Schottky's achievements can be loosely divided into two phases: the first being research into vacuum electronics and the second, starting in 1929, covering semiconductor electronics. Of course, there were side issues to these two generalisations, some of which would alone have guaranteed him a place in the history books. The invention of the ribbon microphone was one, the superhet another. The ribbon microphone dates from 1924 and was invented jointly with Erwin Gerlach. They used an extremely thin concertina ribbon of aluminium placed between the poles of a permanent magnet. Reversing the physical effects led them to invent a ribbon loudspeaker as well, also using a thin ribbon of aluminium. The invention of the superhet is usually credited to the American Edwin Armstrong, but Schottky independently discovered the same principle of the super heterodyne with IF amplification in 1918. Coming second, it would seem, does not provide lasting fame, except perhaps in one's home country.
At Jena, where Schottky began his work on electron physics, he performed both theoretical and experimental studies of the space-charge effects of electrons emitted from cathodes in vacuum tubes. In 1913/14, at about the same time as Irving Langmuir in America, he independently discovered the basic law relating the current in a valve to the applied voltage; the V3/2 law. Here, at least, he gained more lasting fame than with the superhet; it was a pioneering achievement and would have established him as a leading physicist. It may also have helped to determine his temporary career move to Siemens in 1915.
Noise
At Siemens, Schottky further developed his interests in electronic valves. Though he was there only from 1915 to 1919, he seemed to reel off a series of discoveries or inventions. His invention of the screen-grid valve or tetrode (which apparently he originally called the protection grid tube) was a major invention in electronics yet, in the light of hindsight, it was possibly overshadowed by his prediction of thermal and shot noise, two of the fundamental classes of noise in electronic devices.
During the early years of electronic circuitry, especially around the period of WWI, engineers and physicists were trying to solve problems involved in making better vacuum valves. Whilst many of the problems were related to design and manufacturing techniques, such as inadequate vacuum pumping, mechanical resonance, poor welds and the like, the fundamental problem of noise was gaining recognition. Some scientists were trying to discover what the ultimate performance of valve amplifiers might be, once all manufacturing problems were solved and only fundamental problems of physics remained. J.B. Johnson and Harry Nyquist, Swedes working at Bell Laboratories in America, were to provide some of the answers in the 1920s, but the classic paper on noise in valve amplifiers was published by Walter Schottky in 1918 in Germany.
More than 50 years later, Johnson was to remark on the fact that a paper of such quality and technical importance should come out of a Germany facing military defeat and economic collapse. In fact, though the paper was published in 1918, Schottky had been working on the problem of noise since 1915.
Schottky had reached the conclusion that there would be two sources of noise of a fundamental nature in an amplifier. The first would occur in the input circuit and would result from the random motion of charge caused by the thermal motion of the molecules in the conductors. This we know as thermal noise, Schottky called it the Warmeffekt. Because this noise originated in the input circuit it would be amplified appear at the output.
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