Some aspects of Gunn effect oscillators

The general view is that the mechanism responsible for the Gunn effect must be similar to that proposed independently by Ridley and Watkins and by Hilsum several years ago. This theory is described and the experiments that have been reported to substantiate it indirectly are reviewed briefly. A tentative theory to explain the wide tuning range observed when Gunn specimens are placed in a suitable cavity resonator (in some cases almost one octave) is advanced. Some experimental results, giving limited confirmation of the theory are presented.