  Nonequilibrium electron distribution in a superlattice subjected to a
homogeneous electric field (biased superlattice with equipopulated levels) is
studied within the tight-binding approximation, taking into account the
scattering by optical and acoustic phonons and by lateral disorder. It is found
that the distribution versus the in-plane kinetic energy depends essentially on
the ratio between the Bloch energy and the optical phonon energy. The in-plane
conductivity is calculated for low-doped structures at temperatures 4.2 K and
20 K. The negative conductivity is found for bias voltages corresponding to the
Bloch-phonon resonance condition.
