A new non-quasi-static (NQS) MOSFET model, which is applicable for both large-signal transient and small-signal ac analysis, has been developed. It employs a physical relaxation time approach to take care of the finite channel charging time to reach equilibrium and the effect of instantaneous channel charge re-distribution. The NQS model is formulated independently from the dc I-V and the charge-capacitor model, thus can be easily applied to any existing simulators. The model has been implemented in the newly released BSIM3 version 3, and comparison has been made among this model, common quasi-static (QS) SPICE models and PISCES two-dimensional (2-D) numerical device simulator. While predicting accurate NQS behavior, the time penalty for using the new model is only about 20-30% more than the common QS models. It is much less than the time required by other NQS models reported. Limitations and compromises between simplicity, efficiency and accuracy are also discussed.