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In this paper, we demonstrate the size effect on the DC and RF performances of 0.10 μm RF MOSFETs for SOC applications. Our results show that for RF MOSFETs, the input impedance can be represented by a series RC circuit at low frequencies and a "shifted" parallel RC circuit at high frequencies. In addition, the output impedance can be represented by a "shifted" series RC circuit at low frequencies and a "shifted" parallel RC circuit at high frequencies. The appearance of the kink phenomenon of scattering parameters S11 and S22 in a Smith chart is caused by this inherent ambivalent characteristic of the input and output impedances. It was found that an increase of device's width (or gm) enhances the kink effect of S11 and S22. The present study enables RF engineers to understand the behaviors of S-parameters more deeply, and hence are helpful for them to create a fully scalable CMOS model for SOC applications.