The effects of neutron transmutation doping were studied for undoped (residual donor concentrations < 10(15) cm(-3)) GaN films grown by metalorganic chemical vapor deposition. After irradiation with reactor neutrons (equal fluences of 1.5x10(17) n/cm(2) of thermal and fast neutrons) the sample became semi-insulating, with the Fermi level pinned near E-c-0.8 eV. Isochronal annealing from 100 to 1000 degrees C showed three stages-slight recovery of conductivity at 200-300 degrees C, reverse annealing at 300-500 degrees C, and a broad recovery stage from 600 to 1000 degrees C. After annealing at 1000 degrees C, the donor concentration in the sample was close to the expected concentration of Ge donors transformed from Ga atoms upon interaction with thermal neutrons (2x10(16) cm(-3)). Admittance spectroscopy showed that the donors had ionization energies similar to E-a=0.2 eV, much deeper than substitutional Ge donors. For intermediate annealing temperatures of 800 degrees C the donors were deeper (E-a=0.47 eV), but the proximity of concentrations of all these different centers suggests that they are due to transformation of complexes of Ge donors with radiation defects. (C) 2010 American Vacuum Society. [DOI: 10.1116/1.3431083]