In semiconductors at a room temperature

In a semiconductor, the valence band and the conduction band are separated by a small forbidden energy gap $E_g$ (about 1.1 eV for Si and 0.7 eV for Ge).
At absolute zero (0 K), the valence band is completely filled and the conduction band is completely empty, so the semiconductor behaves like an insulator.
At room temperature, thermal energy is enough to break some covalent bonds. A few electrons jump across the small gap from the valence band to the conduction band.
Each electron that leaves the valence band leaves behind a vacancy (a hole). So the valence band is no longer completely filled; it is partially empty.
The electrons that reach the conduction band make it partially filled.
Both electrons in the conduction band and holes in the valence band conduct electricity, which is why the conductivity of a semiconductor increases with temperature.
So at room temperature the valence band is partially empty and the conduction band is partially filled.