We have clarified the effect ofH2andNH3on the passivation of Mg acceptor inp-type GaN films grown by metalorganic chemical vapor deposition. It has been found that the small amount ofH2carrier gas strongly influences the electrical property of the Mg-doped GaN films. Low-resistivityp-type GaN has been obtained byH2-free growth without any post-treatments. Its acceptor concentration is as high as that obtained by conventionalH2-rich growth with subsequent thermal annealing. It has also been clarified that hydrogen produced byNH3dissociation does not prevent Mg from electrically activating inH2-free growth. ©1998 American Institute of Physics.