Low-frequency noise measurements have been performed in the linear range of the I-V characteristics of pseudomorphic Al0.3Ga0.7As/In0.25Ga0.75As/GaAs high electron mobility transistors (HEMTs) grown by molecular beam epitaxy with different channel thicknesses. The results obtained show that the 1/f noise in such devices depends greatly on channel thickness. It is controlled by the penetration of the electron wavefunction into the barrier as well as by Coulombic effects for thin channels and by the increase in dislocation concentration for thick ones. Generation-recombination (G-R) noise is also present. It is mainly due to real-space transfer of electrons between the two-dimensional electron gas in the channel and the conduction band minimum in the AlxGa1-xAs barrier. Similar results were obtained for the G-R noise of Al0.22Ga0.78As/In0.20Ga0.8As/GaAs, Al0.3Ga0.7As/GaAs, and Al0.48In0.52As/In0.47Ga0.53As/lnP HEMTs. (C) 1995 American Institute of Physics.