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 (PAsH3) leads to the empirical relationship: n = aPAsH3 + $bP_{AsH_3}^{-\frac{4}{3}$, which describes the relationship between silicon doping concentration and arsine partial pressure.