Jagadis Chandra Bose, a professor of physics at Presidency College in Calcutta, India, demonstrated the use of galena (lead sulfide) crystals contacted by a metal point to detect millimeter electromagnetic waves. In 1901 he filed a U.S patent for a point-contact semiconductor rectifier for detecting radio signals.
Between 1902 and 1906, American Telephone and Telegraph electrical engineer Greenleaf W. Pickard tested thousands of mineral samples to assess their rectification properties. Silicon crystals from Westinghouse yielded some of the best results. On August 20, 1906, he filed a U.S. patent on "Means for receiving intelligent communication by electric waves" for a silicon point-contact detector; it was awarded that November. With two associates, Pickard founded the Wireless Specialty Apparatus Company to market "cat's-whisker" crystal radio detectors; it was probably the first company to make and sell silicon semiconductor devices.
Many other inventors experimented with alternative materials. Henry Dunwoody received a patent for a carborundum (silicon carbide) detector just two weeks after Pickard. Wichi Torikata of the Electrotechnical Laboratory (ETL) earned Japanese patent No. 15345 for a mineral detector in 1908. In 1922-23 Russian engineer Oleg Losev of the Nizhegorod Radio Laboratory, Leningrad used carborundum and zincite crystal rectifiers in amplifiers and oscillators operating at frequencies up to 5 MHz.
Although crystal rectifiers allowed simple radio sets to operate without external power, by the mid-1920s the predictable performance of vacuum-tubes replaced them in most radio applications. They regained prominence in World War II because of their ability to operate at microwave frequencies. (1941 Milestone)
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