Question

what are forbidden gaps?

Answer 1

When atoms are brought close together, as in a solid, the electrons come under the influence of forces from other atoms, where the energy level merges into bands of energy levels. There are two distinct energy bands in which electrons could exist: the valence band and the conduction band. Separating these two bands is an energy gap , termed the forbidden gap, in which electrons cannot normally exist.  

The energy required to transfer an electron from valence band to conduction band, the external energy which is required is equal to the forbidden energy gap.

Forbidden gap plays a major role for determining the electrical conductivity of material. Based on the forbidden gap, materials are classified in to three types, they are:

Insulators

Conductors

Semiconductors



;-)

Answer 2

In semiconductors and insulators, electrons are confined to a number of bands of energy, and forbidden from other regions. The term "band gap" refers to the energy difference between the top of the valence band and the bottom of the conduction band.