History of the transistor), based on several patents that were issued to him. Erno Borbely states the following: "Field-effect transistors (FETs) have been around for a long time; in fact, they were invented, at least theoretically, before the bipolar transistors. The basic principle of the FET has been known since J.E. Lilienfeld’s US patent from 1930, and Oscar Heil described the possibility of controlling the resistance in a semiconducting material with an electric field in a British patent in 1935." Air Motion Transformer He also invented the "air-motion transformer" audio speaker technology made famous by the amt1 speaker of ESS in the early 1970s. The amt voice coil membrane is made of a polyethylene sheet, embossed with conductive aluminum strips. It is equivalent in surface area to a conventional 7-inch cone type mid-range driver, but is accordion-folded down to less than a 2-inch grouping for point-source dispersion. The low-mass membrane sheet is suspended within a quadratic magnetic housing, concentrating an intense field around the diaphragm. When signal current passes through the aluminum strips, the ensuing bellows-like motion of the folded pleats moves air five times faster than a conventional cone driver. This rapid acceleration of air-motion is claimed to provide enhanced sound reproduction, including high dynamic range and over an extremely broad frequency range. References Notes General references Agnes Arsenjewa, Über den Einfluß des Röntgenlichtes auf die Absorptionsspektra der Alkalihalogenidphosphore, PhD thesis, 1929. Oskar Heil, Auslöschung und Überführung von Resonanzserienspektren ins Bandenspektrum durch Gaszusatz, PhD thesis, 1932. A. Arsenjewa-Heil and O. Heil, Eine neue Methode zur Erzeugung kurzer, ungedämpfter, elektromagnetischer Wellen großer Intensität, Zeitschrift für Physik, Vol. 95, Nos. 11-12 (November 1935), pp. 752–762. External links Heil tube example (manufacturer STC) Oscar Heil Aulos Loudspeakers 1908 births 1994 deaths German electrical engineers 20th-century German physicists University of Göttingen alumni People from Kaiserslautern (district) Engineers from Rhineland-Palatinate