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In this paper, we demonstrate the size effect on the DC and RF performances of 0.10 /spl mu/m 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 S/sub 11/ and S/sub 22/ 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 g/sub m/) enhances the kink effect of S/sub 11/ and S/sub 22/. 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.