  Detailed simulations for Terahertz quantum cascade lasers based on two-well
designs are presented. We reproduce the maximal operation temperatures observed
and attribute the degradation with temperature to the occupation of parasitic
levels and thermal backfilling. Furthermore, we demonstrate that the current
injection can be conveniently studied by using EZ states, which combine energy
selectivity and spatial localisation. Improving the injection allows to achieve
higher maximal operation temperatures around 265 Kelvin.
