  The first global simulation of semiconductor-based quantum-cascade lasers is
presented; Our fully three-dimensional approach allows to study in a purely
microscopic way -without resorting to phenomenological parameters- the
current-voltage characteristics of state-of-the-art unipolar nanostructures.
Based on the proposed theoretical scheme, we are able to give a definite answer
to the long-standing controversial question: is charge transport in
quantum-cascade lasers mainly coherent or incoherent? Our analysis clearly
shows that a proper inclusion of carrier-phonon as well as carrier-carrier
scattering within a semiclassical framework gives excellent agreement with
experimental results.
