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A design method for RF power Si-MOSFETs suitable for low-voltage operation with high power-added efficiency is presented. In our experiments, supply voltages from 1 V to 3 V are examined. As the supply voltage is decreased, degradation of transconductance also takes place. However, this problem is overcome, even at extremely low supply voltages, by adopting a short gate length and also increasing the N- extension impurity concentration-which determines the source-drain breakdown voltage (Vdss)-and thinning the gate oxide-which determines the TDDB between gate and drain. Additionally, in order to reduce gate resistance, the Co-salicide process is adopted instead of metal gates. With salicide gates, a 0.2 μm gate length is easily achieved by poly Si RIE etching, while if metal gates were chosen, the metal film itself would have to be etched by RIE and it would be difficult to achieve such a small gate length. Although the resistance of a Co-salicided gate is higher than that of metal gate, there is no evidence of a difference in power-added efficiency when the finger length is below 100 μm. It is demonstrated that 0.2 μm gate length Co-salicided Si MOSFETs can achieve a high power-added efficiency of more than 50% in 2 GHz RF operation with an adequate breakdown voltage (Vdss). In particular, an efficiency of more than 50% was confirmed at the very low supply voltage of 1.0 V, as well as at higher supply voltages such as 2 V and 3 V. Small gate length Co-salicided Si-MOSFETs are a good candidate for low-voltage, high-efficiency RF power circuits operating in the 2 GHz range.