  Theories describing electrical transport in semiconductor superlattices can
essentially be divided in three disjoint categories: i) transport in a
miniband; ii) hopping between Wannier-Stark ladders; and iii) sequential
tunneling. We present a quantum transport model, based on nonequilibrium Green
functions, which, in the appropriate limits, reproduces the three conventional
theories, and describes the transport in the previously unaccessible region of
the parameter space.
