The two main design schemes for Terahertz quantum cascade lasers, based on tunnelling and scattering injection,
respectively, are theoretically compared. We apply our simulation package based on the non-equilibrium Green’s
function technique. Our results provide a good description of the gain degradation with temperature. Thermal
backfilling contributes to decrease of population inversion in both cases. However, the dropping inversion cannot
account for the total reduction of gain.