In order to characterize interface roughness scattering, two-dimensional electron gas (2DEG) mobility in (4 1 1)A and (1 0 0) selectively-doped In0.53Ga0.47As/In0.52Al0.48As quantum well (QW) HEMT structures, which were grown on InP substrates by molecular beam epitaxy, with a gate contact was measured at 20 K as a function of sheet electron concentration (N-s) by changing gate bias. Thickness of the QW was designed to be rather small (6 nm) for enhancing the interface roughness scattering for both samples. 2DEG mobilities of the (4 1 1)A sample were 21,000-51,400 cm(2)/V S in the range of N-s = 0.7-1.7 x 10(12) cm(-2), which are about more than 2 times higher than the mobilities (8,700-25,400 cm(2)/V s) of the conventional (1 0 0) sample. 2DEG mobility was calculated by taking into account interface roughness scattering and ionized remote impurity scattering. By fitting the calculated results to the observed ones, lateral size (Lambda) and height (Delta) of the interface roughness of the (4 1 1)A sample were determined to be 3.5 and 0.23 nm, respectively, which are 30% and 50% smaller than the corresponding values (Lambda = 5.0 nm and Delta = 0.43 nm) of the (1 0 0) sample. (C) 2002 Elsevier Science B.V. All rights reserved.