Boltzmann's kinetic equations have not been used to date to study nonequilibrium phenomena in semiconductors, and therefore, to obtain analytical expressions for the oscillation frequency inside the semiconductor and the critical external electric field, it is of theoretical interest. In this theoretical work, the frequency of oscillations occurring inside a two-valley semiconductor of the GaAs type in an external constant electric field and in an external strong magnetic field ( , μ-mobility of charge carriers, H-magnetic field strength, c-speed of light) is calculated. It has been proved that the critical values of the external electric field fully correspond to the values of the electric field, which were obtained by the Gunn experiment. It is proved that unstable waves are excited in GaAs if the crystal dimensions are and . Analytical expressions are obtained by theoretical calculation for an external constant magnetic field, when unstable oscillations are excited inside the sample.