The Transferred Electron Effect

In the early sixties Ridley, Watkins, and Hilsum showed theoretically that a semi-conducting wafer with two ohmic contacts on opposite surfaces may exhibit a negative differential resistance and can thus be used for amplifying or generating microwave signals. Particularly well suited are certain III-V compound materials, whose conduction band consists of a high mobility central valley and several energetically higher-lying satellite valleys. This so called "transferred electron effect" was observed in 1963 by Gunn in gallium arsenide and indium phosphide and is now frequently associated with his name. This paper reviews the relevant results obtained at millimeter-wave frequencies with both sandwich and planar TEDs including those having modified cathode and anode contacts.