Question

The energy gap between the valence band and the conduction band for a material is $>9eV$. Identify the material.
A) An insulator B) A metal C) An intrinsic semiconductor D) A superconductor

Answer 1

Hint: The energy gap between the valence band and the conduction band is the measure of the energy required for an electron to jump from the valence band to the conduction band. It determines the electrical conductivity of the material.

Complete step-by-step answer:
The energy gap between the valence band and the conduction band has a direct correlation between the material acting as an insulator, semiconductor, metal (conductor) or a superconductor. The electrical conductivity of the material depends upon this energy gap.
For metals (conductors) the valence band and the conduction band overlap and hence, there is zero energy gap between the valence and conduction band.
For intrinsic semiconductors, there is a moderate energy band gap between the valence and conduction band. When the temperature is more electrons can gain energy and jump up to the conduction band. The energy band gap for intrinsic semiconductors is usually $\le 3.2eV$.
For insulators, the energy band gap is very large and hence electrons cannot jump up from the valence to the conduction band easily and hence cannot partake in electric current flow. Hence, insulators are said to be poor conductors of electricity. Insulators usually have large band gaps $\ge 5eV$.
Based on the above explanation, we can conclude that since the material has such a high energy band gap of $9eV$, it must be an insulator.
Hence, the correct option is A) An insulator.
Note: Students could have gotten confused upon seeing the term superconductors in the options and wondered what it was. Superconductors are actually materials that are even more conducting than conductors. They have almost zero resistance and current flows through them almost without any resistance. Producing superconductors is very hard. However, some diamagnetic materials act as superconductors at very low temperatures.