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We report a new structure of high-responsivity photodetectors that utilizes the transparent and metallic zinc oxide (ZnO) gate in bulk silicon metal-oxide-semiconductor field-effect-transistor photodetectors. The device has a small optical window only in the channel region, and all other regions (depletion) are protected from external light. Whereas the amplification of photocurrent by external light was not significant at the floated or positively biased substrate, the photocurrent was enhanced at the grounded or negatively biased substrate due to the decrement of the recombination rate in the n-channel MOSFET. Responsivity was in excess of 1500 A/W under white-light illumination, which is higher than that of conventional photodetectors with the semitransparent polycrystalline-silicon gate.