We outline a systematic nonperturbative derivation of a hierarchy of closed systems of moment equations that can be applied to any kinetic description of electrons in a semiconductor. This entropy based closure procedure extends one that was introduced in the context of gas dynamics. In the context of semiconductors, this procedure yields generalizations of socalled hydrodynamic models. It is illustrated on the semiclassical Boltzmann-Poisson equation for a single conduction band in the parabolic band approximation.