We model optical parametric oscillation in ring cavities with two-photon absorption, focusing on silicon at 1.55μm. Oscillation is possible if free-carrier absorption can be mitigated; this can be achieved using carrier sweep-out in a reverse-biased p-i-n junction to reduce the carrier lifetime. By varying the pump power, detuning, and reverse-bias voltage, it is possible to generate frequency combs in cavities with both normal and anomalous dispersion at a wide range of wavelengths including

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... 55μm. Furthermore, a free-carrier self-pulsing instability leads to rich dynamics when the carrier lifetime is sufficiently long.