We report a comprehensive study of GaAs–Al 0.15Ga 0.85As quantum cascade electro-luminescence devices designed to emit in the far-infrared. Strong intersubband electro-luminescence is observed at 3.9 THz with a FWHM of typically 0.21 THz . We investigate the intersubband emission as a function of the doping density in the injector regions of the cascade structures. A systematic increase in intersubband emission intensity with doping density is observed owing to the enhanced current flow through the devices, with powers of up to 55 pW detected.