Clear exciton absorption peak has been observed at room temperature by photovoltaic spectra, on a GaN layer grown on (0001)-plane sapphire by metalorganic chemical vapor deposition. From the spectra, we obtained the wurtzite GaN room temperatureAandBexciton transition energies, and the energy gap to be 3.401, 3.408, and 3.426 eV, respectively. We have also performed photovoltaic measurements with varied light incidence angles, and observed the polarization behavior of exciton absorption in GaN. In conjunction with previous room temperature photoreflectance measurements, this work provided direct and reliable assessment of GaN semiconductor crystal layers. ©1997 American Institute of Physics.