The "asymmetric conduction" of electric current across electrical contacts between a crystal and a metal was discovered in 1874 by Karl Ferdinand Braun.[6] Crystals were first used as radio wave detectors in 1894 by Jagadish Chandra Bose in his microwave experiments.[7][8] Bose first patented a crystal detector in 1901.[9] The crystal detector was developed into a practical radio component mainly by G. W. Pickard,[4][10][11] who discovered crystal rectification in 1902 and found hundreds of crystalline substances that could be used in forming rectifying junctions.[2][12] The physical principles by which they worked were not understood at the time they were used,[13] but subsequent research into these primitive point contact semiconductor junctions in the 1930s and 1940s led to the development of modern semiconductor electronics.[1][4][14][15]
The unamplified radio receivers that used crystal detectors are called crystal radios.[16] The crystal radio was the first type of radio receiver that was used by the general public,[14] and became the most widely used type of radio until the 1920s.[17] It became obsolete with the development of vacuum tube receivers around 1920,[1][14] but continued to be used until World War II and remains a common educational project today thanks to its simple design.
Crystal:
Galena crystals sold for use in crystal detectors, Poland, 1930s The most common crystal used was galena (lead sulfide, PbS), a widely occurring ore of lead. Varieties were sold under the names "Lenzite"[19] and "Hertzite".[4][21][22] Other crystalline minerals were also used, the more common ones were iron pyrite (iron sulfide, FeS2, "fool's gold", also sold under the trade names "Pyron"[27] and "Ferron"[19]),[2][21][23]molybdenite (molybdenum disulfide, MoS2),[19][21][23] and cerussite (lead carbonate, PbCO3)[21] Not all specimens of a crystal would function in a detector, often several crystal pieces had to be tried to find an active one.[19] Galena with good detecting properties was rare and had no reliable visual characteristics distinguishing it. A rough pebble of detecting mineral about the size of a pea was mounted in a metal cup, which formed one side of the circuit. The electrical contact between the cup and the crystal had to be good, because this contact must not act as a second rectifying junction, creating two back-to-back diodes which would prevent the device from conducting at all.[28] To make good contact with the crystal, it was either clamped with setscrews or embedded in solder. Because the relatively high melting temperature of tin-lead solder can damage many crystals, a fusible alloy with a low melting point, well under 200 °F (93 °C), such as Wood's metal was used.[4][19][21] One surface was left exposed to allow contact with the cat-whisker wire.
Barretter detector
Electrolytic detector
Magnetic detector
List of historic technological nomenclature
Point-contact transistor
Reginald Fessenden
1909 publication describes experiments to determine the means of rectification (PDF file).
