German physicist Ferdinand Braun, a 24-year old graduate of the University of Berlin, studied the characteristics of electrolytes and crystals that conduct electricity at Würzburg University in 1874. When he probed a galena crystal (lead sulfide) with the point of a thin metal wire, Braun noted that current flowed freely in one direction only. He had discovered the rectification effect at the point of contact between metals and certain crystal materials.
Braun demonstrated this semiconductor device to an audience at Leipzig on November 14, 1876, but it found no useful application until the advent of radio in the early 1900s when it was used as the signal detector in a "crystal radio" set. (1901 Milestone) The common descriptive name "cat’s-whisker" detector is derived from the fine metallic probe used to make electrical contact with the crystal surface. Braun is better known for his development of the cathode ray tube (CRT) oscilloscope in 1897, known as the "Braun tube" (Braunsche Röhre) in German. He shared the 1909 Nobel Prize with Guglielmo Marconi for his "contributions to the development of wireless telegraphy," mainly the development of tunable circuits for radio receivers.
Electronic devices that perform rectification are called diodes. Working in Thomas Edison's laboratory in 1883 William J. Hammer noted this rectifier effect when he added another electrode to a heated filament light bulb. In 1904, John Fleming patented a one-way "oscillation valve" based on the, so called, "Edison effect" that converted alternating radio signal currents into direct currents in the earphones or speaker. Known today as a diode, the Fleming valve was the first practical electronic device. The Braun point-contact rectifier diode performs the same function using semiconducting rather than thermionic properties.
Braun, F. "Uber die Stromleitung durch Schwefelmetalic", Annalen der Physik and Chemie, Vol. 153, No. 4 (1874) pp. 556-563 reprinted in English as "On the current conduction in metal sulphides," in Sze, S.M. Semiconductor Devices: Pioneering Papers. (Singapore: World Scientific Publishing Co., 1991) pp. 377-380.
Braun, Karl Ferdinand. "Electrical Oscillations and Wireless Telegraphy." Nobel Lecture, December 11, 1909. Nobel Lectures in Physics 1901-1921 (Singapore: World Scientific Publishing Co.).
Kurylo, Friedrich. Ferdinand Braun: A Life of the Nobel Prizewinner and Inventor of the Cathode-ray Tube Oscilloscope. Translated and adapted by Charles Susskind (Cambridge, MA: The MIT Press, 1981)
Riordan, M & Hoddeson, L. Crystal Fire: The Birth of the Information Age. (New York: W. W. Norton, 1997) p. 20.