Si-doped GaAs epitaxial layers grown by metal-organic chemical vapour deposition exhibit substantial carrier density loss after rapid thermal annealing (RTA) at temperatures higher than 850-degrees-C. Hall-effect, capacitance-voltage, deep-level transient spectroscopy, and secondary ion mass spectroscopy measurements were performed on samples before and after RTA. We show that the reduction of free-carrier concentration in the entire thickness of the epitaxial layer is accompanied by the deterioration of the mobility and the enhancement of donor-like deep-level concentration at 0.305 eV below the conduction band, which is in good agreement with the model of silicon donor neutralization by formation of neutral silicon-hydrogen complexes.