Silicon-doped InSb films of 100 nm thickness were grown by molecular-beam epitaxy on undoped low-temperature InSb buffer layers on GaAs(100) substrates. For a set of samples grown with the same Si flux, the free carrier concentration also increased with buffer thickness, and an analysis which included an estimate of the donor-doping properties of the dislocations suggests that a fraction of the silicon may also be trapped at the dislocations, where it acts as an acceptor. Hall effect measurements, corrected for the effects of parallel conduction in the buffer layer, and surface depletion in the doped region, showed that the mobility of the Si-doped region improved as the buffer layer thickness increased, and reached corrected values as high as 15 000 cm 2 V −1 s −1 , approaching that of lightly compensated material. These mobility dependences are quantitatively consistent with the incorporation model.