Insulators are materials that resist the flow of electricity. When the valence shell of an atom is full, the electrons are held tightly and are not given up easily. Some good examples of insulator materials are rubber, plastic, glass, and wood. The energy of the moving electron is divided so many times that it has little effect on the atom. Any atom that has seven or eight valence electrons is extremely stable and does not easily give up an electron.
Semiconductors are the materials that are neither good conductors nor good insulators.
Thy contain four valnce electrons and are characterized by the fact that as they are heated, their resistance decreases. Heat has the opposite effect on conductors, whose resistance increases with an increase of temperature. Semiconductors have become extremely important in the electrical industry since the invention of the transistor in 1947. All solid state devices such as diodes, transistors, and integrated circuits are made from combinations of semiconductors materials. The two most common materials used in the production of electronic components are silicon and germanium.