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In this paper, the influence of dielectric polymethylmethacrylate (PMMA) layer on the detectivity of pentacene-based photodetectors with field-effect transistor (FET) configuration were investigated in a visible region. By changing the thickness of the PMMA layer, from 230 nm to 520 nm and 800 nm, electrical parameters, such as the capacitance, “on/off” current ratio, and carrier mobility, of the pentacene-based photodetector decrease with increasing the thickness of the PMMA layer, which influences its detectivity directly. The photosensitivity and responsivity of the FET-based pentacene photodetector with 520-nm PMMA varied with incident monochromatic light from 350 nm to 750 nm, and it showed a maximum responsivity of 149 mA/W with a photosensitivity peak of 1.7 ×104 at 450 nm, which is of the same order as that of the standard Si-based photodetector. Therefore, it is an applicable way to get such kind of FET-based full-organic photodetectors in a full visible region with excellent photosensitivity, responsivity, and selectivity.