The Gunn effect

Microwave current oscillations in n-type polar semiconductors at high electric fields were first observed by Gunn in 1963. The basic physical mechanism responsible for the effect is the progressive transfer of hot electrons from high mobility valleys to low mobility valleys in the conduction-band structure as the field increases. The resulting negative differential mobility makes the homogeneous electron distribution unstable. Narrow domains of extremely high field build up which propagate uniformly through the specimen. Recent theoretical and experimental studies of the electron-transfer mechanism and domain propagation are described. The emphasis is on the basic phenomena in gallium arsenide and microwave applications of the effect are not discussed.