If you felt that Sally's explanation of the conduction properties in different materials was crystal clear, you can skip the following part. But if you're still a little unsure of how it works, I will try to show you another way of looking at this phenomenon. To help my explanation, I am going to use the unrealistic cup with the water-filled compartments again. The compartments equal the energy bands of the material and the water equals the electrons. This time the cup only has two compartments, one for the valence band and one for the conduction band.

In a conductor, the valence band is only partially filled. This means that, in our cup, we are going to have the valence compartment half-filled with water. If we tip the cup from side to side, we will see that it is easy for the water to move back and forth, just as it is easy for the electrons to move within the conductor.

A semiconductor at low temperature is an insulator because there is no place for the electrons to go to. The valence compartment is filled and no matter how we tip the cup there is no room for the water to move into. At room temperature, the heat (energy) makes the atoms vibrate slightly, enough for a few of the electrons to break their bonds and jump into the conduction band. If we take some water (electrons) from the valence band and move it to the conduction band, we will have place for the water (electrons) to move in both bands. If we tip our cup, water will move both in the valence and conduction band. Thus, in a semiconductor at room temperature, a small current will flow.

In an insulator, the valence band is completely filled, and as a result no electrons can move. In the cup, no water will move no matter how we tip it. The band gap between the valence and the conduction band is huge. To move water (electrons) from our valence compartment to the conduction compartment, we would need to add such an amount of energy that our cup (material) would be close to breaking before any water (electrons) would begin to move between the compartments.