In this article, we derive a new SPICE equivalent-circuit model, using the rate equations for a separate confinement heterostructure quantum well laser. In this model, based on physical principles, the frequency chirping phenomenon has been incorporated by the introduction of an exact carrier-dependent linewidth enhancement factor for quantum well lasers. The effects of different separate confinement heterostructure layer lengths and carrier transport effects on the transient and modulation responses are also analyzed through this new model.