InP-based high electron mobility transistors (HEMTs) are grown by solid source molecular beam epitaxy with a valved phosphorus cracker. An InP/InGaAs composite channel improves both the channel mobility and the breakdown voltage for W-band power applications. The optimization of the InP/InGaAs interface quality is found critical to the device performance. A 1.5 V increase in breakdown voltage is observed in a 0.15 mu m T-gate device. A two-stage power amplifier circuit delivered 316 mW output power with 17 dB linear gain at 94 GHz. A pseudomorphic HEMT structure with 75% InGaAs channel is used for extremely high frequency (up to 200 GHz) applications. InP etch-stop layers are used to improve the threshold voltage uniformity, which is closely related to :the gate-recess depth uniformity. By inserting this InP etch-stop layer in the InAlAs barrier layer, the threshold voltage (Vm) standard deviation across a 3-in. wafer is reduced from 50 to 27 mV. An amplifier with 10 dB gain at 190 GHz is fabricated.