Semiconductors\u2014as their name implies, neither conductors nor insulators\u2014are a curious case. In their outer band, atoms that comprise these substances have somewhere between three and five electrons, and they seem to exhibit qualities that are different from those of either a conductor or an insulator. Early in the twentieth century, physicists noted that these materials became better electrical conductors as their temperature increased\u2014the opposite of what happened with metals (and good conductors) like copper. In addition, they could in some circumstances produce an electric current when placed under a light\u2014what was known as a photovoltaic effect. Perhaps most compelling, the materials could rectify, meaning they allowed electric signals to pass in one direction only (they could, in other words, convert alternating electric current, AC, to direct current, DC). This was a useful property familiar to almost any Bell engineer. The early crystal wireless radios that so many Labs scientists grew up with depended on semiconductor crystals like silicon. Silicon crystals would process the incoming radio signal, transforming a weak AC signal into DC, so it could be heard through a headphone.<\/p>\n\n\n\n
Three Bell Labs researchers in particular\u2014Jack Scaff, Henry Theurer, and Russell Ohl\u2014had been working with silicon in the late 1930s, mostly because of its potential for the Labs\u2019 work in radio transmission. Scaff and Theurer would order raw silicon powder from Europe, or (later) from American companies like DuPont, and melt it at extraordinary temperatures in quartz crucibles. When the material cooled, they would be left with small ingots that they could test and examine. They soon realized that some of their ingots\u2014they looked like coal-black chunks, with cracks from where the material had cooled too quickly\u2014rectified current in one direction, and some samples rectified current in another direction. At one point, Russell Ohl came across a sample that seemed to do both: The top part of the sample went in one direction and the bottom in the other. That particular piece was intriguing in another respect. Ohl discovered that when he shone a bright light on it he could generate a surprisingly large electric voltage. Indeed, the effect was so striking, and so unexpected, that Ohl was asked to demonstrate it in Mervin Kelly\u2019s office one afternoon. Kelly immediately called in Walter Brattain to take a look, but none of the men had a definitive explanation. \u201cIn discussing these mysteries Ohl and I decided we needed to characterize them in some way,\u201d the metallurgist Scaff later explained. During a phone call, the two men decided to call one type of silicon p-type (for positive conduction) and the other n-type (for negative).<\/p>\n\n\n\n
It wasn\u2019t necessarily clear at the start why this was so\u2014or whether it was even important. By the early 1940s, however, Scaff and Ohl were increasingly sure that the two differing types of silicon were the product of almost infinitesimal amounts of different impurities. Atoms within semiconductors bond easily with a number of other elements. Scaff and his colleagues knew that when they cut n-type silicon (atomic number 14) into smaller pieces on a power saw, for instance, they could smell something they were sure was phosphorus (atomic number 15). None of the measurement equipment could pick up the taint, but their noses could. Later, the men also determined that p-type silicon often had faint traces of the elements aluminum (13) or boron (5).<\/p>\n\n\n\n
This was the beginning of a larger insight. Ultimately the metallurgists Scaff and Ohl agreed that certain elements added to the silicon (such as phosphorus) would add excess electrons to its outer band of electrons; those extra electrons could, in turn, move around and help the silicon conduct current, just as they might in a conductor such as copper. This was n-type silicon. On the other hand, certain other elements added to the silicon (such as boron) created additional empty spaces for electrons in the outer band\u2014these became known as holes. These so-called holes, much like electrons, could also move about and conduct current, like a stream of bubbles moving air through a liquid. This was p-type silicon. For Scaff and Theurer\u2014and, in time, the rest of the solid-state team at Bell Labs\u2014one way to think of these effects was that purity in a semiconductor was necessary. But so was a controlled impurity. Indeed, an almost vanishingly small impurity mixed into silicon, having a net effect of perhaps one rogue atom of boron or phosphorus inserted among five or ten million atoms of a pure semiconductor like silicon, was what could determine whether, and how well, the semiconductor could conduct a current. One way to think of it\u2014a term that was sometimes used at the Labs\u2014was as a functional <\/em>impurity.<\/p>\n\n\n\n