The relaxations of charge carriers between different subbands of a quantum well can be used to achieve laser action. Especially, laser radiation in far-infrared of electromagnetic spectrum, which can not be achieved by bulk semiconductors, is one of the most important applications of these transitions. Therefore, the motivation of this study is first explained. Then, the theoretical framework for
... [Show full abstract] the relaxation rate calculations is developed. In this part, after introducing the Fermi Golden Rule, the quantum wells are studied using the quantum mechanics relations and the wave functions and energy levels for the present particle are calculated. Then, intersubband transitions in GaAs square, triangular and step quantum wells are studied and the results are physically interpreted. Finally, the relaxation of carriers in an asymmetric Ga-As quantum well with the purpose of achieving the laser action is investigated.