As mentioned earlier, the semiconductor has a conducting capacity somewhere between the conductor and the insulator. If we look closer at the materials we can see why they behave like this. Before we go on, note that contrary to what its name may suggest, the conduction band is not the only band where conduction of a current may occur. Conduction is equally possible in the valence band.

In a good conductor like a metal, the highest energy band with electrons (valence band) is only partially filled. This means that the electrons can accelerate. In other words, they gain energy so that they can transfer to higher energy levels that are empty. Simply put, in a conductor there is plenty of room for the electrons to jump from an occupied state to an empty one.

In a semiconductor, the distance between the valence band and the conduction band is fairly small. At a low temperature the semiconductor has a completely filled valence band. There are no empty energy positions available for the electrons to be accelerated in. Therefore, at low temperatures the semiconductor behaves like an insulator. At room temperature, the atoms in the semiconductor material vibrate enough so that a few electrons may escape from their positions in the valence band into unoccupied positions in the conduction band. The conduction at room temperature, however, is so small that no significant current can be said to pass.

In the insulator, the valence band is completely filled. Also, the band gap to cross between the valence and conduction band is very large. No electron will be able to make the jump to the conduction band at room temperature. Thus, there is no possibility for an insulator to conduct a current.