A SPICE3 model is developed for the resonant-tunnelling diode thar combines a physical formulation of the coherent current component between zero bias and the valley point with an empirical formulation-for the incoherent, or excess, current at bias voltages beyond the valley. The model is designed for type-I RTDs, such as GaAs/AlGaAs structures, and a fitting procedure is developed to derive the parameters of the model From the experimental current/voltage curve.