We have measured the Shubnikov–de Haas (SdH) effect in &dgr;-doped AlAs0.56Sb0.44/In0.53Ga0.47As heterostructures and observed a population of the second subband. Using the persistent photoconductivity effect we increased the electron density from 26.73 to 28.20×1011cm−2in the first subband and 6.61 to 7.20×1011cm−2in the second. The onset of the second subband population occurs when the first subband is filled to a density of 11.56×1011cm−2. From the nonparabolic band approximation we calculated the effective masses in both subbands before illumination. The effective mass for the second subband was evaluated using the temperature dependence of the SdH amplitude. Its value agrees well with the values obtained from thek⋅papproximation and infrared cyclotron resonance measurements. ©1997 American Institute of Physics.