Using transparent conducting oxides (TCOs), like indium-tin-oxide (ITO), for optical modulation attracted research interest because of their epsilon-near-zero (ENZ) characteristics at telecom wavelengths. Utilizing indium-tin-oxide (ITO) in multilayer structure modulators, optical absorption of the active ITO layer can be electrically modulated over a large spectrum range. Although they show advances over common silicon electro-optical modulators (EOMs), they suffer from high insertion losses. To reduce insertion losses and device footprints without sacrificing bandwidth and modulation strength, slot waveguides are promising options because of their high optical confinement. In this paper, we present the study and the design of an electro-optical absorption modulator based on electrically tuning ITO carrier density inside a MOS structure. The device structure is based on dielectric slot waveguide with an ITO plasmonic waveguide modulation section. By changing the dimensions, the effective refractive indices for the slot mode and the off-sate mode of the plasmonic section can be matched. When applying electric field to the plasmonic section (on-state), carriers are generated at the ITO-dielectric interface that result in changing the layer where the electric field is confined from a transparent layer into a lossy layer. A finite difference time domain method with perfect matching layer (PML) absorbing boundary conditions is taken up to simulate and analyze this design. An extinction ratio of 2.3 dB is achieved for a 1-μm-short modulation section, at the telecommunications wavelength (1.55 μm). This EOM has advantages of simple design, easy fabrication, compact size, compatibility with existing silicon photonics platforms, as well as broadband performance.