Semiconductors

Semiconductors: Semiconductors are kind of materials whose properties are different from insulators as well as from conductors.While conductors conduct a fairly high amount of electricity when an electric field is applied across their length and insulators conduct almost none semiconductors on the other hand conduct a small amount of electricity when electric field is applied to them.Lets take the example of silicon which is a semiconductor.Its conductivity is 10¹³ times higher than that of fused quartz (which is an insulator) and 10¹¹ times smaller than that of copper (which is a semiconductor).

Energy Band in Solids: In any solid the electrons are occupied in various shells or energy levels as we all know.Let us consider the example of sodium in its ground state. ( In ground state all the electrons are in their respective energy levels as against the excited state where some electrons may occupy higher energy levels).The electronic configuration of sodium is : (1s²)(2s²)(2p⁶)(3s¹), so sodium has 11electrons and they are divided into four energy levels.So, we see that the 3s energy level is not completely filled as an s orbital can take two electrons.So, this orbital is half filled. So, when as atom is isolated we get the energy distribution as:


But this was the situation in an isolated atom. When there are a lot of atoms together, they interact with each other. But, the  picture doesnsinglequotet remain same in the usual case where there are a lot of atoms in any sample. When atoms are close the outemost electrons (i.e., the 3s electrons in this case) start interacting with the 3s electrons of the neighbouring atoms.As, the atoms interact they exchange energy and so the energy of 3s energy level changes, and the change is not uniform for all the states of 3s energy level.Some states gain energy due to this interaction while some loose energy. So, the boundary of the 3s energy level doesnsinglequotet remain that sharp and instead of energy level it becomes an energy band. The same thing also happens for the inner energy level but since they are not the outermost energy level so the interaction is small and so the in their case the energy band is not as wide as that of the outermost energy level. So, the final picture can be seen as:


If we compare the figure above with the one above it then we can clearly see how the splitting of energy levels take place.So while the inner energy bands remain completely filled the outermost band remains half filled.So the energy level of a band is variable and the difference between the highest energy in a band and the lowest energy in the one above it is called band gap.

So, the energy levels which are completely filled at 0K are called valence bands the ones which are not (and they are at higher energy levels) are called conduction bands.But generally when we talk about valence band, conduction band and band gap,valence band is the completely filled band with the highest energy, conduction band is the one above than the valence band (has more energy than the valence band) and band gap is the energy gap between them.

This band gap is high in insulators, low in conductors such that even by application of electric field they conduct electric current while it is moderately high in insulators such that a small current passes through them by application of electric field.

Actually on application of electric field the electrons in valence band acquire kinetic energy and jump into the conduction band.This constitutes electric current.