Why did early transistors use germanium as their semiconductor material?

Early transistors utilized germanium as their primary semiconductor material because, during and shortly after World War II, researchers had successfully developed the necessary methods to purify it to the extreme degree required for semiconductor devices.1

Because the electrical properties of these materials are highly sensitive to even minuscule traces of impurities—tolerating only about one part per billion of certain elements—the ability to achieve this level of purity was a critical prerequisite for manufacturing functional transistors. While silicon eventually became the preferred material due to its greater abundance, lower cost, and superior performance at higher temperatures, it was significantly more difficult to work with than germanium during the early stages of transistor development.12