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The fluctuation of RF performance (particularly for fT: cutoff frequency) in the transistors fabricated by 90-nm CMOS technology has been investigated. The modeling for fT fluctuation is well fitted with the measurement data within approximately 1% error. Low-Vt transistors (fabricated by lower doping concentration in the channel) show higher fT fluctuation than normal transistors. Such a higher fT fluctuation results from Cgg (total gate capacitance) variation rather than gm variation. More detailed analysis shows that Cgs + Cgb (charges in the channel and the bulk) are predominant factors over Cgd (charges in LDD/halo region) to determine Cgg fluctuation.