A set of room temperature low threshold current densities of a mid-infrared InP-based quantum cascade lasers (QCLs) have been demonstrated. Threshold current densities as low as 1.7 kA/cm(2) for multi-mode Fabry-Perot QCLs at an emission wavelength of 7.6 mu m and 0.9 kA/cm(2) for single-mode distributed feedback QCLs at an emission wavelength of 7.7 mu m have been achieved at 300 K. The results obtained are through the precision control of doping incorporation and concentration, the active region of the structure for suppressing the current leakage, and the low resistance material choice in the wave guide cladding and plasmon for improving heat dissipation. (c) 2007 Elsevier B.V. All rights reserved.