A formula in terms of the equivalent circuit parameters of a resonant tunneling diode (RTD) and its loading for three different equivalent circuit models of RTD’s is derived to describe the apparent change, occurring in the presence of unstable oscillation, in the negative differential conductance (NDC) region of dc current-voltage(I-V)characteristics measurements. The measured experimentalI-Vdata and the related parameters of RTD’s from several fabricated devices are applied to the formula in terms of the stability and instability criteria to do the numerical verification. It has shown good agreement between the derived formula and the experimentalI-Vdata of RTD. Simulated program with integrated circuits emphases (SPICE) simulation shows the flattened and the broken sections, which represent the occurrence of the unstable oscillation, appearing in the NDC region ofI-Vcurve, which also validates the applicability of the derived formula. ©1997 American Institute of Physics.