A HEMT comprises a composite channel, made up of a plurality of channel/barrier layer heterojunctions. That is, two or more channel/barrier layer pairs are deposited on a substrate, under a gate contact. A separate 2DEG is formed in each channel layer at the heterojunction with the barrier layer. The HEMT channel is effectively divided among a plurality of parallel 2DEGs. A high total charge density—required for high power operation—is divided among the plurality of 2DEGs. Since each 2DEG does not have a large charge density, it can sustain the high saturated electron velocity required for very high frequency operation. The composite-channel HEMT thus operates with high gain, at high power levels, and at high frequencies.