Effects of the injector doping densities on lasing properties of mid-infrared InAlAs-InGaAs-InP quantum cascade lasers at 4.3 mu m have been studied. Lasers with different average injector doping between 1.29E17 cm(-3) and 2.07E17 cm(-3) have been grown by gas source molecular beam epitaxy (GSMBE), and their lasing characteristics were investigated. Lasers with low doped injectors (1.68E17 cm(-3)) exhibited lower threshold current density, longer emission wavelength and lower characteristic temperature T-0. The lower injector doping density decreases waveguide loss. On the other hand, the high doping of the injector increases heat transfer through phonon in QCL active region, and thus results in high characteristic temperature T-0. (c) 2013 Elsevier B.V. All rights reserved.