  For different models of the electron-phonon interaction, the asymptotic
behaviour of the moments of the stationary homogeneous solution of the linear
Boltzmann equation is determined in the limit of a high external field. For
Hilbert-Schmidt kernels of a finite rank, a result recently proven for kernels
of rank one is found generally valid; as a consequence velocity saturation is
excluded for these collision models. For a class of singular collision kernels
in contrast, velocity saturation is generally obtained.
