The unity sticking coefficient of Si and the good electrical and optical properties of Si‐doped films make Si a near ideal dopant for GaAs grown by molecular beam epitaxy (MBE). However, the incorporation mechanism of Si at high doping levels differs from that at moderate levels. We found that the maximum doping concentration that can be obtained with Si is 6×1018cm−3. Above this doping concentration Si precipitates, causing a decrease in the free‐carrier concentration and the mobility. A detailed discussion of the incorporation mechanism at high doping, and the effects of the substrate temperature and As4/Ga ratio on Si incorporation at moderate doping is presented.