The influence of longitudinal voltage, applied along semiconductor substrate of planar-oriented liquid-crystal cell, on the birefringence of nematic liquid crystal (NLC) has been studied. It is shown that with the fixed value of transverse voltage the application of longitudinal voltage creates an additional possibility to control the electro-optically induced phase shift of light passing through NLC. The dependence of phase shift vs. control voltages is obtained. The results of simulation of the longitudinal voltage influence on the process of NLC reorientation are given. The obtained results can be used for development and fabrication of liquid-crystal phase modulators.