We report state-of-the-art V-band power performance of 0.15-m gate length InGaAs/InAlAs/InP HEMT's which have 15 m 2 25 m dry-etched through-substrate source vias (substrate thickness 50 m). The 500-m wide InP HEMT's were measured in fixture at 60 GHz and demonstrated an output power of 190 mW with 40% power-added efficiency (PAE) and 6.8 dB power gain at an input power of 16 dBm. These results represent the best combination of power and PAE reported to date at this frequency for any solid state device. The results are achieved through optimization of the InP-based heterostructure which incorporates a graded pseudomorphic InGaAs channel and a graded pseudomorphic InAlAs Schottky barrier layer, and the use of 15 m 2 25 m dry-etched through-substrate source vias.