Schottky diode

He deduced that the Schottky diode would be proportional to the Bolt gathered material which eventually appeared in 1929 in a book on thermodynamics. It was written in collaboration with H. Ulich and C. Wagner and presented the thermodynamic theory of solids with very low impurity content or with small deviations from stoichiometry. It led him naturally to the study of semiconductors.

His other achievement of the period was in finding a wife. He married Elizabeth Lintz in 1923. They had three children.

Bolzmann constant (k) multiplied by the absolute temperature. In the mid-1920s, Johnson experimentally identified thermal noise and Nyquist analysed the discovery mathematically, producing a formula of 4kT watts per unit of bandwidth, confirming Schottky's deduction.

Schottky called his second fundamental source of noise, suggested in his 1918 paper, the Schroteffekt. This, he suggested, would be internal to the valve and would be caused by the randomness of the emission from the cathode and the randomness of the velocity of the emitted electrons. We know it as shot noise and it was first experimentally identified and measured in Schottky's laboratory. Later studies showed it was linked to factors such as the material and design of the cathode. Better understanding of these sources of noise led to better valves and, in the semiconductor age, to better solid-state devices.

This work on noise and thermionic emission in valves represents one of the great periods of Schottky's work. It was near simultaneous with his more engineering contributions of valve and circuit developments, notably the superhet. The next great period of his work was to be with semiconductors, but before that he turned his attention to thermodynamics.

Throughout the 1920s Schottky junction between a metal and a semiconductor instead of a junction between two pieces of semiconductor. Metal-semiconductor junctions are also used for non-rectifying (ohmic) contacts to semiconductor devices.

Ferdinand Braun is usually credited with the first systematic study of metal-semiconductor rectifiers, work which was published in 1874. Point-contact metal-semiconductor rectifiers were used from the early years of this cen­tury, but it was not until 1931 that the theory of current flow in semiconductors was placed on a modern basis by A.H. Wilson. Seven years later, Schottky published his diffusion theory of current transport in metal-semiconductor junctions it was from this theory that modern understanding grew, hence a metal-semiconductor diode is usually known as a Schottky diode. Their importance lies in the speed with which they can be switched off from the saturated state. Being majority carrier devices they do not suffer from the minority carrier storage problems which slow down p-n junction switching.

In thermionic valves, the current emitted from the metal cathode into the vacuum depends, in part, on the metal's work function. Schottky discovered that this work function was lowered from its "normal" value by the presence of image forces and by the electric field at the cathode. This effect became known as the Schottky effect. In practical thermionic diodes it meant that, even when the current saturated, there would still be some increase in current if the anode voltage was further increased. However, unless very high anode voltages were used the Schottky effect could be neglected.

If we regard the filament in a thermionic valve as part of a metal-vacuum "junction" then the Schottky effect theory can be extended to a metal-semiconductor junction. The "barrier lowering" (as it is then called) that takes place is less than in the equivalent metal-vacuum "junction" but the effect is profound. Schottky used this as the basis of his explanation of the metal-semiconductor rectifier, work that was published in 1938. Other work by H. A. Bethe, Neville Mott, B. Davidov and others further clarified the conduction processes within metal-semiconductor rectifiers, but even so the term Schottky diode seems to be used synonymously with metal-semiconductor diode.

Two further examples of Schottky's vision are worth recounting: one to do with electronics, one not.

Schottky's obituaries recount that, in 1929, whilst studying semiconductors, he perceived 01 anticipated what we now call "holes". He wrote: "To a certain extent the places available for conduction electrons are occupied by static electron space charges and thus the passage of conduction electrons is blocked." These static space charges he called "defect electrons". It was another two years before Werner Heisenberg clarified the phenomenon of holes using quantum mechanics. Again Schottky's work paralleled that of someone else. Rudolph Peierls, at Bell Telephone. Laboratories, conceived the same idea also in 1929.

The final example of Schottky's vision relates to man's use of natural resources. In the preface to his 1929 book on thermodynamics he commented: "The time when man could dispose freely over the resources of energy and materials given to us by Nature will one day appear to belong to an era past, probably in the lifetime of our children." The world is now witnessing the truth of that prophecy.

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