Schottky Diode Technology & Structure

Basic Schottky diode structure

The Schottky barrier diode can be manufactured in a variety of forms. The most simple is the point contact diode where a metal wire is pressed against a clean semiconductor surface. This was how the early Cat's Whisker detectors were made, and they were found to be very unreliable, requiring frequent repositioning of the wire to ensure satisfactory operation. 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 resulting forming process.

Point contact Schottky diode structure

Although some diodes still use this very simple format, any diode requiring a long term reliability needs to be fabricated in a more reliable way.

Vacuum deposited Schottky diode structure

Although point contact diodes were manufactured many years later, these diodes were also unreliable and they were subsequently replaced by a fabrication technique in which metal was vacuum deposited.

Deposited metal Schottky barrier diode structure

This format for a Schottky diode is very basic and is more diagrammatic than actually practical. However it does show the basic metal-on-semiconductor format that is key to its operation.

Schottky diode structure with guard ring

One of the problems with the simple deposited metal diode is that breakdown effects are noticed around the edge of the metallised area. This arises from the high electric fields that are present around the edge of the plate. Leakage effects are also noticed.

To overcome these problems a guard ring of P+ semiconductor fabricated using a diffusion process is used along with an oxide layer around the edge. In some instances metallic silicides may be used in place of the metal.

The guard ring in this form of Schottky diode structure operates by driving this region into avalanche breakdown before the Schottky junction is damaged by large levels of reverse current flow during transient events.

Schottky diode rectifier structure showing with guard ring

This form of Schottky diode structure is used particularly in rectifier diodes where the voltages may be high and breakdown is more of a problem.

Schottky diode structure notes

There are a number of points of interest from the fabrication process.

• The most critical element in the manufacturing process is to ensure a clean surface for an intimate contact of the metal with the semiconductor surface, and this is achieved chemically. The metal is normally deposited in a vacuum either by the use of evaporation or sputtering techniques. However in some instances chemical deposition is gaining some favour, and actual plating has been used although it is not generally controllable to the degree required.
• When silicides are to be used instead of a pure metal contact, this is normally achieved by depositing the metal and then heat treating to give the silicide. This process has the advantage that the reaction uses the surface silicon, and the actual junction propagates below the surface, where the silicon will not have been exposed to any contaminants. A further advantage of the whole Schottky structure is that it can be fabricated using relatively low temperature techniques, and does not generally need the high temperature steps needed in impurity diffusion.

The Schottky diode is used in a variety of forms for many different applications. Obviously those used for signal applications are in much smaller packages, often in SMT ones these days. Those devices used for power applications are in much larger packages, often ones which can be bolted to a heat-sink.

The Schottky diode is a very useful form of diode. It is widely used within electronics circuits because it has some particularly useful characteristics.

Its characteristics mean that it can be used where other forms of diode do not perform so successfully.

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 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 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.

Schottky diode IV characteristic

The use of a guard ring in the fabrication of the diode has an effect on its performance in both forward and reverse directions. [see page on structure and fabrication]. 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.

Key specification parameters

In view of the particular properties of the Schottky diode there are several parameters that are of key importance when determining the operation of one of these diodes against the more normal PN junction diodes.

• 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 IV 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.
• 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 sued with ordinary silicon diodes. Care should be taken to ensure heatsinking of power diodes does not allow this figure to be exceeded.
• 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.

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.

COMPARISON OF CHARACTERISTICS OF SCHOTTKY DIODE AND PN DIODE
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.