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 material.

2. **Understanding Energy Bands**:
- **Valence Band**: This is the highest range of energy levels that electrons can occupy in a material at absolute zero temperature. Electrons in this band are involved in bonding and are not free to conduct electricity.
- **Conduction Band**: This is the range of energy levels where electrons are free to move and conduct electricity. Electrons in this band can move freely, allowing for electrical conduction.

3. **Visual Representation**:
- Draw a diagram with two horizontal lines representing the valence band and conduction band. The valence band is at a lower energy level, while the conduction band is at a higher energy level. The gap between these two bands represents the forbidden energy gap (Eg).

4. **Examples of Different Materials**:
- **Conductors**: In conductors, the conduction band and valence band overlap, meaning there is no band gap (Eg = 0). Electrons can move freely without needing additional energy.
- **Semiconductors**: In semiconductors, the band gap is relatively small (approximately 1 eV). Electrons can jump from the valence band to the conduction band with a small amount of energy (e.g., thermal energy at room temperature).
- **Insulators**: In insulators, the band gap is large (approximately 6 eV). Electrons require a significant amount of energy to jump from the valence band to the conduction band, making them poor conductors of electricity.

5. **Importance of Band Gap**:
The size of the band gap determines the electrical conductivity of the material. A smaller band gap allows for easier electron movement, leading to better conductivity, while a larger band gap restricts electron movement, resulting in poor conductivity.