Enhancement of phonon-drag thermopower at relatively high temperatures

Two mechanisms of electron-phonon drag in nondegenerate semiconductors are considered. In a superlattice, the quasimomentum conservation law is violated because of the confinement of charge carriers in quantum wells. Therefore electrons interact with thermal phonons and the phonon-drag thermopower of a superlattice can be a few orders of magnitude greater than the corresponding thermopower of a bulk semiconductor. The second mechanism under consideration is the optical-phonon drag in polar semiconductors. The obtained results are valid for any temperature. It is shown that the contribution of optical-phonon drag to the Seebeck coefficient can be large even at a temperature exceeding the Debye one