Temperature dependent conductivity and Hall‐effect measurements were carried out on molecular‐beam epitaxial GaAs layers grown at 420 °C and separated from the substrate. The layers exhibit semi‐insulating properties with a room temperature resistivity of (6–7)×106 Ω cm. The electron Hall mobility is proportional to temperature as ∼T−1.1 and the room temperature value is 5900 cm2 V−1 s−1. From the (nHT−3/2) vs T−1 plot, a donor activation energy of 0.68 eV, different than in bulk semi‐insulating GaAs, has been evaluated. A similar activation energy is observed in as‐grown and annealed GaAs layers grown at 200–350 °C. This indicates that the high resistivity of GaAs grown at low temperatures might be explained by deep donor defects rather than As precipitates.