Julius Lilienfeld files a patent describing a three-electrode amplifying device based on the semiconducting properties of copper sulfide. Attempts to build such a device continue through the 1930s.

Polish-American physicist and inventor Julius E. Lilienfeld filed a patent in 1926, “Method and Apparatus for Controlling Electric Currents,” in which he proposed a three-electrode structure using copper-sulfide semiconductor material. Today this device would be called a field-effect transistor. While working at Cambridge University in 1934, German electrical engineer and inventor Oskar Heil filed a patent on controlling current flow in a semiconductor via capacitive coupling at an electrode – essentially a field-effect transistor. Although both patents were granted, no records exist to prove that Heil or Lilienfeld actually constructed functioning devices.

In 1938 Robert Pohl and Rudolf Hilsch experimented with potassium-bromide crystals with three electrodes at Gottingen University, Germany. They reported amplification of low-frequency (about 1 Hz) signals, but their research did not lead to any applications.

Stimulated by research into copper-oxide rectifiers at Bell Telephone Laboratories and by explanations of semiconductor rectification by Mott and Schottky (1931 Milestone), William Shockley wrote in December 1939 that “It has today occurred to me that an amplifier using semi conductors rather than vacuum is in principle possible.” Under his direction, Walter Brattain and others performed experiments on such three-electrode devices but did not achieve amplification. On his return to Bell Labs after the war in 1945 Shockley resumed his work on semiconductor devices. Again he failed to achieve his predicted results. In 1946 physicist John Bardeen calculated that surface effects could account for the failure of these attempts to build working devices. (1947 Milestone)