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A new PIN photodiode (PD) structure with deep n-well (DNW) fabricated in an epitaxial substrate complementary metal-oxide-semiconductor (epi-CMOS) process is presented. The DNW buried inside the epitaxial layer intensifies the electric field deep inside the epi-layer significantly, and helps the electrons generated inside the epi-layer to drift faster to the cathode. Therefore, this new structure reduces the carrier transit time and enhances the PD bandwidth. A PD with an area of 70 times 70 mum2 fabricated in a 0.18-mum epi-CMOS achieves 3-dB bandwidth of 3.1 GHz in the small signal and 2.6 GHz in the large signal, both with a 15-V bias voltage and 850-nm optical illumination. The responsivity is measured 0.14 A/W, corresponding to a quantum efficiency of 20%, at low bias. The responsivity increases to 0.4 A/W or 58% quantum efficiency at 16.2-V bias in the avalanche mode.