  The effect of plasma oscillations on acoustic wave amplification by carrier
drift in semiconductors is discussed. A continuum theory is developed on the
basis of hydrodynamical equations for the conduction electrons, the equations
for the elastic vibrations of the crystal, and the Poisson equation. It is
shown that if the plasma frequency is much lower than the reciprocal value of
the electron relaxation time for the scattering on phonons and impurities, the
results are in agreement with available calculations based on the electrical
conductivity - diffusion equation. However, in the opposite case the
amplification factor is orders of magnitude larger compared to the value
predicted by those calculations, while the optimum drift velocity is much
smaller.
