Silicon MOSCAP ring modulator：arbitrary design rule of the modulation bandwidth， phase efficiency and compactness

By constructing a metal oxide semiconductor capacitor (MOSCAP) structure and embedding the optical waveguide, the carrier accumulation modulator allows the free carriers to accumulate on both sides of the dielectric layer, providing a large modulation bandwidth while maintaining a high modulation efficiency, and also allowing a smaller size and lower driving voltage. Meanwhile，the speed of MOSCAP modulator based on carrier accumulation is no longer limited by the carrier lifetime, but depends on the capacitance and resistance of the device structure, which is the advantage of MOSCAP modulator over carrier injection modulator and more attractive to the next generation of optical communication.

This paper aims to solve the problems of high transmission loss of optical modulators based on MOSCAP structure and the relationship between capacitance value and modulation efficiency. When the bias voltage is applied to the lower electrode and gate of the MOSCAP, the charge accumulation occurs on the lower electrode and gate, which affects the effective refractive index of the material at the rib waveguide location through the plasma dispersion effect.