Nowadays, a transistor about the size of a pencil eraser can perform the functionsof a tube in most situations. Often, the power supply can be a couple of AA cells or a 9-V“transistor battery.”Figure 19-1 is a size comparison drawing between a typical transistor and a typicalvacuum tube.360 Introduction to semiconductors19-1Transistors are smallerthan tubes.Integrated circuits, hardly larger than individual transistors, can do the work ofhundreds or even thousands of vacuum tubes. An excellent example of this technologyis found in the personal computer, or PC. In 1950, a “PC” would have occupied a largebuilding, required thousands of watts to operate, and probably cost well over a milliondollars. Today you can buy one and carry it in a briefcase. Integrated-circuit technologyis discussed in chapter 28.Semiconductor materialsThere are numerous different mixtures of elements that work as semiconductors. Thetwo most common materials are silicon and a compound of gallium and arsenic knownas gallium arsenide (often abbreviated GaAs).In the early years of semiconductor technology, germanium formed the basis formany semiconductors; today it is seen occasionally, but not often. Other substancesthat work as semiconductors are selenium, cadmium compounds, indium compounds,and various metal oxides.Many of the elements found in semiconductors can be mined from the earth. Oth-ers are “grown” as crystals under laboratory conditions.SiliconSilicon (chemical symbol Si) is widely used in diodes, transistors, and integrated cir-cuits. Generally, other substances, or impurities, must be added to silicon to give it thedesired properties. The best quality silicon is obtained by growing crystals in a labora-tory. The silicon is then fabricated into wafers or chips.Gallium arsenideAnother common semiconductor is the compound gallium arsenide. Engineers andtechnicians call this material by its acronym-like chemical symbol, GaAs, pronounced“gas.” If you hear about “gasfets” and “gas ICs,” you’re hearing about gallium-arsenidetechnology.