Annealed GaAs layers grown at low temperatures (180–300 °C) by molecular‐beam epitaxy (MBE) were studied by transmission electron microscopy (TEM). These layers were used as buffers for a field‐effect transistor (FET) device structure and effectively eliminated sidegating effects. All these layers were found to contain As precipitates. Precipitate size and separation between them differed from sample to sample. The smallest precipitates were coherent ‘‘pseudocubic.’’ Larger precipitates had hexagonal structure. The distance between precipitates was estimated to be in the range of 10–40 nm. These results are consistent with the buried Schottky model by Warrenetal., although other explanations cannot be excluded at present.