We studied capacitance and parallel resistance in resonant tunneling diodes as a function of the doping density in the emitter and the collector regions. Capacitance was obtained by analyzing resonance in the admittance measured. Our data show that the capacitance varies with the doping density, and that the capacitance is smaller than the value expected from the growth parameters. Electron density modulation exists around the barriers, and capacitance has doping density dependence as a result of the modulation. ©1997 American Institute of Physics.