At school, we are learning about semi-conductors and their applications in modern electronics. One of the features of semi-conductors is that there is a small energy gap between the valence and conduction band. My teacher explains that he bands occur because in the compound, since there are millions of atoms close together, due to the Pauli Exclusion Principle the energy levels of the atoms slightly change in value, forming band. When electrons are excite form the valence to conduction band, the substance is able to conduct an electric current.
However I fairly certain this is a superficial explanation as it doesn't really explain what the conduction and valence band is and doesn't explain why electrons in the conduction band are able to conduct electricity while electrons in the valence band can't.
After some researching on the web this is my rough understanding of semi-conductors now:
The s and p orbitals of the metals combine together to form MOs. As the AOs of millions of atoms combine together, these MOs become a continuum, forming two bands. One band is the valence band which consists of 2s sigma bonding and anti-bonding orbitals while the other band is the conduction band which consists of 2p pi bonding and anti-bonding orbitals. This explains why electrons in the conduction band are able to conduct an electric current since they are conjugated (not sure if this is the correct term) across the entire substance and hence explains why electrons in metals are described as a 'sea of de-localised electrons'.
This is just my understanding which isn't that solid. Could someone please offer a more in-depth explanation into how these bands form and correct any wrong things that I said. Also I still have some questions.
One website stated that 'it is only when these bands become filled with 2p electrons that the elements lose their metallic character'. Which I interpret as that when the conduction band is completely filled with electrons, the substance isn't able to conduct electricity any more. However why is this so? Aren't there still electrons in the pi MOs which are 'de-localised' along the substance?
Also the above explanation using MOs seems to explain why these bands form. Does this mean that the explanation that my teacher gave me about the Pauli Exclusion Principle being responsible for the formation of bands is wrong and not needed?