Problem 123 - 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 of band gap energy or higher is shone on a semiconductor, it promotes electrons from the valence band to the conduction band, creating more charge carriers (electrons and holes) in the material. With an increased number of charge carriers, the conductivity of the semiconductor increases. Therefore, the correct answer is (b) the conductivity of the semiconductor will increase.

Step 1: Understanding Band Gaps and Conductivity

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.

Step 2: Effect of Shining Light on The Semiconductor

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.

Step 3: Predicting the Change in Conductivity

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.

Key Concepts

Band Gap Energy

In the realm of semiconductors, the concept of band gap energy is a cornerstone. It represents the energy required to shift an electron from a lower energy state in the valence band to a higher energy state in the conduction band. This energy difference is crucial as it determines the electrical properties of the material. The size of the band gap helps classify materials into conductors, insulators, and semiconductors.

• Conductors have no band gap; electrons flow freely.
• Insulators have a large band gap, hindering electron flow.
• Semiconductors possess a moderate band gap, making them useful for control applications.

In semiconductors, if the incident energy is equal to or greater than the band gap energy, electrons can be "excited" to jump to the conduction band. This transition is vital for semiconductor functionality. It allows the material to respond dynamically to external energies like light, making it conductive under certain conditions.

Valence Band and Conduction Band

In a semiconductor, understanding the valence band and conduction band is necessary to grasp how conduction occurs. The valence band is filled with electrons that are bound tightly to their respective atoms. These electrons do not have adequate energy to break free. Thus, their ability to move and conduct electricity is limited. On the other hand, the conduction band consists of electrons having enough energy to move freely through the semiconductor. When high energy, such as light with energy equal to or greater than that of the band gap, is applied, electrons are excited from the valence band to the conduction band. This movement generates free electrons capable of conducting electricity effectively.
This transition isn't just crucial for conduction, but also for the creation of "holes" in the valence band. As electrons leave this band, they leave behind positive holes which also act as charge carriers. Understanding the behavior and interplay between these bands is vital for designing semiconductor devices.

Charge Carriers in Semiconductors

Charge carriers play an integral role in the conductivity of semiconductors. The main charge carriers are electrons in the conduction band and holes in the valence band. When energy is supplied, either by heat or light, electrons are excited from the valence band to the conduction band. This results in two crucial effects:

• Increase in free electrons - These electrons are now able to move freely and conduct electricity in the conduction band.
• Creation of holes - As electrons vacate the valence band, vacant spots or "holes" form, which are positively charged and can also facilitate conduction.

Both electrons and holes contribute to the movement of charge within the material, effectively enhancing its conductivity. This makes the semiconductor responsive to external stimuli, hence it's extensively used in technology such as solar cells, transistors, and diodes. By controlling the number of charge carriers, various electrical properties, like conductivity, can be finely tuned to suit specific applications.