Any oscillator is inherently a nonlinear system: it is nonlinearity that limits the unbounded growth of oscillation amplitude. The resonant-tunneling-diode (RTD) oscillator offers perhaps the purest and simplest model for studying this phenomenon. Moreover, it is a device that simultaneously operates at record-high frequencies for electronic devices and holds direct relevance for future THz systems and applications. Remaining within the domain of analytical and semi-analytical methods, one analyses such a model using the van der Pol equation framework, where the RTD current–voltage (I–V) characteristic is approximated by a cubic polynomial. In this work, we generalise this model to extend it to an arbitrary RTD I–V curve, deriving an analytical solution to first order in perturbation theory. The solution yields a surprising immediate outcome: it provides a natural explanation for a puzzling behaviour observed experimentally in certain RTD oscillators, namely that the oscillation frequency varies non-monotonically with bias, despite the expectation that frequency should vary monotonically if governed solely by the well-known reduction of RTD capacitance with bias. We show instead that this frequency variation can arise from the RTD nonlinearity itself.

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