Carrier dynamics in a MOSFET channel under fast time-varying gate input is included in the modeling for circuit simulation and implemented in SPICE3f5 at only 7% increased computational runtime cost. Correct reproduction of transient drain currents as well as harmonic-distortion characteristics are verified. While the carrier dynamics under low-frequency operation is mostly governed by the carrier mobility in the channel, the dominant factor under high-frequency operation changes to channel charging and discharging.