Active Q‐switching was studied in a two‐segment AlGaAs/GaAs DH laser with a modulation‐doped GaAs QW placed in the n‐region of a p‐n junction. The modulator uses the blue shift of the QW absorption edge due to band filling by a 2D electron gas. The electron gas concentration could be varied in the range 0–8×1011 cm2 by applying bias voltage. Only about 100 mV of the modulating voltage was necessary to provide stable active Q‐switching so that every period of the modulating microwave signal gave rise to one optical laser pulse less than 50 ps FWHM. The threshold injection current density was very low and ranged from 400 to 800 A/cm2. The modulation bandwidth has been estimated to be not less than 4–5 GHz.