The subband electronic structure of Si&dgr;-doped GaAs grown by metal organic vapor phase epitaxy was investigated using magnetotransport measurements. The work focused on the effect of illumination. We found that illumination leads to a slight increase of the quasi-two dimensional electron gas density in the well. This increase does not depend on the illumination intensity. The illumination-generated electron density weakly increases with an increase in the Si&dgr;-doping concentration. We also experimentally confirmed that illumination only slightly alters the electron densities of the occupied subbands and the illumination-generated electrons populate one previously empty subband. Only a fraction of the illumination-generated electrons persist but the newly occupied subband under illumination remains populated in the dark after removal of the illumination. The experimental results suggest that the DX centers are unlikely to be populated in our Si&dgr;-doped GaAs regardless of the doping concentration. The weak and partially persistent photoconductivity effect observed in Si&dgr;-doped GaAs may arise from ionization of other Si localized states.©1997 American Institute of Physics.