Band gap

A band gap is like a gap between two levels in a playground. Just like a playground may have two slides, one on the ground and one on a platform two levels up, a material's electrons can be thought of as sitting in two different levels, the ground and the platform. The gap between the two levels is called the band gap. Materials with a large band gap don't allow any electrons to move around very easily, and so they don't conduct electricity well. Materials with a small band gap, however, allow their electrons to move around easily, and so they make great materials for things like cell phones and computers.