How did diffusion technology improve transistor performance during the 1950s?
Negative-channel metal-oxide semiconductors (NMOS) employ a positive secondary voltage to switch a shallow layer of p-type semiconductor material below the gate into n-type. For positive-channel metal-oxide semiconductors (PMOS), all these polarities are reversed. NMOS transistors are more expensive, but faster, than PMOS transistors.
During the 1950s, diffusion technology significantly advanced transistor performance by allowing for the precise creation of extremely thin semiconductor layers, often measured in microns.1
By diffusing impurity atoms into a semiconductor surface from a hot gas, engineers could exert fine control over the penetration depth by adjusting the gas's density, temperature, pressure, and the processing time. This capability was critical for improving high-frequency performance: because the operating frequency of a junction transistor is inversely related to the thickness of its base layer, the ability to create these narrow, microns-thick bases enabled transistors to function at frequencies exceeding 100 megahertz for the first time.12
These diffused-base transistors became essential for high-frequency applications, such as FM radio and television receivers and transmitters. Furthermore, the development of a protective silicon dioxide layer during the diffusion process allowed for the stabilization of these sensitive electrical properties, eventually paving the way for the planar manufacturing process and the development of integrated circuits.1