Abstract Multiple energy ion implantation facilities have been used to prepare essentially box-shaped dopant profiles in GaAs with plateau concentrations between 2 x 10 16 and 8 x 10 17 cm -3. The implanted species was 28Si +. The ion energies ranged between 40 and 400 keV. The depletion depth correction applied to the carrier and mobility profiles in the plateaux did not affect the data values but only their depth scale. Thus, in these regions the measured data (without any feed-back correction) were used to study the electrical properties of the implanted layers. Doping efficiencies between 65% and 81% and carrier mobilities in the range 3000 to 6000 cm 2 V -1s -1 were measured. Activation thresholds in the range 10 15 to 10 16 cm -3 were estimated.