# A level Physics Help

Hi,I'm doing Edexcel AS Physics.The topic is about Electricity.
So,I have a question.Conduction is caused due to free electrons,and a substance can conduct if it only has free electrons.
Whether or not the substance can have free electrons,is determined by the band theory.
So,when many atoms join together to form a crystal,it forms an energy band.The outermost shell band is specifically called the valence band.The electrons in the valence band has a range of energy levels,and if supplied with the right energy,it will move to conduction band.

First of all,was my explanation correct till this point?And second of all,what does conduction band actually mean?Does it mean the electrons(valence electrons) go to higher energy levels ,or what?
I mean,what exactly is the conduction band?
Yes your explanation is correct up to that point. The conduction band is the range of energy levels that electrons can occupy above the valence band. Electrons in the conduction band are free to move and can carry an electric current. When an electron in the valence band absorbs enough energy it can jump to the conduction band leaving behind a hole in the valence band. The energy required for this transition is called the band gap. In conductors the valence and conduction bands overlap allowing electrons to move freely between them. In semiconductors the band gap is small enough that electrons can be excited into the conduction band by thermal energy or light. In insulators the band gap is large making it difficult for electrons to jump from the valence to the conduction band.
Original post by 17sagill
Yes your explanation is correct up to that point. The conduction band is the range of energy levels that electrons can occupy above the valence band. Electrons in the conduction band are free to move and can carry an electric current. When an electron in the valence band absorbs enough energy it can jump to the conduction band leaving behind a hole in the valence band. The energy required for this transition is called the band gap. In conductors the valence and conduction bands overlap allowing electrons to move freely between them. In semiconductors the band gap is small enough that electrons can be excited into the conduction band by thermal energy or light. In insulators the band gap is large making it difficult for electrons to jump from the valence to the conduction band.

Thank you so much for the explanation!
By the way,what do these conduction 'holes' do?What is their significance for conductivity?