Photoluminescence spectra of heavily Si-doped GaAs grown by mole cular beam epitaxy (MBE) were investigated at 20 K as a function of electron concentration. We found that the two peaks in the electron populations of the conduction band and the donor band, respectively, merge as the doping concentration increases, and can not be distinguished at the electron concentration of the order of 10(18) cm(-3). Copyright (C) 1996 Elsevier Science Ltd