Hall-effect measurements were conducted on unintentionally dopedn-type GaN films grown on sapphire substrates by hydride vapor phase epitaxy. Film nucleation involved either a GaCl or ZnO pretreatment. Variable temperature Hall-effect measurements reveal a dependence of both the electron concentration and the Hall mobility on film thickness. We demonstrate that this dependence is indicative of a nonuniform distribution of electrically active defects. For GaCl-pretreated sapphire the presence of a highly conductive, 200-nm-thick near-interface layer is likely to account for the observed phenomena. For ZnO-pretreated sapphire the Hall-effect data clearly indicate a continuous reduction of the defect density with increasing film thickness. ©1998 American Institute of Physics.