All-silicon (Si) photodiodes have drawn significant interest due to their single and simple material system and perfect compatibility with complementary metal-oxide semiconductor photonics. With the help from a cavity enhancement effect, many of these photodiodes have shown considerably high responsivity at telecommunication wavelengths such as 1310 nm, yet the mechanisms for such high responsivity remain unexplained. In this work, an all-Si microring is studied systematically as a photodiode to unfold the various absorption mechanisms. At -6.4V, the microring exhibits responsivity up to ∼0.53A/W with avalanche gain, a 3 dB bandwidth of ∼25.5GHz, and open-eye diagrams up to 100 Gb/s. The measured results reveal the hybrid absorption mechanisms inside the device. A comprehensive model is reported to describe its working principle, which can guide future designs and make the all-Si microring photodiode a promising building block in Si photonics.