High selectivity succinic acid: H 2 O 2 solution ͑SA͒ 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 pHEMT͒. High uniformity of threshold voltage ͑V th ͒ 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 m long and 1.2 mm wide gate FETs exhibited a V th of +0.1 V and a maximum drain current ͑I d max ͒ 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.

GaAs-based pseudomorphic high electron mobility transistors (pHEMTs) in which the field-plate (FP) is connected to the gate terminal and the source terminal, were developed and evaluated experimentally to determine their microwave and power performance. The small gate-to-drain feedback capacitance (C gd ) and the stable FP-induced depletion region at high input power (P in ) of the source-terminated FP pHEMT (FP-S pHEMT) greatly improve the power and linearity of the FP-S pHEMT above those of the gate-terminated FP pHEMT (FP-G pHEMT). The power ratio of the fundamental to the third-order inter-modulation product (IM3) is 18.8 dBc for FP-S pHEMT for P in = 0 dBm; the corresponding value for FP-G pHEMT is 12.4 dBc. These experimental results indicate that the FP architecture is more effective at high-power operation and exhibits high linearity in high-power pHEMT applications.

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High uniformity enhancement-mode (E-mode) and depletion-mode (D-mode) InGaP/InGaAs pseudomorphic high electron mobility transistors (pHEMTs) have been developed on 6 inch GaAs substrate using a selective succinic acid gate recess process. The 0.5 µm long gate fingers of E-mode and D-mode pHEMTs are deposited simultaneously in this process simplification. The InGaP/InGaAs E-mode pHEMT exhibits a maximum drain-to-source current (I ds ) of 460 mA mm −1 , and a maximum transconductance (g m ) of 430 mS mm −1 . At 5.2 GHz operation, 216 mW mm −1 power density, 40% power-added efficiency and a 0.81 dB minimum noise figure (NF min ) are also achieved for the E-mode device. In this study, D-mode pHEMTs are applied for a switch monolithic microwave integrated circuit which provides an insertion loss of −1.8 dB and isolation of −9.2 dB under 28 dBm input power (P in ) and 5.5 GHz operation. From these measured results, this uniform E/D-mode InGaP/InGaAs pHEMT technology exhibits a high potential for WLAN applications.