High selectivity succinic acid: H 2 O 2 solution (SA) wet etching processes were used in the gate recess process to fabricate (Al 0 . 3 Ga 0 . 7 ) 0 . 5 In 0 . 5 P quaternary enhancement-mode pseudomorphic high electron mobility transistors (E-mode pHEMT). High uniformity of threshold voltage (V t h ) was achieved due to a high wet etching selectivity during gate recess process. In this study, we improved the power density and uniformity of the device by using wide bandgap (Al 0 . 3 Ga 0 . 7 ) 0 . 5 In 0 . 5 P Schottky layers. In addition, the best wet etching selectivity between GaAs and (Al 0 . 3 Ga 0 . 7 ) 0 . 5 In 0 . 5 P was obtained for large area power transistors. The developed 0.5 μm long and 1.2 mm wide gate FETs exhibited a V t h of +0.1 V and a maximum drain current (I d m a x ) of 385 mA/mm. The maximum output power at 1.9 GHz was 95 mW/mm with a linear power gain of 22 dB and a power-added efficiency of 60%. These characteristics demonstrate this novel E-mode pHEMTs have great potential for use in microwave power device applications.