Designing interband and intersubband quantum well lasers

Summary form only given. Reports of new and improved semiconductor lasers incorporating quantum well (QW) active layers continue to appear. Of particular note has been the extension of the wavelength range over which electrically driven devices can operate at room temperature. At the short wavelength end near 0.4 /spl mu/m, we have the InGaN MQW laser which operates based on electron energy transitions between conduction and valence bands (interband lasers). At the long wavelength end near 11.5 /spl mu/m, we have the InGaAs MQW laser (the quantum cascade laser) which operates based on electron energy transitions between subbands in the conduction band (intersubband lasers). In designing interband lasers, one usually assumes that the carriers in the quantum wells occupy energy levels determined by the Fermi-Dirac distribution and that optical gain is achieved based on nonequilibrium quasi-Fermi level concepts. For intersubband lasers, one usually ignores the Fermi-Dirac distribution and utilizes four level laser concepts instead. In the tutorial, the methods used to estimate threshold current density for both laser types will be outlined and the reasons for the different approaches discussed.