A note on the theory of rectification

Abstract

It has long been known that certain crystal contacts will rectify alter­nating currents, but no entirely satisfactory theory has yet been given. The most generally accepted theory is that due to Schottky, which has recently been discussed by van Geel. If a metal, and a semi-conductor such as cuprous oxide or selenium, are joined together in series in such a way that the contact between them is not perfect, a large resistance is found, which varies enormously with the direction of the current. Schottky’s theory assumes that, if a potential difference is created between the ends of the combination of metal and semi­-conductor, most of the drop in potential takes place in the transition layer between the two crystals, and electrons are drawn from one substance to the other by the high field set up. Further, owing to the differences in the inner potentials and the work functions of the two substances, the electrons are pulled out selectively, and the current-voltage curve is asymmetrical. The theory was by no means complete, the most serious objection to it being that it did not explain why one of the components must necessarily be a semi­-conductor. This is not surprising, as at that time the nature of semi-conductors was unknown, but recently a theory has been proposed which enables a start to be made on the many problems which involve semi-conductors. When subjected to alternating current, the rectifier copper/cuprous-oxide behaves in many ways like a condenser, and the capacity of the equivalent condenser was measured by Schottky and Deutschmann. They found that the capacity of the transition layer corresponds to a thickness of about 1.3 X 10 -6 cm. This fact has led Schottky to doubt the adequacy of the theory. He say “In any discussion of the asymmetry of the current across the transition layer, the question is met, how is it possible for an additional specific resistance to occur which is a hundred to a thousand times greater than the specific resistance of a massive oxide layer of the maximum thickness of the transition layer, while at the same time the outer oxide layer has practically the same material properties as the inner ones.