A Perspective on Acoustoelectric Instabilities

The acoustoelectric instabilities appear in diverse forms and seem complex because of the large number of factors which influence them. A perspective on the instabilities is provided in terms of experiments on the III–V semiconductors, selected to limit or control the variables. Detailed evidence is presented that the source of the acoustic flux is the thermal equilibrium phonon spectrum. The individual contributions of various factors to the instabilities are identified and discussed; e.g., 1) circuit conditions (constant current or constant voltage) which determine the degree of feedback between buildup of flux and gain everywhere in the sample, 2) boundary conditions, including destruction or reflection of flux, 3) inhomogeneities in resistivity of the samples, to which the current oscillations and initial stages of domain formation are particularly sensitive, and 4) the form of the acoustoelectric gain curve, which determines whether the feedback on the gain is positive or negative when the current is changed by the flux buildup. Also, several aspects of deviation from small signal theory are identified. In strong flux, 1) the peak of the frequency spectrum of the flux is shifted down, 2) the acoustoelectric gain coefficient is changed, and 3) the carrier distribution function may become hot in the high field of the domain. It is concluded that the strong flux effects modify but do not basically determine the form of the resistance instabilities.