Silicon and semiconductors

When we left the early history of computers in Chapter 2, we had seen that logic gates were first implemented using electromechanical relays – as in the Harvard Mark 1 – and then with vacuum tubes – as in the ENIAC and the first commercial computers. These early computers with many thousands of vacuum tubes actually worked much better and more reliably than many engineers had expected. Nevertheless, the hunt was on for a more dependable technology. After World War II, Bell Labs (Fig. 7.1) initiated a research program to develop solid-state devices as a replacement for vacuum tubes. The focus of the program was not on materials that were metals or insulators but on strange, “in-between” materials called semiconductors.

In a solid, it is the flow of electrons that gives rise to electric currents when a voltage is applied. One of the great successes of quantum physics has been in giving us an understanding of the way in which different types of solids – metals, insulators, and semiconductors – conduct electricity. This quantum mechanical understanding of materials has led directly to the present technological revolution, with its accompanying avalanche of stereo systems, color TVs, computers, and mobile phones. A good conductor, such as copper, must have many conduction electrons that are able to move and thus constitute a current when a voltage is applied. By contrast, an insulator such as glass or carbon has very few conduction electrons, and little or no current flows when a voltage is applied. Semiconductors are solids that conduct electricity much better than insulators but much worse than metals. The elements germanium and silicon are two examples. The importance of silicon for computer technology is evident in the naming of California’s “Silicon Valley,” home to many of the earliest electronic component manufacturers (Fig. 7.2).