The main difference between conductors, semiconductors, and insulators is because of the width of the forbidden energy gap.

Conductors, semiconductors, and insulators are three different types of materials based on their electrical conductivity. The conductivity of a material depends on the movement of electrons within its structure.

Conductors:
- Conductors are materials that have high electrical conductivity.
- In conductors, the valence band and the conduction band overlap, or the energy gap between them is very small.
- This means that there are plenty of available energy levels for electrons to move freely.
- When an electric field is applied, electrons can easily move from the valence band to the conduction band, resulting in the flow of current.
- Examples of conductors include metals like copper, silver, and aluminum.

Semiconductors:
- Semiconductors are materials that have moderate electrical conductivity.
- In semiconductors, the valence band and the conduction band are separated by a small energy gap called the forbidden energy gap.
- The forbidden energy gap is small enough that electrons can be excited from the valence band to the conduction band under certain conditions, such as high temperatures or the application of an external electric field.
- This allows semiconductors to conduct electricity, but not as efficiently as conductors.
- Examples of semiconductors include silicon and germanium, which are widely used in electronic devices.

Insulators:
- Insulators are materials that have very low electrical conductivity.
- In insulators, the valence band and the conduction band are separated by a large energy gap.
- The forbidden energy gap in insulators is so wide that electrons cannot easily jump from the valence band to the conduction band, even under extreme conditions.
- This makes insulators poor conductors of electricity.
- Examples of insulators include rubber, glass, and wood.

Conclusion:
The main difference between conductors, semiconductors, and insulators lies in the width of the forbidden energy gap. Conductors have no energy gap, semiconductors have a small energy gap, and insulators have a large energy gap. This difference in the energy gap determines the ability of electrons to move freely and conduct electricity in these materials.