  Stationary electric transport in semiconductor nanostructures is studied by
the method of nonequilibrium Green functions. In the case of sequential
tunneling the results are compared with density matrix theory, providing almost
identical results. Nevertheless, the method of Green functions is easier to
handle due to the availability of an absolute energy scale. It is demonstrated,
that the transport in complicated structures, like quantum cascade lasers, can
be described in reasonable agreement with experiment.
