Using the multiple-carrier model, the process for thermal/electric-field po
ling in the fused silica is analyzed. The depletion region is proved to be
composed of a positive charged layer, an ionosphere layer and a negative de
pletion layer. The space-charges in two former layers are H3O+, O+, Al++ an
d others, and the space-charge in the latter is drop Si--O-. The thermal st
ability of the depletion region is determined by the strength of the ionosp
here. In addition, the ion density distribution in the depletion region is
derived. The results show that both the positive and the negative space-cha
rge layers are approximately well-distributed, and the self-built electric
field in the depletion layer is approximately distributed in triangle-form,
the maximum of which is located inside the sample. This model proves that
the second order susceptibility and the conversion efficiency of the second
harmonic are proportional to the square-root and square of the applied de
voltage, respectively.