Transport in nanostructures: A comparison between nonequilibrium Green functions and density matrices

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.