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Chapter 9: Problem 23

Why are some materials semiconductors?

Short Answer

Expert verified
Some materials are semiconductors because they possess a filled valence band and an empty conduction band, which are separated by a band gap. At lower temperatures they behave as insulators, but upon heating, the electrons gain enough energy to jump the band gap and reach the conduction band, turning the material into a conductor.

Step by step solution

01

Understand what semiconductors are

Semiconductors are materials that have properties both of conductors and insulators. The capability of a semiconductor to conduct electric current can be changed drastically by the addition of certain impurity elements, which creates an imbalance in the electron population. Understanding these properties provides the basis for the comprehension of why certain materials are semiconductors.
02

Understand the role of the energy band

In many materials, electrons occupy energy bands. The bands closest to the nucleus are core bands and are generally filled with electrons. The outermost 'band' often referred to as the valence band, can either be filled with electrons or some electrons can be missing. When this outer band is partially filled, it allows for jumping of electrons and thus the material can conduct electricity – such materials are conductors. If the outer band is completely filled, the electrons aren’t free to move – rendering these materials as insulators.
03

Discern the role of the energy gap

In semiconductors, the outer band – valence band – is filled, but it's followed by an empty conduction band. The conduction band is separated by an energy gap or band gap from the valence band. At absolute zero temperature, the electrons occupy the valence band thus making the semiconductor an insulator. However, when heat is applied, the electrons gain energy to jump the band gap and reach the conduction band, turning the semiconductor into a conductor. This is why some materials are semiconductors - because of this middle ground between being an insulator at lower temperatures and a conductor at higher temperatures.

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