Why was zone refining important for early germanium transistors?

Zone refining was critical for early germanium transistors because it provided a method to achieve unprecedented levels of purity, which was essential for the device's functionality.1

The electrical properties of semiconductors are extremely sensitive to even the slightest trace of impurities; in fact, only about one part per billion of certain elements can be tolerated in materials used for semiconductor devices. Zone refining—a technique first described by U.S. scientist W.G. Pfann in the early 1950s—allowed for the purification of germanium to a degree where there was less than one part of detectable impurity in 10,000,000,000 parts of the material.21

By passing a series of molten zones through a solid ingot, the process effectively redistributed and concentrated impurities at the end of the charge, leaving the remainder of the material in a highly purified state. This breakthrough enabled the reliable manufacture of transistors, as it allowed scientists to control the material's electrical properties with the necessary precision.12