The Invention of the Transistor
Scientists continued to improve their understanding of electrons and electricity. In the late 1920s, Felix Bloch proposed the band theory, dis- cussed briefly in Chapter 4. The theory suggested that the electrons in materials are in energy bands. In some materials, especially metals, key bands overlap, and electrons can flow through the material to provide electrical-conduction behavior. In other materials, including most ceramics and organics, the bands are widely separated(a wide band gap), and the material is an electrical insulator(highly resistant to the flow of electrons). And in some ceramic materials the key bands(the valence band and conduction band) are close enough together(a small band gap) that a small external energy source, such as heat or light, can boost electrons from the valence band to the conduction band. These materials are semiconductors