The characteristics of thermally oxidized p-GaN metal-oxide-semiconductor (MOS) capacitors fabricated on sapphire substrates have been investigated electrically. 100-nm-thick beta-Ga2O3 was grown by thermal dry oxidation of Mg-doped GaN at 880 degreesC for 5 h. Capacitance-voltage measurements at room temperature show a low interface trap density of similar to 1 X 10(10) eV(-1) cm(-2) and display peculiar behavior that is different from a deep depletion feature. These results indicate some possibilities of surface inversion in thermally grown beta-Ga2O3 /p-GaN MOS structures without any n(+) source regions. (C) 2003 American Vacuum Society.