We report the result of first-principles calculations on Se &dgr;-doped GaAs and propose a carrier compensation mechanism in the region of high Se concentrations. It is found that introducing Ga vacancies near Se atoms makes the system extremely stable and semiconducting. In the high-concentration region where Se atoms are close to each other, the Se atoms are likely to get together by introducing Ga vacancies and form clusters consisting of three Se atoms and one Ga vacancy. The extra electrons of the Se atoms transfer to the dangling bonds of the Se or As atoms, and become electrically inactive. ©1997 American Institute of Physics.