Intrinsic population inversion in biased multiband superlattices

The electric field induced carrier redistribution in multiband superlattices is studied within an equation of motion analysis of the density matrix. Semiclassical intrasubband and quantum-mechanical tunneling contributions are identified. It is shown that there is no global population inversion for superlattices with only two minibands. However, we predict that an intrinsic global population inversion may occur at particular electric field strengths in superlattices with at least three minibands. Conditions under which global population inversion is expected to emerge are discussed. It is proposed to exploit the global population inversion that may appear due to resonant tunneling into a wide third miniband to fabricate unipolar midinfrared semiconductor lasers without any injector regions.