Semi-insulating GaN samples, grown by ammonia-based molecular beam epitaxy and doped with carbon, were investigated by thermally stimulated current spectroscopy and photoluminescence at 4.2 K. In addition to a dominant trap at 0.90 eV, thought to be related to the N interstitial, a trap at 0.50 eV, presumably related to C-Ga, was observed in the samples with high carbon concentrations. For all of the carbon-doped samples, strong photoluminescence (PL) bands were observed in the yellow (YL), blue (BL), and near-band-edge regions, with the YL dominating, and the BL decreasing as the carbon concentration increased. Besides the PL and trap properties, the carbon doping also influenced the resistivity and effective carrier lifetime. (c) 2005 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.