The microscopic electrical activation efficiency of implanted Si atoms in undoped, liquid-encapsulated Czochralski-grown GaAs substrates was investigated by means of the threshold voltage (Vth) of implanted metal-semiconductor field-effect transistors as a function of stoichiometry and impurities, especially of carbon as determined by Fourier transform infrared (FT-IR) spectroscopy (≤2.5×1016 cm -3). The Vth scattering, which is related to the electrical activation efficiency dispersion, was small for an As-rich substrate grown from an As-rich melt, whether or not there existed a horizontal 3400-Oe magnetic field. The smallest amplitude of the Vth scattering occurred for the As-rich melt and did not vary with magnetic field. The V th scattering with magnetic field was independent of carbon concentration (≤2.5×1016 cm- 3). However, without magnetic field, an effect clearly appeared at low carbon concentration (nearly nondetectable). It seems that the V th scattering or the electrical activation efficiency dispersion is dominated by the dispersion of stoichiometry, that is, composition, of the GaAs substrate because the electrical activation efficiency depends on the ratio of the Ga vacancy concentration and As vacancy concentration and the magnitude of their concentrations at the annealing temperature for the Si-implanted GaAs substrate. Therefore small Vth scattering means that the dispersion of the composition (Ga and As) is small. The composition dispersion for the most As-rich melt is small because the solidification is controlled by the straight As-rich side solidus line of the Ga:As composition-temperature phase diagram, and thus temperature variations of the GaAs melt in the pyrolytic boron nitride crucible are not as important. Also, there was no correlation between V th scattering and dislocation density.