Question

What is forbidden energy ( $ {{E}_{g}} $ ) or band gap?
A. Forbidden energy gap $ {{E}_{g}} $ is the difference of energy levels of conduction band and within conduction band.
B. Forbidden energy gap $ {{E}_{g}} $ is the difference of energy levels of conduction band and valence band.
C. when there is no energy
D. conduction band

Answer 1

Hint :The band gap can be thought of as the gap between electron energy levels or the forbidden energy levels that cannot be occupied by the electrons. The energy levels which can be occupied by electrons are conduction band and valence band. Therefore, the band is not the conduction band nor does it lie within the conduction band.

Complete Step By Step Answer:
The group of discrete but closely spaced energy levels for the orbital electrons in the atoms is called the energy band gap. The energy bands are formed when the atoms are packed together as in solids.
The upper empty energy band is called the conduction band and the lower completely filled energy band is called the valence band. In between the conduction band and valence band, there is a group of energy levels that cannot be occupied by the electrons. It is called the forbidden energy () or band gap. That is, electrons are forbidden to occupy it, electrons mainly lie in the valence band.
To make the element conducting, we have to provide some energy to the element so that the valence electrons of its atoms get excited and become free to conduct electricity. So, the electrons can jump from the valence band to the conduction band when they are excited by absorbing energy. Also, they can make a transition from conduction band to valence band by emitting energy.
The band gap can also be defined as the energy between the conduction band and the valence band. The electrons can either be in the valence band or they may jump to the conduction band skipping the energy levels of the band gap. Thus, a forbidden gap is defined as the difference in the energy levels of the conduction band and valence band.
Hence, option (B) is the correct answer.

Note :
The value forbidden energy ( $ {{E}_{g}} $ ) or band gap differs for conductors, semiconductors and insulators. For conductors, the band gap may be negligibly small or zero. For insulators it may be very large and can range from $ 5eV $ to $ 15eV $ and semiconductors, the band gap is of the order of $ 1eV $ .