The first global quantum simulation of semiconductor-based quantum-cascade
lasers is presented. Our three-dimensional approach allows us to study in a
purely microscopic way the current-voltage characteristics of state-of-the
-art unipolar nanostructures, and therefore to answer the long-standing con
troversial question: Is charge transport in quantum-cascade lasers mainly c
oherent or incoherent? Our analysis shows that (i) quantum corrections to t
he semiclassical scenario are minor and (ii) inclusion of carrier-phonon an
d carrier-carrier scattering gives excellent agreement with experimental re
sults.