The electrons in atoms are contained in energy levels. When the atoms come together to form solids, the electrons then become contained in energy bands separated by gaps.
Every solid has its own characteristic energy band structure.
Solids can be categorised into conductors, semiconductors or insulators by their band structure and their ability to conduct electricity. The electrical properties of conductors, insulators and semiconductors can be explained using the electron population of the conduction and valence bands and the energy difference between the conduction and valence bands.
For a solid to be a conductor, both free electrons and accessible empty states must be available —usually the solids which meet these criteria are metals. For metals, we have the situation where one or more bands are partially filled. Some metals have free electrons and partially filled valence bands, therefore they are highly conductive. Some metals have overlapping valence and conduction bands, where each band is partially filled and therefore they are conductive.
In an insulator, the valence band is full. The gap between the valence band and the conduction band is large and at room temperature there is not enough energy available to move electrons from the valence band into the conduction band where they would be able to contribute to conduction (there are no electrons in the conduction band). Therefore, there is no electrical conduction in an insulator.
In a semiconductor, the gap between the valence band and conduction band is smaller and at room temperature there is sufficient energy available to move some electrons from the valence band into the conduction band allowing some conduction to take place. An increase in temperature increases the conductivity of a semiconductor.
During manufacture, semiconductors may be doped with specific impurities to increase their conductivity.
Doping results in two types of semiconductor: p-type and n-type.
When a semiconductor contains the two types of doping (p-type and n-type) in adjacent layers, a p-n junction is formed. There is an electric field in a p-n junction. The electrical properties of this p-n junction are used in a number of devices, e.g. transistors, LEDs, solar cells, etc.
Forward bias reduces the electric field in the p-n junction.
Reverse bias increases the electric field in the p-n junction.
LEDs are forward biased p-n junction diodes that emit photons.
The forward bias potential difference across the junction causes electrons to move from the conduction band of the n-type semiconductor towards the conduction band of the p-type semiconductor. Photons are emitted when electrons ‘fall’ from the conduction band into the valence band either side of the junction.
As LED, only function when forward biased in a circuit, LEDs connected in opposite directions in a circuit cannot conduct at the same time.
