In 1931 Alan Herries Wilson, at Cambridge, publishes "The Theory of Electronic Semi-Conductors", applying Bloch's band theory (1928) to rigorously explain, for the first time, the difference between conductors, insulators, and semiconductors via the band-gap concept. Crucially, a semiconductor's conductivity is controllable via temperature, doping, or electric fields — exactly the physical principle enabling the transistor. Bardeen, Brattain, and Shockley at Bell Labs applied Wilson's theory in their transistor research.