Semiconductors and p-n junctions

semiconductor

a material that act as an insulator when pure but can act as a conductor when impurities are added to them

conductor

a material that allows current to flow easily

insulator

a material that does not allow current to flow easily

outermost shell full 

electrons are tightly bound to the nucleus and are not free too move, making the material an insulator 

outermost shell incomplete

electrons are loosely bound to the nucleus and are free to move, making the material a conductor

valence band

outermost band that contains electrons, completely filled. electrons within this band are found to the atom

conduction band

electrons with energies in this band are free to move through the material, giving an electric current (partially filled)

insulator explanation

there is a large band gap between the full valance band and the empty conduction band. as there is no electrons in the conduction band and it takes a large amount of energy to move a valence electron up to it no charges can move through the material and no current flows

conduction explanation

the valence band and conduction bands overlap meaning there are electrons within the conduction band therefore electrons can move through it allowing a current to flow

semiconductor explanation 

the gap between the valence band and conduction band is smaller. At room temperature there is sufficient energy available to move some electrons from the valence band into the conduction band this allows some conduction to take place. as a higher temperature would give more energy there will be more electrons in the conduction band and as such as temperature increases the resistance of a semiconductor decreases