Abstract High-quality GaN epilayers were consistently obtained using a home-made gas-source MBE system on sapphire substrates. Room-temperature electron mobility of the grown GaN film is 300 cm 2/V s with a background electron concentration as low as 2×10 17 cm −3. The full-width at half-maximum of the GaN (0 0 0 2) double-crystal X-ray rocking curve is 6 arcmin. At low temperature (3.5 K), the FWHM of the near-band-edge photoluminescence emission line is 10 meV. Furthermore, using piezoelectric effect alone with the high-quality films, two-dimensional electron gas was formed in a GaN/AlN/GaN/sapphire structure. Its room-temperature and low-temperature (77 K) electron mobility is 680 cm 2/V s and 1700 cm 2/V s, and the corresponding sheet electron density is 3.2×10 13 and 2.6×10 13 cm −2, respectively.