As shown in the figures below, there are four possible band configurations.
In cases of (c) and (d), insulator and semiconductor, the bands are completely filled and electrons have no mobility, because like the cars in the figure below, the electrons cannot move because there are no available open spaces to move to.
In the cases of (a) and (b), empty energy states are readily available and electrons (with a little bit of thermal energy) are able to speed through the material, similar to the cars pictured below.
In the case of semiconductors, applying a voltage can boost the electrons across the gap. This would be like kicking one of the cars from the traffic jam over a medium to an unpacked highway. Thus, the semiconductor can be changed from being an insulator (off) to a conductor (on). We will look at one aspect of this behavior in the next section.