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This paper presents a CMOS-compatible gate-assisted photonic demodulator with contrast enhancement (GAPD-CE) techniques. To form an asymmetric field inside the substrate that will facilitate the transfer of photogenerated electrons, p-well and channel doping techniques are applied under the polysilicon guides. Modulation contrast (MC) values extracted from TCAD simulation show that the modified structure effectively collects electrons generated deep within the substrate. A prototype of the GAPD-CE is fabricated in a 0.18- $\mu \text{m}$ standard CMOS foundry process. An MC of 83% and 57% at 1 MHz and 30 MHz, respectively, are achieved under 850 nm illumination, demonstrating the potential of the proposed device for time-of-flight (ToF) sensing applications.