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A 90GHz power amplifier implemented with three series-connected (stacked) FETs in 45-nm SOI CMOS is reported. Stacking FETs allows increasing voltage handling capability of circuits with highly scaled CMOS transistors. This work shows for the first time that the stacking strategy is effective up to W band. The amplifier achieves power gain of 8 dB at 91 GHz with 3 dB bandwidth of 18 GHz using a supply voltage of 4.2 V. It delivers saturated output power of 17.3 dBm in 88-90 GHz range with peak PAE of 9 %. The PA chip occupies 0.256 mm2 including the pads. This chip demonstrates the highest output power from a CMOS PA in this frequency regime.