Problem 116 - Chemistry: The Central Science

Question

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?

Step-by-Step Solution

Answer

When light with energy equal to or greater than the band gap energy is shone on a semiconductor, electrons are promoted from the valence band to the conduction band. This process increases the number of free electrons in the conduction band, resulting in a larger number of charge carriers available for the movement of electric charge. Consequently, the conductivity of the semiconductor will increase. Thus, the correct answer is (b) increase.

Step 1: Understanding the band gap

The band gap is the energy difference between the valence band (where the valence electrons reside) and the conduction band (where the electrons are free to move and conduct electricity). In semiconductors, the band gap is smaller compared to insulators. When light with energy equal to the band gap energy or higher is shone on a semiconductor, electrons in the valence band can absorb this energy and jump to the conduction band.

Step 2: Understanding the effect of promoting electrons on conductivity

In a semiconductor, the conductivity depends on the number of free electrons available in the conduction band. When electrons are promoted from the valence band to the conduction band, the number of free electrons in the conduction band increases. This results in an increased number of charge carriers available for the movement of electric charge, thus increasing the conductivity of the semiconductor.

Step 3: Answering given options (a), (b) and (c)

As we have established that promoting electrons to the conduction band increases the number of charge carriers available for electric charge movement, we can conclude the following: (a) The conductivity of the semiconductor will not remain unchanged, as the number of free electrons in the conduction band will increase. (b) The conductivity of the semiconductor will increase, due to the increased availability of free electrons in the conduction band. (c) The conductivity of the semiconductor will not decrease, as promoting electrons to the conduction band leads to a greater number of charge carriers available for electric charge movement. Therefore, the correct answer is that the conductivity of the semiconductor will (b) increase when light of band gap energy or higher is shone upon it.