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?

A band gap is the energy difference between the valence band (where electrons are bound to their atoms) and the conduction band (where electrons are free to move). In a semiconductor, the band gap is small enough that electrons can be easily excited from the valence band to the conduction band. Such excitation occurs when a sufficient amount of energy is supplied, for example by shining light on the semiconductor. The conductivity of a semiconductor refers to its ability to conduct electricity. The excitation of electrons from the valence band to the conduction band creates both negatively charged electrons and positively charged holes that can move through the semiconductor, carrying electric current as they do so.

When light of band gap energy or higher is shone on a semiconductor, it provides sufficient energy for electrons to overcome the band gap and get promoted from the valence band to the conduction band. This process creates more charge carriers (electrons and holes) in the material which can contribute to the conduction of electric current.

As more charge carriers are created by shining light of band gap energy or higher on the semiconductor, both the number of free electrons and holes increase. This increase in charge carriers leads to an increase in the conductivity of the semiconductor. Hence, the correct answer is (b) the conductivity of the semiconductor will increase.