Excellent n-type GaN layers have been grown by all of the major epitaxial techniques: MBE, MOCVD, and HVPE. In this work, we analyze the band conduction in such samples by temperature-dependent Hall-effect measurement and theory, and determine quantitative information on donor and acceptor concentrations, as well as donor activation energies. In HVPE layers it is necessary to take account of a degenerate n-type layer at the GaN/sapphire interface: in order to correctly analyze the bulk material. We also investigate hopping conduction, which occurs at low temperatures in conductive material, and at both low and hi-h temperatures in semi-insulating material. Finally, we show by analysis of electron-irradiation data that both the N vacancy and the N interstitial are electrically active, demonstrating donor and acceptor character, respectively. (C) 1997 Elsevier Science S.A.