We present here the results of a study on the effect of substrate temperatureTson the electrical and physical characteristics of low temperature molecular‐beam epitaxy GaAs layers. Based on the x‐ray results, three temperature ranges have been defined forTs: 1) high range (Ts≥ 460 °C), 2) intermediate range (260 ≤Ts≤ 450 °C), and 3) low range (the amorphous range) (Ts≤ 250 °C). Hall measurements have been performed on the as‐grown samples and on samples annealed at 610 and 850 °C. Si implantation into these layers has also been investigated. From an electrical stand point, the most striking difference between the low range and the intermediate range is the fact that after annealing at 850 °C, the undoped layers grown below or at 250 °C have a low resistivity (net electron concentrations as high as 1.5×1018cm−3and mobilities as high as 920 cm2 V−1 s−1), while after anneal the undoped layers grown in the intermediate range have extremely high resistivity (about 8×105Ω cm).