Ian Hickman

Overview

The semiconductor diode, like its predecessor the thermionic diode, conducts current in one direction only. It is arguable that diodes in general are not really active devices at all, but simply nonlinear passive devices. The earliest semiconductor diode was a point contact device and was already in use before the First World War, being quite possibly contemporary with the earliest thermionic diodes. It consisted of a sharp pointed piece of springy wire (the "cat's whisker") pressed against a lump of mineral (the "crystal"), usually Galena, an ore containing sulfide of lead. The crystal detector was widely employed as the detector in the crystal sets that were popular in the early days of broadcasting. Given a long aerial and a good earth, the crystal set produced an adequate output for use with sensitive headphones, while with so few stations on the air in those days the limited selectivity of the crystal set was not too serious a problem. The crystal and cat's whisker variety of point contact diode was a very hit and miss affair, with the listener probing the surface of the crystal to find a good spot. Later, new techniques and materials were developed, enabling robust preadjusted point contact diodes useful at microwave frequencies to be produced. These were used in radar sets such as AI Mk.10, an airborne interceptor radar that was in service during (and long after!) the Second World War. Germanium point contact diodes are still manufactured and are useful...