Materials that allow electric current to flow through easily are called conductors. Materials that do not allow electric current to flow well are insulators. And in between those two, with a fitting name, are semiconductors.
They can either be crystalline (meaning they have an orderly atomic structure) or amorphous (meaning they don’t) and just like how their electrical conductivity lies between that of a conductor and insulator, their resistivity does too.
But why do they demonstrate these properties?
This is where the band structure comes in. The band theory states that atoms have energy levels that correspond to a stationary state. These levels can be filled with electrons or partially empty.
When multiple atoms are nearby, their energy level shift due to each others’ influence.
If a large number of atoms come together, the levels form bands of energy.
In solid-state physics, the band structure of a solid describes those energy bands, or ranges of energy states, that an electron within the solid may have as allowed bands and ranges of energy that are between those bands but forbidden, as band gaps.
The valence band, also called the highest occupied band, is the range of permissible energy values that are the highest energies an electron can have. The conduction band is the lowest occupied band or the lowest range of vacant electronic states.
In the conduction band, electrons are able to roam around freely and take part in an electrical current.
In metals, the valence band and conduction band overlap, and electricity flows freely and easily through them. In insulators, there is a wide gap between the valence band and the conduction band, making it almost impossible for an electron to get excited enough to jump from one to the other, so they block the flow of electricity.
In semiconductors however, the band gap is neither too big or too small. This is beneficial because too big would mean that electrons wouldn’t be able to reach the conduction band, and too small would result in high electrical conductivity and high thermal conductivity.
Ultimately, semiconductors are highly desirable because their unique properties and atomic structure allow for control of the electron flow (conductivity), opening up the door for 100s of applications, especially in electronics.
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