Van der Pauw-Hall effect measurements were carried out to determine the carrier density and the mobility of Si-doped Al0.25Ga0.75As epilayers. The epilayers were grown on semi-insulating GaAs (100) substrates using molecular beam epitaxy (MBE). Photoluminescence (PL) measurements were performed to investigate the optical behaviors in the layers. It was found that as the annealing temperature increased to 700 °C, the mobilities of the layers decreased and their carrier concentration increased. Both the mobility and the carrier concentration decreased while annealing at 950 °C, and the PL peak related to the carrier compensation was observed.