Herein, we present a detailed analysis of the effects of O2 plasma treatment on the AlGaN barrier volume trap states in an Al0.45Ga0.45N/GaN high-electron mobility transistor. Compared to that of the as-grown sample, the single short-pulse <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">${I}_{D}$ </tex-math></inline-formula> – <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">${V}_{\text {GS}}$ </tex-math></inline-formula> characterization of the plasma-treated sample exhibited lower charge trapping inside the AlGaN barrier. The 1/ <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">${f}$ </tex-math></inline-formula> low-frequency noise characterization revealed a significant reduction of approximately 67% in the volume trap density of the AlGaN barrier layer after O2 plasma treatment. This was achieved by the formation of Al–O and Ga–O bonds via the penetration of oxygen ions into the AlGaN bulk, which resulted in reduced trap state density in the AlGaN barrier. In addition, the Schottky characteristics were improved notably. Consequently, the O2 plasma-treated sample did not display current collapse and showed steady drain current output under the reverse-sweep drain-stress bias conditions. Furthermore, the plasma treatment significantly reduced the RF transconductance ( <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">${g}_{m}$ </tex-math></inline-formula> ) collapse in the as-grown sample, and significantly increased the <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">${f}_{T}/{f}_{\text {max}}$ </tex-math></inline-formula> of the plasma-treated sample from 65/70 to 120/230 GHz for <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">${L}_{g}$ </tex-math></inline-formula> = 80 nm devices, respectively. Last, the O2 plasma-treated sample showed substantial improvements in <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">${P}_{\text {out}\_\max}$ </tex-math></inline-formula> , power added efficiency (PAE), and linear gain from 1.25 W/mm, 20%, and 15 dB to 2.4 W/mm, 50%, and 19 dB, respectively.