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A fully integrated 31-GHz power amplifier (PA) fabricated in 0.18-μm CMOS process is presented in this paper. This PA consists of two common-source stages. The first stage is the driver amplifier, and the second stage is the power amplifier. In order to achieve high output power, power cell with large device size is adopted in this design. To overcome the substrate loss, the thin-film microstrip (TFMS) lines are used in this work. In addition, the TFMS lines can provide large current capacity for PA operation. Operating at the 2.0-V supply voltage, this Ka-band PA achieves the measured 12.33-dBm saturation output power (Psat) with 8.24-dBm output 1-dB compression point (OP1dB) at 31 GHz. The input return loss (S11) and output return loss (S22) are -9.5 dB and -5.2 dB, respectively. The power density is 20.36 mW/mm2. According to authors' knowledge, this 31-GHz PA achieves the highest Psat, highest OP1dB, and highest power density compared to other reported CMOS amplifiers above 30 GHz in Ka and Q band.