We propose an ultra-thin (similar to3 nm) InGaN-channel high electron mobility transistor (HEMT) structure with two-dimensional electron gas (2DEG) mobility of over 1000cm(2)/V s at room temperature (RT). This structure suppresses alloy-disorder scattering in the channel because part of the 2DEG penetrates into the GaN buffer layer. All the AlGaN/ InGaN/GaN heterostructures were grown on a (000 1) sapphire substrate using two-flow atmospheric metalorganic chemical vapor deposition (MOCVD). As a result, we achieved 2DEG mobility of 1110 cm(2)/V s with a sheet carrier density of 1.31 X 10(13) cm(-2) at RT, which is the highest mobility reported to date for an InGaN-channel structure. Excellent electron confinement was also observed compared to a conventional AlGaN/GaN heterostructure, despite the use of a very thin InGaN-channel. We also investigated the relationship between the density of the 2DEG and strain relaxation in InGaN-channel structures. Strain relaxation in the InGaN layer was enhanced by an increase in thickness with the result that the negative polarization charge drastically decreased at the InGaN/GaN interface. Thus, the potential at the interface dropped and the 2DEG density increased. (C) 2004 Elsevier B.V. All rights reserved.