A crucial semiconductor technology developed at Bell Labs in the early 1950s is zone refining, which leads to ultra-pure samples of germanium and silicon with impurities as low as one part in ten billion - equivalent to a pinch of salt in three freight cars of sugar. Such ultra-pure semiconductor samples allow exquisite control of n-type and p-type regions by adding small amounts of impurities.
Chemical engineer William Pfann pioneered zone refining in 1950-51. By repeatedly passing a long tube filled with germanium horizontally through a series of electrical heating coils, he melted portions of the germanium and allowed them to recrystallize. The newly crystalline material was purer than what came before, while impurities became steadily concentrated in the molten portions, which were swept away to the tube’s ends.
This technique did not work for silicon, however, because it melts at higher temperatures (1415°C versus 937°C for germanium) and reacts with almost all other materials. Beginning in 1952, Bell Labs chemist Henry Theurer developed a variation on this technique called float-zone refining in which a rod of silicon clamped at both ends passes vertically through a heating coil. The small molten segment remains fixed in place between the solid portions of the rod due to surface tension. Using steam refining to remove the most stubborn impurities, such as boron, he produced silicon with impurity levels below one part per billion in early 1955. The process was developed independently at two other laboratories; by P. H. Keck and M. J. E. Golay, at the U. S. Army Signal Corps, Fort Monmouth, NJ and by R. Emeis working under the direction of Eberhard Spenke at Siemens in Pretzfeld, West Germany.
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