We present a new design for quantum cascade lasers (QCLs) without the typically used injector between two consecutive active stages. The lasers are realized with the InP-based material system AlInAs/GaInAs. With additional AlAs and InAs layers a significant optimization of the structure can be realized. In this improved structure the possibility of electrons escaping into the quasi-continuum is drastically reduced by the AlAs-blocking layer. On the other hand, InAs, a material with a very low effective mass, significantly Prolongs the carrier lifetime, enhancing the population inversion and increasing the dipole matrix element of the transition. Both inserted layers result in an overall improvement of the device properties, basically the threshold Current density (j(th)), maximum operating temperature (T-max), output power, slope efficiency and characteristic temperature To. With high reflection coated facets a record threshold current density as low as 450 A/cm(2) at 300 K was achieved in the pulsed mode. (C) 2008 Elsevier B.V. All rights reserved.