An exploration of THz range causes the necessity of dynamic control over the parameters of the radiation - modulation of its intensity, phase and polarization. In this range optical properties of semiconductors are mainly defined by the interaction of radiation with semiconductor plasma. A strong influence of the external field on parameters of a plasma of any kind, including the carrier plasma, opens possibilities for controlling parameters of THz radiation. The characteristic modulation time is defined by the carriers relaxation time (about 10-12 s), i.e. has the same order of magnitude as a THz wave period. For the calculation of an influence of an external quasi-static electric field on the parameters of the plasma and on the optical properties of the semiconductor all main mechanisms of carrier scattering and relaxation were accounted. For AIIIBV semiconductors the calculations of the response of the complex permittivity of carrier plasma on the external field were done numerically. An analysis of the reflection coefficients based on generalized Fresnel equations showed a variation of the reflection coefficient of the semiconductor under the influence of the field by dozens percent. Static parameters of semiconductors which provide a maximum modulation depth were defined. In case when an external field is a THz band field itself, the dependence of the permittivity on the intensity results in the harmonics generation. An analysis of the specific features of the generation showed an appearance of frequency resonances and a unique non-monotonic way of the dependence of the harmonic intensity on the pump wave field.© (2010) COPYRIGHT SPIE--The International Society for Optical Engineering. Downloading of the abstract is permitted for personal use only.