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In this paper, an ultrafine pixel size (2.0×2.0 μm2) MOS image sensor with very high sensitivity is developed. The key technologies that realize the MOS image sensor are a newly developed pixel circuit configuration (1.5 transistor/pixel), a fine 0.15-μm design rule, and an amorphous Si color filter (Si-CF). In the new pixel circuit configuration, a unit pixel consists of one photodiode, one transfer transistor, and an amplifier circuit with two transistors that are shared by four neighboring pixels. Thus, the unit pixel has only 1.5 transistors. The fine design rule of 0.15 μm enables reduction of wiring area by 40%. As a result, a high aperture ratio of 30% is achieved. A newly developed Si-CF realizes the 1/10 thickness of that of the conventional organic-pigment CF, giving rise to high light-collection efficiency. With these three technologies combined, a high sensitivity of 3400 electrons/lx·s is achieved even with a pixel size of 2.0×2.0 μm2.