Crystalline silicon (c-Si) exhibits two electro-optical effects that are suitable for device-application in guided-wave modulators and switches. The effects are electrorefraction and free-carrier dispersion. The dispersion is controlled by electrical injection or electrical depletion of charge carriers. The magnitude of these effects has been calculated in this paper. The results for c-Si are compared to those for GaAs and InP. Low-loss guided-wave electro-optical phase modulation structures in c-Si are proposed here.