In this paper, promising experimental results for the p-type electrical properties of carbon-doped (C-doped) AlGaN are discussed. P-type conductivity was experimentally achieved in C-doped (0001) plane AlGaN layers with from a small amount to 55% solid Al composition, but not in (0001) plane GaN. The maximum free hole density (determined by van der Pauw geometry-Hall effect measurement) achieved for an AlGaN layer with 10% solid Al composition was p ¼ 3.2 Â 10 18 cm
À3. The maximum net ionized acceptor densities (NIAD),, which were determined by capacitance-voltage measurement, for AlGaN with 6, 10, 27, and 55% solid Al compositions, were all in the range of (6-7) Â 10 18 cm
À3. Moreover, the electrical activity of the carbon acceptors was estimated to be 55-71% from the NIAD and secondary-ion microprobe mass spectrometry analysis data on the carbon concentration. A p-n junction was also fabricated using the C-doped p-type AlGaN.