Semiconductors are a group of materials classified according to their electrical conductivity properties. Other classifications are conductors and insulators. In their pure form, semiconductors are known as 'intrinsic'. The addition of certain foreign material, (known as a dopant), can create an extrinsic semiconductor. Introduction of a dopant material creates an excess of either electrons or 'holes', (depends on valence of dopant and intrinsic materials), within the material thereby increasing the material's conductivity.

In terms of energy bands of electrons, semiconductors are characterised by the size of the band gap between their valence and conduction bands. Put simply, this gap is the amount of thermal energy an electron requires to jump from the lower energy valence band to the higher energy conduction band in a material. Electrons in the conduction band are free to move within the material, thus contributing the conduction of electricity through the material. The size of the energy gap varies from material to material. An insulator is a material with a large band gap, with consequently fewer electrons in its conduction band. A conductor, on the other hand, has no band gap so electrons are free to move in and out of the conduction band and the material is therefore highly conductive.

Here's an interesting characteristic of semiconducting materials: As you heat up an intrinsic semiconductor, more electrons are able to jump the gap to the conduction band and the conductivity of the material increases. On the other hand, increasing the temperature of an extrinsic semiconductor causes a decrease in conductivity because the effect of the increase in number of electrons inhabiting the conduction band is outweighed by the reduction in conductivity caused by increased vibration of the material's crystal lattice. The latter is the same mechanism which causes the conductivity of conductor materials, such as metals, to decrease with increasing temperature.

The really interesting thing about semiconductors is not the size of the gap between conduction & valence energy bands but the fact that two different types of semiconductor can be created that have overlapping conducting / covalent energy levels. A junction of these two different materials (one doped to give extra holes, one doped for extra electrons), will allow electrons to pass through _only_ one way. For the electrons to flow backwards (reverse bias), there must be enough Potential for them to tunnel through the barrier, which happens in zener diodes and chips when they go 'phwttt' but not much else. Conductivity of semiconductors is _much_ worse than any metal, but is the fact that semiconductor's can make a switch, where the tunneling energy is reduced at command of a small electric flow, is the only reason that they are valuable.

This is a list of nodes related to semiconductors and semiconductor physics. The majority of these nodes do exist, but in some cases I have linked to nodes which I think should but don't exist. The node integrated circuit contains tons of links to writeups about the use of semiconductors in technology. I created a node semiconductor physics that, along with the other writeups in this node, could be a useful introduction to the topic.

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