The incorporation of silicon from SiH₄ in GaAs has been studied on a wide range of growth conditions by atmospheric pressure metalorganic vapor phase epitaxy (AP-MOVPE). The highest free carrier concentration exceeds 1 × 10¹⁹ cm⁻³. Compensation ratios appear to be strongly dependent on the SiH₄ partial pressure. The relatively high compensation ratios essentially originate from the amphoteric behaviour of silicon. The temperature dependence of silicon doping has been investigated in the range 650 °C to 765 °C with various silane partial pressures. The activation energy of Si incorporation varies from 0 to 2.2 eV. The dependence of the free electron concentration on the arsine partial pressure (P _AsH₃) leads to the empirical relationship: n = aP _AsH₃ + $bP_{AsH_3}^{-\frac{4}{3}$, which describes the relationship between silicon doping concentration and arsine partial pressure.