Systematic studies have been made for the first time on the basic properties of AlxGa1-xAs (x=0.2–0.7) grown by molecular beam epitaxy in the wide growth temperature range of 540–780 °C with As4 and As2. The forbidden growth temperature region (FTR), where the specular smooth surface cannot be obtained, has been found to depend strongly upon both the As species and the AlAs mole fraction x. FTR does not change with x in the case of As4; however, in the case of As2, FTR does not exist for x=0.2 and it increases with x from 0.3–0.7. Photoluminescence of n‐Al0.3Ga0.7As (Si=1×1018 cm-3) grown with As2 shows lower intensity and higher sensitivity to growth temperature than those of samples grown with As4. Deep level transient spectroscopy has been measured on n‐Al0.7Ga0.3As (Si=2×1016 cm-3). New electron traps are found in layers grown with As2.