The purpose of this study is to improve the output power (P-out) and third-order inter-modulation distortion (IM3) of planar type GaAs power FET, which is used for RF power amplifiers for base stations of digital wireless communications systems. We focused on the GaAs surface state and attempted to improve the FET performance by intentional GaAs surface oxidation just prior to SiN cap deposition. Thermal or plasma oxidation has been found to improve P-out and IM3 characteristics. Distinct improvement of FET performance was observed in intentionally oxidized FET, and the highest P-out = 34 dB and P-out (@IM3=-55 dB(c)) = 23 dB(m) was measured on plasma oxidized FET with W-g = 5.2 mm. These results are shown in this article together with the results of photoelectron analysis. (C) 2003 Elsevier Science B.V. All rights reserved.