When you shine light of band gap energy or higher on a semiconductor and promote electrons from the valence band to the conduction band, do you expect the conductivity of the semiconductor to (a) remain unchanged, (b) increase, or (c) decrease?

Short Answer

Expert verified
When light with energy equal to or greater than the bandgap energy (\(E_g\)) is shined on a semiconductor, it promotes electrons from the valence band to the conduction band, creating additional free charge carriers (electrons and holes) available for conduction. As a result, the conductivity of the semiconductor (b) increases.

Step by step solution

01

Understanding Energy Bands and Conductivity in Semiconductors

Semiconductors have two distinct energy bands: the valence band and the conduction band. The valence band is fully occupied by electrons, whereas the conduction band is typically empty at low temperatures. The bandgap (Eg) is the energy difference between the highest occupied energy level in the valence band and the lowest unoccupied energy level in the conduction band. Conductivity in a semiconductor mainly depends on the number of free charge carriers (electrons and holes) available to carry current through the material. The more free charge carriers there are, the higher the conductivity.
02

Effect of Light on Semiconductors

When light with energy equal to or greater than the bandgap energy (Eg) is shined on a semiconductor, it can be absorbed by electrons in the valence band. As a result, these electrons gain enough energy to be promoted to the conduction band, leaving behind holes in the valence band. This process creates additional free charge carriers (electrons in the conduction band and holes in the valence band) available for conduction.
03

Effect of Increasing Charge Carriers on Conductivity

As a direct result of the light absorption, there are now more free charge carriers available in the semiconductor to carry current. This increase in the number of free charge carriers leads to an increase in the conductivity of the semiconductor.
04

Conclusion

So, when light with energy equal to or greater than the bandgap energy (Eg) is shined on a semiconductor, the conductivity of the semiconductor (b) increases.

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