What is the forbidden energy gap?

The energy gap between the valence band and conduction band is known as forbidden energy gap. It is a region in which no electron can stay as there is no allowed energy state. Magnitude of forbidden energy gap in germanium and silicon is 0.72 eV and 1.12 eV respectively at 300 K and 0.785 eV and 1.21 eV respectively at absolute zero temperature.