In semiconductors at a room temperature

the valence band is completely filled and the conduction band is partially filled

In semiconductors, the conduction band is empty and the valence band is completely filled at 0 K. No electron from valence band can cross over to conduction band at 0 K. But at room temperature some electrons in the valence band jump over to the conduction band due to the small forbidden gap, i.e., 1 eV.