For an insulator, the forbidden energy gap is

For an insulator, $E_{\mathrm{g}} > 3 \mathrm{eV}$, that is why electron transition from valence band to conduction band is not possible. For semiconductor $E_{\mathrm{g}}$ is $0.2 \mathrm{eV}$ to $0.3 \mathrm{eV}$, while for metals $E_g > 5 \mathrm{eV}$.