In materials science we often divide materials into distinct classes. The primary classes of solid materials are ceramics, metals, and polymers. This classification is based on the types of atoms involved and the bonding between them. The other widely recognized classes are semiconductors and composites. Composites are combinations of more than one material and often involve ceramics, such as fiberglass. Semiconductors are materials with electrical conductivities that are very sensitive to minute amounts of impurities. As we will see later, most materials that are semiconductors are actually ceramics, for example,gallium nitride, the blue–green laser diode material