We have investigated the band gap shift of Si-doped AlxGa1−xAs alloys as a function of both silicon concentration and Al composition. Correlation and impurity scattering effects were included in the theoretical model. The optical absorption of Al0.3Ga0.7As:Si at the fundamental band gap energy has been measured by a photoacoustic spectroscopy technique. We found the energy gap at about 1.8 eV. The samples were grown by molecular beam epitaxy and the Al fraction was measured by reflection high energy electro-diffraction technique.