We propose and demonstrate a new doping approach, i.e. intrinsic doping, for n-type modulation doping in InP-based heterostructures. Instead of the conventional method of n-type doping by shallow donor impurities, grown-in intrinsic defects are utilized to provide the required doping without external doping sources. The success of this approach is clearly demonstrated by our results from InGaAs/InP heterostructures, where the required n-type doping in the InP barriers is provided by Pin antisites, preferably introduced during off-stoichiometric growth of InP at low temperatures (LT-InP) by gas source molecular beam epitaxy. A twodimensional electron gas (2DEG) is shown to be formed near the InGaAs/InP heterointerface as a result of electron transfer from the LT-InP to the InGaAs active layer, from studies of Shubnikov-de Haas oscillations and photoluminescence. The concentration of the 2DEG is determined to be as high as 1.15×1012 cm−2, where two subbands of the 2DEG are readily occupied.