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High frequency (HF) distortion of MOSFETs has been characterized at different frequencies and bias conditions with a single tone measurement system. The results show that a MOSFET has much higher "low frequency limit" (LFL) than a bipolar transistor with similar critical dimensions, implying that the HF distortion characteristics of MOSFETs operating at a frequency lower than LFL is dictated by its low-frequency behavior. This discovery is useful for designers and modelers to validate the distortion of a MOSFET model for RF application. It has also been found that the second harmonic Pout2 reaches to its minimum as fT peaks, due to a similar non-linearity cancellation as in bipolar transistors. Furthermore, the measured data shows fairly "constant" distortion characteristics over a wide range of drain biases as the device operates in the saturation region. Simulation with a BSIM3v3 based sub-circuit model demonstrates that the distortion behavior of MOSFETs can be well predicted by an RF model if it can accurately describe both DC and AC characteristics with proper parameter extraction.