In this paper we propose a novel silicon optical amplitude- phase modulator integrated into a Silicon-On-Insulator waveguide and based on a three terminal electronic structure, which gives rise to definite advantages in comparison with classical p-i-n diode based modulator. The proposed device utilizes the free carrier dispersion effect to produce the desired complex refractive index variations. The MEDICI device simulator has been employed to analyze the electrical operation, with reference to the injected free carriers concentration into the optical channel, its uniformity and the required current density and electrical power. The optical investigation was carried out by means of Finite Difference Method, Effective Index Method and Beam Propagation Method tools, giving rise to a complete evaluation of the properties of our device. We report the results for both the amplitude and phase modulators, paying attention to the static and the dynamic behavior. In particular, a modulation depth of 20%, with an injection power expense of about 126 mW, and a switching time of 5.6 ns is achieved. Furthermore, as a phase modulator, the device exhibits a very high figure of merit, predicting an induced phase shift per volt per millimeter of about 215 degree(s)/V(DOT)mm, for a injection power of about 43 mW, and a switching time shorter than 3.5 ns. The most attractive characteristic of the proposed device is the new bias operation mode which is based on the drift of the plasma injected into the optical channel. Respect to the p-i-n based modulator, based on the free carriers injection and depletion, the switching speed is almost one order of magnitude smaller.