A lumped-circuit nonquasi-static (NQS) model, that is applicable for both large-signal transient simulations and a small-signal ac analysis, is developed in this paper. An improved physical equivalent circuit, capturing NQS effects in the millimeter waveband, is derived using a transmission line model, by incorporating the high-frequency longitudinal gate electrode and a channel distributed RC network. The proposed model is implemented in a BSIM-BULK MOSFET model and validated with dc and RF data, obtained from technology computer-aided design device simulations and experimental data. The proposed model is in very good agreement with the data up to {50}{f}-{t}. The transient currents, for a gate-voltage switching rate of {5}\times {10}^{{10}} V/s, show excellent match with the data. The dc, transient, and ac simulations using the proposed model are much faster than a 10-segmented MOSFET model. This shows that the proposed model is better than other computationally complex compact models, for most RF applications. © 1963-2012 IEEE.