1951 - Development of Zone Refining William Gardner Pfann and Henry Theurer Develop Zone Refining Techniques for Production of Ultra-Pure Semiconductor Materials

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1951 - Development of Zone Refining William Gardner Pfann and Henry Theurer develop zone refining techniques for production of ultra-pure semiconductor materials.

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 T. H. Keck and M. J. C. Golay, at the U. S. Army Signal Corps, Fort Monmouth, NJ and R. Emies working under the direction of Eberhard Spenke at Siemens and Halske in Pretzfeld, West Germany.

Zone-refining was first introduced in 1952 [W. G. Pfann. Trans. AIME, 194, 747 (1952)],

A small-scale float-zone refining experiment at Bell Laboratories

Illustration of the process of zone refining.

1954 floating-zone system designed by R. Emeis, at Siemens. Courtesy of: Siemens Corporate Archives, Munich.

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In 1951 William G. Pfann discovered a technique for purifying germanium. This ultrapurifying method, known as "zone refining," solved the inherent problem of melting materials such as germanium and silicon at extremely high temperatures. Pfann's method brought the purity of materials under control to the extent that it could then be applied to manufacturing semiconductors. During the latter half of the 1950s, two scientists at Bell Labs, C.H. Townes and A.L. Schawlow, collaborated on experiments that ultimately led to the discovery of the laser (light amplification by stimulated emission of radiation) and its principles. Later, additional research in this field led to the invention of the first helium-neon laser and the first continuous wave solid-state laser....it in one direction. Each zone carries a fraction of the impurities to the end of the solid charge, thereby purifying the remainder. Zone refining was first described by the U.S. scientist W.G. Pfann and was first used in the early 1950s to purify germanium for transistors. The purity achieved was hitherto unheard of—less than one part of detectable impurity in 10,000,000,000 parts of...