Single-crystal GaN thin films (500 nm) were grown by low-pressure metal-organic vapour-phase epitaxy (LP-MOVPE) on Si(111) substrates using AIN buffer layers. Depending on the AlN growth temperature, the growth mode of GaN can be either two- or three-dimensional (2D or 3D) and structural, electronic and optical properties of GaN layers are consequently changed. 2D growth leads to the best material with a full-width at half-maximum (FWHM) of the (0002) X-ray diffraction (XRD) line in rocking curve for GaN of about 656 arcsec, a dislocation density in the low 10(10)cm(-2) range and a surface root-mean-square (RMS) roughness as low as 0.3 nm. AlGaN/GaN two-dimensional electron gas (2DEG) were grown on such layers and mobility of mu = 813 cm(2)/V s at 300 K and mu = 2200 cm(2)/V s at 77 K were obtained.