Certain aspects of the present disclosure generally relate to a semiconductor device implemented with multiple channels in a gate-all-around (GAA) high-electron-mobility transistor (HEMT) and techniques for fabricating such a device. One example semiconductor device generally includes a substrate; a first gate layer disposed above the substrate; a first barrier layer disposed above the first gate layer; a first channel region disposed above the first barrier layer; a second barrier layer disposed above the first channel region; a second gate layer disposed above the second barrier layer; a third barrier layer disposed above the second gate layer; a second channel region disposed above the third barrier layer; a fourth barrier layer disposed above the second channel region; a source region; and a drain region.