Carrier‐concentration profiles of Si in semi‐insulating GaAs have been obtained byC‐Vmeasurement techniques. Si+or Si++ions were implanted at energies ranging from 50 to 600 keV, and annealing was carried out with Si3N4encapsulants. Range parameters such as the projected rangeXpand the projected standard deviation &Dgr;Xpwere experimentally determined by use of depths at the peak carrier concentration and at the 1/&sqrt;evalue of the peak carrier concentration of the profiles. It was found that &Dgr;Xpwas strongly dependent on the Cr contents of substrates whileXpwas not. The measured carrier concentrations could be approximated by the Gaussian distribution, and values ofXpwere in good agreement with the theoretical value of the projected rangeRp. However, &Dgr;Xptended to saturate as the incident energies increased and deviated from the theoretical value of the projected standard deviation &Dgr;Rp.