  The frequency dependence of negative differential conductance (NDC) is an
important property for the resonant-tunneling-diode terahertz source. Among
several phenomena determining the frequency dependence, this paper shows that
the effect of potential change of the quantum well due to electron charge can
be analyzed with a simple and tractable model based on the tunneling admittance
and capacitance. The result is identical to that of Feiginov's analysis based
on more fundamental equations, showing a one-to-one correspondence between the
parameters of the two analyses. Similar to Feiginov's analysis, our analysis
also shows that NDC remains finite even at infinitely high frequency. It is
shown in our model that this result is attributed to neglecting the tunneling
time at the emitter barrier. Comprehensive analysis of the frequency dependence
of NDC will be possible by incorporating the tunneling time into the present
model.
