AlxGa1-xN/GaN (0.20less than or equal toxless than or equal to0.52) heterostructures (HSs) were grown on a sapphire substrate by atmospheric pressure metalorganic chemical vapor deposition with good uniformity and two-dimensional-electron-gas (2DEG) mobilities of 936, 1163, 1310, 1274, and 911 cm(2)/Vs for different-Al-contents of 20%, 27%, 34%, 42%, and 52%, respectively. 2DEG mobility increase up to the Al content of 34% and then it slowly decreases for high Al-content AlGaN/GaN HSs. An increase of sheet carrier density with the increase of Al content has been observed. A small hump photoluminescence peak of e(2DEG)(1)-h has been observed in both 34% and 42% Al-content AlGaN/ GaN heterostructures. High Al-content (52%) heterostructure has exhibited a distinguished e(2DEG)(1)-h peak. The increase of surface roughness and granular size of AlGaN/GaN heterostructures, with the increase of Al content is due to the increase of lattice mismatch between GaN and AlGaN layers. High-electron-mobility transistors (HEMTs) have been fabricated and characterized using AlxGa1-xN/GaN heterostructures with different-gate lengths (2.0-5.0 mum). An increase of extrinsic transconductance (g(m)) and drain current density has been observed up to the Al content of 34% and it slowly decreases for higher Al-content AlxGa1-xN/GaN HEMTs. The maximum g(m) of 202 mS/mm with maximum drain-source current density of 525 mA/mm has been observed for 2.0-mum-gate-length Al0.34Ga0.66N/GaN HEMT structure. About 3-4 orders of magnitude, low, gate-leakage current has been observed on 42% and 52% Al-content AlGaN/GaN HEMTs when compared with the low Al-content (20%) AlGaN/GaN HEMTs. (C) 2003 American Vacuum Society. [DOI: 10.1116/1.1556398].