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In this letter, we present an 850-nm Si octagonal photodiode (PD) with deep n-well implantation formed using the CMOS process without process modifications. Two different bias schemes (normal bias and extra bias) on the deep n-well were used to analyze the effects of deep n-well bias on the bandwidth and gain-bandwidth performances of Si PDs. The avalanche gain, frequency response, and optical pulse measurements in this letter demonstrate that the extra bias on deep n-well improves the PD performance. To the best of our knowledge, this design achieves the highest bandwidth (8.7 GHz) and a large gain-bandwidth product of 542 GHz with a reverse bias of 11.45 V and an extra voltage of 11.45 V in standard CMOS technology.