We present a comprehensive study of the thermal behavior of GaAs/AlGaAs quantum cascade lasers as a function of the aluminum content in the barrier layers, and results in a new QC-structure designs based on the possibility to change the Al content during the MBE growth. In the first sample, we found a steady improvement of the threshold current dependence with the temperature when the Al content in the barriers is increased. The influence of indirect barrier states seems to play no significant role, which can be explained by the strong quantum confinement of the barrier states impeding carrier transport through the satellite valleys. Moreover, these results show that the GaAs/AlGaAs material system may be exploited over its entire composition range for QC lasers. Also, for a sample with AlAs "spikes" of 1-2 monolayers in the Al0.45Ga0.55As barriers, we found an improvement of 45 K in the T-0, depending on the structures. We also studied a new sort of injector barrier made of two different compositions of Al. The "step"' in energy is chosen such that when the injector ground state aligns with the upper active region state, it provides optimal injection; at the same time it inhibits parasitic injection from the injector state to the lower states of the active region. As a result we observe a noticeable reduction of the threshold current. (C) 2003 Elsevier Science B.V. All rights reserved.