A composite-channel high-electron mobility transistor (HEMT) on GaAs substrate is designed and fabricated, using the following methodology to improve device performance: (1) an AlGaAs buffer layer, (2) an AlGaAs/GaAs superlattice layer, and (3) an In0.1Ga0.9As/In0.22Ga0.78As/In0.1Ga0.9As composite channel. For comparison, a control HEMT without the composite channel is fabricated in parallel (whose channel comprises only a 125-angstrom-thick In0.22Ga0.78As layer). These two devices are grown by metal-organic chemical vapor deposition. The peak extrinsic transconductance (g(m,ext)) of the control HEMT with a gate length of 1 mu m,m is 160 mS/mm, while the peak g,,ext of the composite-channel HEMT of the identical gate length is measured to be 186 mS/mm. The on-state breakdown voltage of the composite-channel HEMT is as great as 9.7 V, which represents an improvement of 1.4 V over the control HEMT. The control HEMT exhibits a current gain cutoff frequency (f(T)) of 12.5 GHz and a maximum frequency of oscillation (f(max)) of 31.5 GHz, while the composite-channel HEMT exhibits an f(T) of 16.9 GHz and an f(max) of 37.4 GHz. Experimental data reveal that the composite-channel structure provides improved gate-to-source voltage swing, improved saturation current density, enhanced f(T), enhanced f(max), and enhanced breakdown voltage without compromising electron mobility. The composite-channel HEMT is thus far superior to the control HEMT and is highly promising for use in high-frequency applications. (C) 2009 American Vacuum Society. [DOI: 10.1116/1.3093883]