Magnetotransport properties of Si &dgr;-doped pseudomorphicIn0.2Ga0.8As/GaAsheterostructures grown by metalorganic vapor phase epitaxy have been investigated in magnetic fields up to 12 T in the dark at 1.7 K. Different &dgr;-doping configurations, in which the same Si &dgr;-doped layer was placed at different positions with respect to theIn0.2Ga0.8Aswell, have been studied to clarify their effect on subband electron densities in the well. Very high electron densities of>4×1012 cm−2were obtained when placing a Si &dgr;-doped layer at the well center or the well–barrier interface. We found that one subband was occupied in the well-center-doped structure, but when the Si &dgr;-doped layer was at the well–barrier interface, the second subband in the well became occupied. The electron density of Si &dgr;-modulation-dopedIn0.2Ga0.8As/GaAsheterostructures, in which the cap barrier or the buffer barrier was Si &dgr; doped, was in the order of<1.2×1012 cm−2.The Si &dgr; doping in both of the barriers led to an increase of the electron density by almost a factor of 2. Owing to an incomplete transfer of the electrons from the Si &dgr;-doped layers to the well, parallel conduction was observed in the Si &dgr;-modulation-doped structures. ©1997 American Institute of Physics.