Problem 123 - Chemistry The Central Science In Si Units

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

The conductivity of the semiconductor will increase.

Step 1: Understand the Basics of Semiconductor Conductivity

In a semiconductor, conductivity depends on the availability of charge carriers. Electrons in the conduction band and holes in the valence band are the charge carriers. Promotion of electrons from the valence band to the conduction band increases the number of charge carriers.

Step 2: Identify the Effect of Shining Light

When light with energy equal to or greater than the band gap energy is shone on a semiconductor, it provides enough energy to move electrons from the valence band to the conduction band, creating electron-hole pairs.

Step 3: Assess the Impact on Conductivity

As the number of electrons in the conduction band increases, the number of holes in the valence band also increases. This leads to more free charge carriers being available to conduct electricity, thereby increasing the semiconductor's conductivity.

Step 4: Conclude the Expected Change in Conductivity

Since the process of promoting electrons increases the number of charge carriers, the conductivity of the semiconductor is expected to increase when exposed to light with energy equal to or greater than its band gap energy.

Key Concepts

Band Gap Energy

Band gap energy is a crucial concept in understanding how semiconductors work. It is defined as the difference in energy between the valence band, which is the highest range of electron energies where electrons are normally present, and the conduction band, where electrons can move freely and contribute to conductivity.
This energy gap can vary between different materials and determines whether a material behaves as a conductor, an insulator, or a semiconductor. The band gap provides a barrier that electrons need energy to overcome to move from the valence band to the conduction band.
When light with energy equal to or greater than the band gap energy is absorbed by a semiconductor, it can provide the necessary energy for electrons to jump from the valence band to the conduction band. This process generates charge carriers and is pivotal to increasing the conductivity of the semiconductor.

Charge Carriers

Charge carriers are the particles that carry electrical charge through a conductive material. In semiconductors, these are mainly electrons and holes:

• Electrons are negatively charged particles that move to higher energy levels when they gain sufficient energy.
• Holes, on the other hand, represent the absence of an electron in the valence band and can be thought of as positive charge carriers.

Both types of charge carriers are essential for the flow of electricity in semiconductors. When electrons are excited from the valence band to the conduction band, more charge carriers are created, which improves the semiconductor's ability to conduct electric current. The number of available charge carriers directly affects the material's conductivity, which increases with the creation of additional electron-hole pairs.

Electron-Hole Pairs

Electron-hole pairs play a vital role in the function of semiconductors. An electron-hole pair is created when an electron gains enough energy to leave its position in the valence band and move to the conduction band.
Upon this transition, the electron contributes to conduction while leaving behind a hole. These holes appear as positive charges in the valence band because they are sites that can accept electrons, allowing for the flow of electric current.
Therefore, when a semiconductor is illuminated with light at or above its band gap energy, numerous electron-hole pairs are generated. This significantly increases the number of charge carriers, thereby enhancing the semiconductor's conductivity.
The increase in electron-hole pairs also allows for more efficient energy transfer and electrical efficiency, which is crucial in applications like solar cells and semiconductor devices.