High-breakdown and high-linearity Ga0.5lIn0.49P Ilno. 15Ga0.8sAs pseudomorphic high electron mobility transistors (PHEMT's) have been successfully fabricated and demonstrated in both dc and ac performance. Together with a wide-gap Ga0.51In0.49P gate-insulator, a gate-to-drain breakdown voltage of 33 V is. further improved to over 40 V by selectively removing mesa sidewalls. The transconductance and output current density of a 1 ×100 μm2 device at room temperature (77 K) are 90 (J 20) mSlmm and 646 (780) mAlmm, respectively. In addition to the broad-plateau gm vs. IDS profile for the fabricated device, the associated fr and fmax vs. IDS profiles are also wide andflat. The measured frand fmax of 12 and 28.4 GHz are obtained, respectively.