their theories, and later making manufacture of modern diodes possible. The theory of rectification in a metal-semiconductor junction, the type used in a cat whisker detector, was developed in 1938 independently by Walter Schottky at Siemens & Halske research laboratory in Germany and Nevill Mott at Bristol University, UK. Mott received the 1977 Nobel Prize in Physics. In 1949 at Bell Labs William Shockley derived the Shockley diode equation which gives the nonlinear exponential current–voltage curve of a crystal detector, observed by scientists since Braun and Bose, which is responsible for rectification . The first modern diodes The development of microwave technology during the 1930s run up to World War II for use in military radar led to the resurrection of the point contact crystal detector. Microwave radar receivers required a nonlinear device that could act as a mixer, to mix the incoming microwave signal with a local oscillator signal, to shift the microwave signal down to a lower intermediate frequency (IF) at which it could be amplified. The vacuum tubes used as mixers at lower frequencies in superheterodyne receivers could not function at microwave frequencies due to excessive capacitance. In the mid-1930s George Southworth at Bell Labs, working on this problem, bought an old cat whisker detector and found it worked at microwave frequencies. Hans Hollmann in Germany made the same discovery. The MIT Radiation Laboratory launched a project to develop microwave detector diodes, focusing on silicon, which had the best detecting properties. By about 1942 point-contact silicon crystal detectors for radar receivers such as the 1N21 and 1N23 were being mass-produced, consisting of a slice of boron-doped silicon crystal with a tungsten wire point pressed firmly against it. The cat whisker contact did not require adjustment, and these were sealed units. A second parallel development program at Purdue University produced germanium diodes. Such point-contact diodes are still being manufactured, and may be considered the first modern diodes. After the war, germanium diodes replaced galena cat whisker detectors in the few crystal radios being made. Germanium diodes are more sensitive than silicon diodes as detectors, because germanium has a lower forward voltage drop than silicon (0.4 vs 0.7 volts). Today a few galena cat whisker detectors are still being made, but only for antique replica crystal radios or devices for science education. See also Barretter detector Electrolytic detector Magnetic detector List of historic technological nomenclature Point-contact transistor Reginald Fessenden