We have investigated the Si dopant site in the GaN lattice using ion channeling in combination with Rutherford backscattering spectrometry (RBS), particle induced x-ray emission (PIXE), and nuclear reaction analysis (NRA). Metalorganic chemical vapor deposition grown GaN on a c-plane sapphire substrate implanted with Si-28 at dose of 7 x 10(14)cm(-2) with postimplant annealing was investigated. Channeling measurements were performed by taking angular scans around the (0001) and (<10(1)1over bar>) axes and recording RES, PIXE, and-NRA yields for Ga, Si, and N, respectively. The channeling results indicate that almost 100% of Si goes into the Ga site when the samples are annealed at 1100 degrees C for 30 min while for annealing at 1050 degrees C and below, Si is distributed almost randomly. This suggests that a drastic change of Si substitutionality takes place in a narrow temperature region near 1100 degrees C. Our results directly indicate that the electrical activation of Si implanted GaN with postimplant annealing is due to the formation of substitutional Si at this temperature.