Heavy Si doping was studied for low-pressure metalorganic chemical vapor deposition of GaAs by using tertiarybutylarsine (tBAs) as a group-V source and silane (SiH4) as a doping source gas. The Si doping efficiency was higher by one order of magnitude when tBAs was used instead of arsine (AsH 3). The maximum electron concentration was 9.0×1018 cm-3, which is slightly higher than that obtained for AsH3 (5.9×1018 cm-3). The slight increase of the maximum concentration is considered to be due to a reduction of the carrier compensating center generated in high SiH4 partial pressure conditions. Generation of the electrical compensating center is assigned to be related with the carbon incorporation from alkylsilanes during growth.