AMPLIFICATION AND DISPERSION OF OPTICAL PHONONS IN POLAR SEMICONDUCTORS

The interaction between conduction electrons and optical phonons is studied. It is shown that as soon as the electron drift velocity exceeds the phase velocity of the phonons an instability occurs which appears to be strong enough to overcome damping effects. The dielectric constant of the electrons is a function of wave number and due to this effect the group velocity of the optical phonons is greatly increased. One would therefore expect an increased thermal conductivity due to optical phonons in polar semiconductors with a high density of conduction electrons. (Author)