272 CHEMICAL SENSORS: FUNDAMENTALS. VOLUME 3: POLYMERS & OTHER MATERIALS Figure 5.5. Energy band gaps in materials. idea, the differences in the behavior of metals, insulators, and semiconductors were easily distinguished (Figure 5.5). Metals have energy levels very close to each other, insulators have levels very far away from each other, and semiconductors have levels in between. In addition, the theory showed that the addition