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This paper presents new MOS image sensor technologies that realize ultra-small pixel size, i.e., 2.25×2.25 μm2, with high sensitivity and low supply voltage. A 1/4-inch 2-Mpixel MOS image sensor has been developed by a new pixel configuration and by a new pixel design with a 0.25-μm CMOS process. In the new pixel configuration, a unit pixel consists of one photodiode (PD), one transfer transistor, and an amplifier circuit with three transistors which are shared by four pixels. As a result, the unit pixel has 1.75 transistors. High sensitivity has been achieved by a high aperture ratio of 25%. In the new pixel design, the low supply voltage of 2.5 V has been realized by optimizing both the potential profile in the PD and the gate length of the transfer transistor.