Al(0.26)Gao(0.74)N/AlN/GaN high-electron-mobility transistor (HEMT) structures with AIN interfacial layers of various thicknesses were grown on 100-mm-diameter sapphire substrates by metalorganic vapor phase epitaxy, and their structural and electrical properties were characterized. A sample with an optimum AlN layer thickness of 1.0 nm showed a highly enhanced Hall mobility (mu(Hall)) of 1770 cm(2)/Vs with a low sheet resistance (rho(s)) of 365 Omega/sq. (2DEG density n(s) = 1.0 x 10(13)/cm(2)) at room temperature compared with those of a sample without the AlN interfacial layer (mu(Hall) = 1287 cm(2)/Vs, rho(s) = 539 Omega/sq., and n(s) = 0.9 x 10(13)/cm(2)). Electron transport properties in AlGaN/AlN/GaN structures were theoretically studied, and the calculated results indicated that the insertion of an AlN layer into the AlGaN/GaN heterointerface can significantly enhance the 2DEG mobility due to the reduction of alloy disorder scattering. HEMTs were successfully fabricated and characterized. It was confirmed that AlGaN/AlN/GaN HEMTs with the optimum AlN layer thickness show superior DC properties compared with conventional AlGaN/GaN HEMTs. (c) 2006 Elsevier Ltd. All rights reserved.