High-mobility GaN thin films were grown by RIF plasma-assisted molecular beam epitaxy on (0 0 0 1) sapphire. A conventional low-temperature buffer layer and an intermediate-temperature buffer layer (ITBL) were first deposited before the growth of the epitaxial layer. Electron mobility is found to vary strongly with the ITBL thickness with value as high as 460 cm(2)V(-1)S(-1) obtained from the sample grown on a 800 nm ITBL on top of a low-temperature buffer layer. A systematic shift in the photoluminescence peak position, following the same trend as the mobility, suggests the relaxation of residual strain in the top GaN epitaxial layer by utilizing an ITBL. Detailed characterizations of G-R noise indicate reduction of deep levels by over an order of magnitude for the sample with 800 nm ITBL compared to the control sample which has the same total thickness but with only the low-temperature buffer layer.