The impedance dependence of resonant tunneling diodes (RTDs) based on the GaAs/AlAs heterosystem is investigated in the range of 0.1–40 GHz. The analysis shows that the impedance of about 90% of unbiased RTDs is well described by an equivalent circuit (EC) consisting of parallel-connected resistance and capacitance and an additional resistance connected in series with this parallel combination. When a bias voltage is applied to these RTDs, one needs a “quantum” inductance LQ to describe the impedance behavior. We find the value of LQ and calculate the delay time of electrons in the quantum well (QW) of an RTD. The impedance of the rest 10% of the RTDs is well described by an EC that takes into account the recharge of localized electron states at the heterointerfaces of the active layers. Expressions for the cut-off frequencies that take into account the delay of electrons in the QW and the localized electron states at the heterointerfaces are derived. It is shown that the delay of electrons in the QW and localized electron states at the heterointerfaces may significantly reduce the cut-off frequency of RTDs.