During the first three decades of radio, from 1888 to 1918, called the
wireless telegraphy or "spark" era, primitive
radio transmitters called
spark gap transmitters were used, which generated radio waves by an
electric spark. Instead spark gap transmitters transmitted information by
wireless telegraphy; the user turned the transmitter on and off rapidly by tapping on a
telegraph key, producing pulses of radio waves which spelled out text messages in
Morse code. Therefore, the
radio receivers of this era did not have to
demodulate the radio wave, extract an
audio signal from it as modern receivers do, they merely had to detect the presence or absence of the radio waves, to make a sound in the earphone when the radio wave was present to represent the "dots" and "dashes" of Morse code. the first primitive radio wave detector, called a
coherer, developed in 1890 by
Édouard Branly and used in the first radio receivers in 1894–96 by Marconi and
Oliver Lodge. Made in many forms, the coherer consisted of a high resistance electrical contact, composed of conductors touching with a thin resistive surface film, usually oxidation, between them. motivating much research to find better detectors. He studied
copper pyrite (Cu5FeS4),
iron pyrite (iron sulfide, FeS2), galena (PbS) and copper antimony sulfide (Cu3SbS4). This was before radio waves had been discovered, and Braun did not apply these devices practically but was interested in the
nonlinear current–voltage characteristic that these sulfides exhibited. Braun's method of making contact with the crystal may have been crucial to his results: he placed the sample on a circle of wire, then touched it with the end of a slender silver wire, a "cat's whisker" contact. Like other scientists since Hertz, Bose was investigating the similarity between radio waves and light by duplicating classic
optics experiments with radio waves. He experimented with many substances as contact detectors, focusing on
galena. His detectors consisted of a small galena crystal with a metal point contact pressed against it with a thumbscrew, mounted inside a closed
waveguide ending in a
horn antenna to collect the microwaves. In 1906
L. W. Austin invented a silicon–
tellurium detector, and in 1911 Thompson H. Lyon invented the
cerussite detector. In Germany a tellurium-carbon detector became popular, called "the Bronc cell".
Lee De Forest,
George Washington Pierce, and
William Henry Eccles also studied mineral detectors. The fine wire catwhisker contact invented early in the detector's history was patented in 1911 by Pickard Until the triode vacuum tube began to be used during World War I, crystals were the best radio reception technology, used in sophisticated receivers in wireless telegraphy stations, as well as in homemade crystal radios. In transoceanic radiotelegraphy stations elaborate inductively coupled crystal receivers fed by mile long wire antennas were used to receive transatlantic telegram traffic. Much research went into finding better detectors and many types of crystals were tried. The goal of researchers was to find rectifying crystals that were less fragile and sensitive to vibration than galena and pyrite. Another desired property was tolerance of high currents; many crystals would become insensitive when subjected to discharges of atmospheric electricity from the outdoor wire antenna, or current from the powerful spark transmitter leaking into the receiver. Carborundum proved to be the best of these; Use continued to grow until the 1920s when vacuum tube radios replaced them. and Pickard. They noticed that when their detectors were biased with a DC voltage to improve their sensitivity, they would sometimes break into spontaneous oscillations. He realized that amplifying crystals could be an alternative to the fragile, expensive, energy-wasting vacuum tube. He used biased negative resistance crystal junctions to build solid-state
amplifiers,
oscillators, and amplifying and regenerative
radio receivers, 25 years before the invention of the transistor. Later he even built a
superheterodyne receiver. While investigating crystal detectors in the mid-1920s at Nizhny Novgorod,
Oleg Losev independently discovered that biased carborundum and zincite junctions emitted light. Losev was the first to analyze this device, investigate the source of the light, propose a theory of how it worked, and envision practical applications. and the 16 papers he published on LEDs between 1924 and 1930 constitute a comprehensive study of this device. Losev did extensive research into the mechanism of light emission. He measured rates of evaporation of benzine from the crystal surface and found it was not accelerated when light was emitted, concluding that the luminescence was a "cold" light not caused by thermal effects. He wrote to Einstein about it, but did not receive a reply. AM
radio broadcasting spontaneously arose around 1920, and radio listening exploded to become a hugely popular pastime. The initial listening audience for the new broadcasting stations was largely owners of crystal radios, because many consumers couldn't afford the new tube radios. Commercial and military wireless telegraphy stations had already switched to more sensitive vacuum tube receivers. Vacuum tubes put an end to crystal detector development. The temperamental, unreliable action of the crystal detector had always been a barrier to its acceptance as a standard component in commercial radio equipment The German word
halbleiter, translated into English as "
semiconductor", was first used in 1911 to describe substances whose conductivity fell between
conductors and
insulators, such as the crystals in crystal detectors.
Felix Bloch and
Rudolf Peierls around 1930 applied quantum mechanics to create a theory of how electrons move through a crystal. In 1930
Bernhard Gudden and Wilson established that electrical conduction in semiconductors was due to trace impurities in the crystal. A "pure" semiconductor did not act as a semiconductor, but as an
insulator (at low temperatures). at
Siemens & Halske research laboratory in Germany and
Nevill Mott at
Bristol University, UK. ==See also==