The effects of the addition of a small amount of O-2 to SiCl4/SiF4 plasma and the thickness of the GaAs capping layer on gate recess etching of GaAs pseudomorphic high electron mobility transistor (p-HEMT) devices have been studied. During gate recessing of GaAs/AlGaAs p-HEMTs using a selective reactive ion etching (RTE) process in SiCl4/SiF4 plasma [see S. K. Murad, N. I. Cameron, P. D. Wang, S. P. Beaumont, and C. D. W. Wilkinson, Microelectron. Eng. 27, 439 (1995)], it was found that the profile of the gate changes dramatically from undercut to nearly vertical with no lateral etching when the thickness of the capping layer was reduced below 40 nm. This vertical profile puts the gate metal too close to the recess edges in devices with a less than or equal to 30 nm capping layer. The addition of a very small amount of O-2 to SiCl4/SiF4 plasma was seen to increase GaAs etch rates remarkably, while maintaining the high selectivity over AlGaAs. This increase in the etch rate agrees well with optical emission spectroscopic observations which indicate that the Cl emission has increased by more than an order of magnitude for the addition of only 1.5% of O-2 to SiCl4/SiF4 plasma. This selective RIE process of SiCl4/SiF4/O-2 was applied to gate recess etching of GaAs/AlGaAs/InGaAs p-HEMTs with various capping layer thicknesses. The profile of the T gates changed from nearly vertical (with no undercut or gate offset) to undercut with a lateral etch rate depending on the thickness of the capping layer. The lateral etching rate also strongly depends on the O-2 flow. The p-HEMT devices gate recessed using this process (SiCl4/SiF4/O-2) exhibited an f(t) of 120 GHz, g(m) of 700 mS/mm and a gain of 9.5 dB at 94 GHz.