Electrical measurements have been made on films of lightly Si‐doped GaAs and on (AlGa)As/GaAs two‐dimensional electron gas (2DEG) structures grown by MBE. These have shown residual donor and acceptor concentrations of ∼2×1014cm−3in the GaAs, and mobilities for 2DEG layers which varied systematically with carrier densitynsand undoped spacer layer thicknessd; the highest mobility obtained was 2.2×106cm2 V−1 s−1for a sample withns=2×1011cm−2andd=800 Å. Analysis of the data shows that scattering by remote ionized impurities is the dominant mechanism in most of our 2DEG samples. In such layers, the mobility μ at 4 K varied with photoexcited carrier densitynsapproximately asns1.5. This behavior is expected for photoexcitation of carriers from DX centers in the doped (AlGa)As, and results from increased screening in the 2DEG and increased scattering due to the greater number of charged impurities in the (AlGa)As. However, some samples showed a much steeper variation of μ withns, and also different temperature dependences of μ for high and lownsvalues. Comparison with previously reported data on Si–SiO22DEG systems suggests that this effect is due to strong localization effects occurring at the (AlGa)As–GaAs interface.