Si optical phase shifters are key components for efficient phase modulation utilizing carrier modulation. Among many structures for carrier modulation in them, a semiconductor-insulator-semiconductor (SIS) capacitor can achieve high phase modulation efficiency because of its large carrier accumulation. However, due to the limitation caused by relatively small plasma dispersion in Si, its size and voltage for the pi phase shift are large to integrate into conventional CMOS technology. To enhance its phase modulation efficiency, material engineering of a SIS capacitor is indispensable. Although semiconductor engineering has been proved to realize efficient phase modulation, there are few reports on the insulator engineering of a SIS capacitor for efficient phase modulation. Therefore, we have introduced new functionalities for a SIS optical phase shifter using insulator engineering. In this paper, we have investigated the charge enhancement effect in a SIS capacitor with ferroelectric (FE) materials utilized by the negative capacitance (NC) effect. We will discuss the design of an NC SIS optical phase modulator to enhance this effect for a new efficient phase modulation scheme.