Schottky Diode Technology & Structure
- a summary or tutorial of the Schottky Barrier Diode giving its structure and how it can be used RF circuits as well as rectifier and other electronic circuits.
Although it may appear a rather straightforward form of component when compared to others, Schottky diode technology has much to offer.
The Schottky diode structure while appearing straightforward in its most simple version is still able to offer performance that no other form of diode can provide.
As a result, Schottky diode technology has developed to enable it to be used in areas that would otherwise not have been possible. Many diodes incorporating elements including guard rings enhance the performance in some areas by considerable degrees.
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.
By Ian Poole
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