Abstract
<jats:p>A theory of unstable wave excitation in a two-valley semiconductor subjected to a temperature gradient and constant external electric and magnetic fields is developed. The effects of the external electric field, the tem perature gradient, the magnetic field generated within the sample by hydrodynamic motion, and the electric field arising from charge-carrier redistribution are taken into account. It is shown that the sample size plays an important role in the excitation of unstable waves. The frequency of hydrodynamic waves is shown to be twice that of the thermomagnetic waves excited in the sample. Analytical expressions for the frequencies and growth rates of the unstable waves are obtained. Analytical conditions for the external magnetic field required to excite hydrodynamic unstable waves are derived, and the ranges of the external electric field corresponding to wave excitation are determined. It is established that the transition time of charge carriers from the lower valley to the upper valley is shorter than the transition time from the upper valley to the lower valley. The analysis is based on a linear theory and assumes that carrier mobilities differ only slightly from their equilib rium values. For the first time, the electric field generated within the semiconductor is taken into account, demonstrating the feasibility of developing new Gunn-effect devices, including generators and amplifiers. The proposed mechanisms are consistent with available experimental data on the Gunn effect. It is also shown that the combined action of a temperature gradient and an external magnetic field can facilitate the de sign and optimization of high-frequency devices and amplifiers.</jats:p>