Question

When electric field is applied across a semiconductor
A. Electrons move from lower energy level to higher energy level in the conduction band.
B. Electrons move from higher energy level to lower energy level in the conduction band.
C. Holes in the valence band move from higher energy level to lower energy level.
D. Holes in the valence band move from lower energy level to higher energy level.

Answer 1

The electrons remain in the conduction band of the semiconductor while the holes remain in the valence band of the semiconductor. When an electric field is applied, then the potential energy decreases. The holes are positively charged in nature while the electrons are negatively charged in nature.

Complete answer:
There are two types of bands in a semiconductor; the conduction band and the valence band. In a semiconductor, electron and holes pairs are created. When an electron leaves its position inside a semiconductor, then a hole is created with a positive charge. The conduction band is at a higher energy level while the valence band is at a lower energy level. The electrons remain at the conduction band and it is called the conduction band because the electrons in the conduction band are the reason for producing electric current. Holes remain in the valence band.

When an electric field is applied across the semiconductor, the electrons being negatively charged oppose the direction of the electric field and hence move in the opposite direction of the electric field. The holes being positively charged move in the direction of the electric field. This is because when an electric field is applied, the potential energy decreases, and hence electrons move from lower energy level to higher energy level in the conduction band while holes in the valence band move from higher energy level to lower energy level.

Thus option A and C are correct.

Note: There is a portion of space between the energy levels of the valence band and the conduction band and it is known as bandgap. At the bandgap, there are no electrons or holes present. The bandgap of different semiconductors is different, and only after the application of sufficient voltage, can the electrons from the valence band move to the conduction band.