In a semi conductor at OK

At T=0 K in a semiconductor, the valence band is full, and the conduction band is empty. The energy gap of a semiconductor is small, in the range 0.5eV<Eg<3eV. In the presence of a small applied voltage, electric field, or optical field, a semiconductor acts as an insulator. In the presence of a large applied voltage or other energy source, a semiconductor acts as a conductor, and electrons flow.