On the validity criteria of the phenomenological theory of electron absorption and amplification of sound in semiconductors

By using the kinetic equation for the electron distribution function in the acoustic wave field, the criteria are established for the validity of the phenomenological (“hydrodynamic”) theory of electron absorption and amplification of sound (acoustic waves) in the semiconductors. It is shown that the upper frequency limit of the validity of this theory lies, generally speaking, in the region of considerably lower (maybe by orders of magnitude) frequencies than it follows from the usually accepted criterion ql ? 1.