The fabrication and characterization of the p-i-n optical waveguide modulators on silicon-on-insulator (SOI) substrate were demonstrated. The modulation was based on the mechanism of carrier injection, or plasma dispersion effect. The corresponding p and n regions were defined in both types of silicon substrates (conventional p-doped and highly resistive SOI substrates with respective resistivities of ρ∼7-10Ω-cm and ρ∼7000- 10000Ωcm) using the spin-on-dopant (SOD) technique. The SOD diffusion process was conducted at 900-1000°C in nitrogen ambient. The diffusion time and temperature, and the resistivity of SOI substrate used were the primary parameters dictating the resultant dopant concentrations and diffusion depths. For the modulators fabricated with various waveguide widths and electrode lengths, the corresponding modulation index was enhanced in response to an increase in the electrode (or modulation) length and/or a decrease in waveguide width. The highest modulation index of ∼4.15% was successfully achieved for a silicon p-i-n waveguide modulator with 5μm-wide waveguide and 7mm-long modulation electrode.