The microwave impedance of high current Al0.3 Ga0.7 As‐GaAs‐Al0.3 Ga0.7 As resonant tunneling diodes is measured from 0.1 to 12.1 GHz over a large bias range at 300 K. Using an equivalent circuit model, the bias dependence of intrinsic conductance and capacitance suffice to explain the pronounced variations of impedance.