Monolithically grown planar nanowire (NW) high-electron-mobility transistors (NW-HEMTs) are demonstrated using self-aligned 〈110〉 GaAs NWs capped with Si-doped AlxGa1-xAs shell as the channel on semi-insulating (100) GaAs substrates. The planar Al0.35Ga0.65As-GaAs NW-HEMT with ~ 1-μm-long gate exhibits excellent dc characteristics, with extrinsic Gm of ~ 80 mS/mm and estimated intrinsic Gm of ~ 260 mS/mm, where the device width is defined as the entire periphery of the NWs. The ION/IOFF ratio is ~104 and the threshold is -1.5 V operating in depletion mode. The output current increases linearly with the number of NWs in the channel, while the threshold voltage does not change at all. This indicates excellent uniformity and scalability of the bottom-up-grown NW devices. Compared to field-effect transistors with doped NWs as channels, the structure reported here circumferences the inherent doping nonuniformity issues in NWs grown by the vapor-liquid-solid mechanism, and self-aligned lateral epitaxy nature of our NW structure makes scaling up to NW array-based transistors from the bottom up feasible.