AlGaN/GaN heterostructures were grown on (0 0 0 1) sapphire substrates by metalorganic vapor phase epitaxy. The dependences of electrical properties on the SiH4 flow rate, and the At composition and thickness of the AlxGa1-xN layer were studied. For Al compositions less than or equal to 15%, sheet electron densities were proportional to the SiH4 flow rate. At ail At compositions of 15%, a mobility of 1220 cm(2)/Vs (sheet electron densities 5.2 x 10(12) cm(-2)) was obtained, indicating the formation of high-quality AlGaN/GaN hetero-interfaces. Sheet electron densities continue to increase in proportion to the At composition. Even when the AlGaN was undoped, sheet electron density as high as 2.6 x 10(13) cm(-2) was obtained at an At composition of 45%. This is due to the effect of piezoelectric and spontaneous polarizations. When the AlGaN layer thickness exceeds the critical thickness, the sheet electron density decreased, because the piezoelectric polarization was also reduced by the relaxation. The dependence of sheet carrier density on the At composition can be explained by the polarization at the heterointerface and the effect of relaxation. The critical thickness estimated by Xray diffraction was higher than the theoretical calculation, implying that the elastic properties might be affected by the presence of high-density dislocations. (C) 2002 Elsevier Science B.V. All rights reserved.