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Large-signal operation of HEMT (High Electron Mobility Transistor) switch device is found to be much affected by trap-induced slow dynamic effects. Off-state gate and drain capacitances derived from pulsed S-parameter measurements with a large negative gate quiescent voltage are less voltage-dependent than capacitances measured by a continuous bias. Two-dimensional device simulation suggests that surface traps near the gate edge are responsible for the observed dynamic effect. A new large-signal device model is developed that takes both C-V and I-V pulsed dynamic behavior into account. The new model is shown to accurately predict harmonics generated from off-and on-state switches.