In terms of band theory,what is the difference:
$(i)$ Between a conductor and an insulator
$(ii)$ Between a conductor and a semiconductor

(N/A) $(i)$ The valence band of a conductor is partially-filled or it overlaps with a higher energy,unoccupied conduction band.
On the other hand,in the case of an insulator,the valence band is fully-filled and there is a large energy gap between the valence band and the conduction band.
$(ii)$ In the case of a conductor,the valence band is partially-filled or it overlaps with a higher energy,unoccupied conduction band. So,the electrons can flow easily under an applied electric field.
On the other hand,the valence band of a semiconductor is filled and there is a small energy gap between the valence band and the next higher conduction band. Therefore,some electrons can jump from the valence band to the conduction band and conduct electricity.