Electrical and optical properties of epitaxial layers grown from trimethylgallium and arsine were studied by Hall and photoluminescence measurements. The conduction type of the layers grown on substrates was changed from p‐ to n‐type with increase of , the mole ratio of arsine to trimethylgallium introduced, without intentional doping. An emission (P2) at 1.488 eV due to shallow acceptors was observed on most of the samples.