This paper presents the design, fabrication, and measurement results of a novel lateral p-i-n silicon photodetector (Si-PD) for 850 nm in a silicon-on-insulator (SOI) platform. In the proposed photodetector, the incident light is directed horizontally using a grating coupler, significantly increasing optical absorption in the depletion area thereby increasing the PD's responsivity. The measurement results show that the grating coupler increases the responsivity by 40 times compared with the Si-PD without a grating coupler. The grating-assisted Si-PD with 5µm intrinsic width has a responsivity of 0.32 A/W and a dark current of 1 nA at 20 V reverse-bias voltage. Further, it shows an open eye diagram for 10 Gb/s PRBS‑31 non-return-to-zero on-off keying (NRZ-OOK) data and has a 3-dB bandwidth of 4.7 GHz at this bias voltage. In addition, the design parameters of three variations of the grating-assisted Si-PD for high-speed applications (>25 Gb/s) are presented. The optimized grating-assisted Si-PD uses a focusing grating coupler and its p-i-n diode has a 0.3 µm intrinsic width. It has a responsivity of 0.3 A/W, an avalanche gain of 6, a dark current of 2 µA, and a 3-dB bandwidth of 16.4 GHz at 14 V reverse-bias voltage that, to the best of our knowledge, is the largest 3-dB bandwidth reported for a Si-PD. As a result, it has a figure of merit (FoM) of 4920 GHz×mA/W. Further, it shows an open eye diagram of 35 Gb/s PRBS‑31 NRZ-OOK data.