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.

This Schottky diode tutorial is split into several pages:

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 is 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.

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.

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 technique in which metal was vacuum deposited.

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 metalised 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 Scottky diode structure is used in many forms of Schottky, but particularly in rectifier diodes where the voltages may be high and breakdown could be 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.

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Further pages from this tutorial
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