Are Quantum-Cascade Lasers really quantum?

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.