|Maximum recurrent peak reverse voltage||40||V|
|Maxim DC blocking voltage||40||V|
|Average forward current, IF (AV)||15||A||T = 100°C|
|Peak forward surge current, IFSM||500||A|
|Maximum instantaneous forward voltage, VF||0.5||V||At IFM = 15A and Tj = 25°C|
|Maximum instantaneous reverse current at rated blocking voltage, IR||10
Tj = 125°C
1N5711 Schottky barrier switching diode
This diode is described as an ultra-fast switching diode with high reverse breakdown, low forward drop voltage and a guard ring for junction protection.
|Typical 1N5711 Characteristics / Specifications|
|Max DC Blocking Voltage, Vr||70||V|
|Max forward continuous current, Ifm||15||mA|
|Reverse breakdown voltage, V(BR)R||70||V||@ reverse current of 10µA|
|Reverse leakage current, IR||200||µA||At VR=50V|
|Forward voltage drop, VF||0.41
|V|| at IF = 1.0 mA
|Junction capacitance, Cj||2.0||pF||VR = 0V, f=1MHz|
|Reverse recovery time, trr||1||nS|
Even though the examples here gives a reverse voltage characteristic of 40 V which is fairly typical, the maximum that can normally be obtained is around 100 V.
It should be noted that even though these figures are given as examples of the figures that may be expected for typical Schottky diodes, figures even for a given device number will also vary slightly between different manufacturers.
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