1. Conductor (Metal)
In conductor the conduction band and valence band overlap each other.
Electron can easily jumped to conduction band and available for conduction of electricity.
In insulator, The Valence band is completely filled and conduction band is completely empty and the energy gap is large.
In semiconductor, The valence band is totally filled and the conduction band is empty but the energy gap between valeance band and conduction band is very small. So at very small temperature electron can jump from valence band to conduction band.