What is the concept of valence and conduction bands in semiconductors?

The valence and conduction bands in semiconductors determine their electrical conductivity.

Semiconductors are materials that have electrical conductivity between that of conductors and insulators. The valence band is the highest energy band in which electrons are normally present at absolute zero temperature. It is filled with electrons that are tightly bound to the atoms in the crystal lattice. The conduction band is the lowest energy band that is empty at absolute zero temperature. It is separated from the valence band by an energy gap, called the band gap.

When an external energy is applied to the semiconductor, such as heat or light, electrons can be excited from the valence band to the conduction band, leaving behind a hole in the valence band. The movement of these electrons and holes through the semiconductor is what allows for electrical conductivity.

The size of the band gap determines the electrical conductivity of the semiconductor. In materials with a small band gap, such as silicon, thermal energy is enough to excite electrons across the band gap and allow for electrical conductivity. In materials with a larger band gap, such as diamond, more energy is needed to excite electrons across the band gap and allow for electrical conductivity.

In summary, the valence and conduction bands in semiconductors determine their electrical conductivity. The movement of electrons and holes through the semiconductor is what allows for electrical conductivity, and the size of the band gap determines the amount of energy needed for this movement to occur.