In addition to amplifying an electric signal, triodes can work as a switch, using the grid voltage to simply turn a current on or off. During the 1930s several researchers identified rapid switching as a way to carry out complex calculations by means of the binary numbering system—a way of counting that uses only ones and zeros rather than, say, the 10 digits of the decimal system. For computing purposes, zeros and ones could be expressed as the two states of a switch, on or off. Moreover, just a few different arrangements of switches would be sufficient to perform any mathematical or logical operation.

Vacuum tubes, being much faster than any mechanical switch, were soon enlisted for the new computing machines. But because a computer, by its nature, requires switches in very large numbers, certain shortcomings of the tubes were glaringly obvious. They were bulky and power hungry; they produced a lot of waste heat; and they were prone to failure. The first big, all-electronic computer, a calculating engine known as ENIAC that went to work in 1945, had 17,468 vacuum tubes, weighed 30 tons, consumed enough power to light 10 homes, and required constant maintenance to keep it running.

By that time the search for a semiconductor alternative to vacuum tubes was well under way, aided by the insights of quantum mechanics—the physics of elementary particles. Scientists now knew that conductivity was determined by how tightly electrons were bound to an atom or molecule. In the case of the in-between semiconductors, researchers also saw that electrical behavior was strongly affected by the presence of impurities in the crystal. Some impurities, such as phosphorus, provide a surplus of electrons that are free to wander and contribute to a current. Others, such as boron, create areas of electron deficiency known as holes, and these holes can move about in the lattice structure of the crystal—another sort of electric current. During the years of World War II, researchers at AT&T's Bell Laboratories and elsewhere made great strides in handling semiconductors and various impurities, fashioning them into rectifiers in radar receivers, which had to handle frequencies beyond the reach of vacuum tubes.