Early passivated AlGaN/GaN heterostructure field effect transistors (HFETs) have shown superior device performance compared to their unpassivated counterparts. The opening of gate trenches in the passivation layer by a fluorine based dry-etch process is the crucial step to obtain high-performance devices. Even though oxygen addition to this process has been reported, its effect on the device characteristics has not been studied yet. In this paper, we report the impact of oxygen addition to a fluorine based plasma on two-dimensional electron gas (2DEG) properties as well as on device characteristics of passivated AlGaN/GaN HFETs. It is shown that oxygen addition reduces the fluorine-induced degradation of the 2DEG density. Yet, recovery of the 2DEG density is prevented. Additionally, it is shown that oxygen addition increases the current collapse and therefore decreases the RF performance. (C) 2011 Elsevier Ltd. All rights reserved.