Semiconductors Part 1: Introduction & Band Theory

 

zathris (pixabay user): https://pixabay.com/photos/cpu-chip-semiconductor-condenser-3061923/

Intrinsic to modern electronics, semiconductors are defined as ‘crystalline solids intermediate in electrical conductivity between a conductor and an insulator.’ ^[1] They have been incorporated in numerous devices including computers, smartphones, bank ATM’s, televisions, washing machines, trains and LED bulbs just to name a few.^[2]

Band theory

Band theory is helpful in visualising the difference between conductors, semiconductors and insulators. Quantum physics dictates that for isolated atoms, the energy states that electrons can occupy are quantised. However, when atoms are brought together to make solids, ‘these discrete energy levels become perturbed through quantum mechanical effects’ ^[3] As a result, the electrons can occupy a range of energies within a certain energy band.

The electrons in an atoms outer shell are situated in an energy band called the valence band. Meanwhile, the conduction band is defined as the ‘energy band that consists of free electrons that are responsible for conduction.’ ^[4] For metals, which can conduct electricity, their outer electrons are delocalised and free to move. When a voltage is applied, these electrons flow towards the positive terminal so a current (the rate of flow of charge) is produced. In the language of band theory, we can say that the valence band and conduction band overlap:

For insulators, such as wood, there is a large ‘forbidden energy gap’ between the conduction and valence bands. This makes it very difficult for electrons to move into the conduction band as too much external energy is required. Therefore, these electrons are not free to move to generate an electrical current:

In semiconductor materials like silicon, there is still an energy gap, but it is much smaller than the forbidden gap in insulators. ^[5] This means that only a low amount of energy is required to move electrons into the conduction band. In fact, this energy can be provided through thermal energy: increasing the temperature sufficiently means that ‘a finite number of electrons can reach the conduction band and provide some current.’ ^[6] At 0K, since no energy is provided, silicon behaves as an insulator as all the electrons are trapped in the valence band:

In Part 2 (https://phys-talk.blogspot.com/2020/08/semiconductors-part-2-doping-types-of.html), I will look at doping and different types of semiconductors.

Sources:

1. Encyclopædia Britannica: https://www.britannica.com/science/semiconductor

2. Hitachi: https://www.hitachi-hightech.com/global/products/device/semiconductor/life.html

3. Encyclopædia Britannica: https://www.britannica.com/science/band-theory

4. Circuit Globe: https://circuitglobe.com/difference-between-valence-band-and-conduction-band.html

5. High School Physics Explained (YouTube channel): https://www.youtube.com/watch?v=zdmEaXnB-5Q

6. HyperPhysics: http://hyperphysics.phy-astr.gsu.edu/hbase/Solids/band.html#c6