We report on the achievement of a two-dimensional electron gas in completely undoped In0.75Al0.25As/In0.75Ga0.25As metamorphic quantum wells. Using these structures we were able to reduce the carrier density, with respect to reported values in similar modulation-doped structures, to about 2 - 3 x 10(11) cm(-2) with mobilities of up to 2.15 x 10(5) cm(2) (V s)(-1). We found experimentally that the electronic charge in the quantum well is likely due to a deep-level donor state in the In0.75Al0.25As barrier band gap, whose energy lies within the In0.75Ga0.25As/In0.75Al0.25As conduction band discontinuity. This result is further confirmed through a Poisson-Schrodinger simulation of the two-dimensional electron gas structure. (C) 2004 American Vacuum Society.