Historians often label epochs of human history according to their material technologies—the bronze age, the iron age, and, most recently, the silicon age. From a physicist’s perspective, the silicon age began with the theory, experiment, and device prototyping of a new type of material: the semiconductor. Although semiconductors had been known since the late 1800s as materials with unusual sensitivities to light, direction of current flow, and method of synthesis, not until the early 1930s did Alan Wilson make the radical proposal to describe their conduction in terms of the filling of their electronic bands.1