What is forbidden energy gap (Eg) or band gap?

Answer

### Step-by-Step Solution

1. **Definition of Forbidden Energy Gap (Eg)**:
The forbidden energy gap, also known as the band gap (Eg), is defined as the energy difference between the highest energy level of the valence band and the lowest energy level of the conduction band in a solid material.

2. **Understanding Energy Bands**:
In solids, electrons occupy energy bands. The two most important bands are the valence band (where electrons are present) and the conduction band (where electrons can move freely and contribute to electrical conduction).

3. **Energy Band Structure**:
- **Conductors**: In conductors, the conduction band and valence band overlap, meaning there is no band gap. Electrons can move freely, which allows for high electrical conductivity.
- **Insulators**: In insulators, there is a large band gap (typically around 6 eV), which means that electrons require a significant amount of energy to jump from the valence band to the conduction band, resulting in very low electrical conductivity.
- **Semiconductors**: In semiconductors, the band gap is smaller (around 1 eV). This allows some electrons to gain enough energy (from thermal excitation or other means) to jump from the valence band to the conduction band, enabling moderate electrical conductivity.

4. **Importance of Band Gap**:
The band gap is crucial in determining the electrical properties of materials. It influences how materials respond to external energy sources (like heat or light) and their ability to conduct electricity.

5. **Summary**:
The forbidden energy gap (Eg) is a key concept in understanding the electrical properties of materials, distinguishing conductors, insulators, and semiconductors based on their band structure.