We investigate experimental dependence of the third harmonic generation efficiency in the n-type Si crystals on the geometrical dimensions of the sample, polarization and power of the fundamental wave. The efficiency increases monotonically with the rise of the sample thickness up to a threshold value, and decreases dramatically above the threshold. At shorter propagation distances the generation efficiency could be correctly simulated using the layered medium approximation and the numerically calculated electron drift velocity response to the pumping wave electric field to describe the change of the semiconductor properties under high-power microwave irradiation.