A series of GaAs/InGaAs quantum wells with a silicon δ-doped layer in the top barrier was investigated by Shubnikov–de Haas measurements as a function of the illumination time of the samples. During the illumination process strong modifications of the electronic density and the quantum mobility of each occupied subband were observed. Based on self-consistent calculations, the dominant mechanism which caused the changes in the subband quantum mobilities with illumination was elucidated.