The general analysis of the forward AC behavior of a semiconductor diode under series mode is pre-sented for the first time.A new method without any particular assumption to characterize a diode was developed. This method can accurately measure the dependence of series resistance, junction capacitance, junction vol-tage, ideality factor, and interfacial layer impedance on forward biases. The measurements confirm that the ne-gative capacitance (NC) of Schottky diode is an effect of the junction, and the interfacial layer can be consi-dered as a layer structure with nonlinear resistance and capacitance.