Energy Gap, Valence Shell and Valence electrons, Forbidden Energy Gap

Energy Gap

The Energy gap between the valence band and conduction band is known as forbidden energy gap. Forbidden energy gap is a region in which no electron can stay as there is no allowed energhy state. The greater the forbidden energy gap more tightly the valence electrons are bound to the nucleus. 

Valence Shell and Valence electrons

The outermost shell of an atom is called the valence shell and electrons in this shell are called the valence electrons. Formation of energy bands occur owing to overlapping of energy levels of these valence electrons in valence shells. With the decrease in interatomic distance between the atoms in a crystal, the energy levels of electrons in outermost shells of atoms overlap to form energy bands.

Forbidden Energy Gap

The energy gap between the valence band and conductions 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 zerro temperature.