We report a sub-bandgap linear-absorption-based photodetector in avalanche mode at 1550 nm in a PN-diode-integrated silicon microring resonator. The photocurrent is primarily generated by the defect-state absorption introduced by the boron and phosphorous ion implantation during the PN diode formation. The responsivity is enhanced by both the cavity effect and the avalanche multiplication. We measure a responsivity of ∼72.8  mA/W upon 8 V at cavity resonances in avalanche mode, corresponding to a gain of ∼72 relative to the responsivity of ∼1.0  mA/W upon 3 V at cavity resonances in normal mode. Our device exhibits a 3 dB bandwidth of ∼7  GHz and an open eye diagram at 15  Gbit/s upon 8 V.