Diffusion transistor

A diffusion transistor is a bipolar junction transistor (BJT) formed by diffusing dopants into a semiconductor substrate. The diffusion process was developed later than the alloy junction and grown junction processes for making BJTs.

Bell Labs developed the first prototype diffusion transistors in 1954.

The earliest diffusion transistors were diffused-base transistors. These transistors still had alloy emitters and sometimes alloy collectors like the earlier alloy-junction transistors. Only the base was diffused into the substrate. Sometimes the substrate formed the collector, but in transistors like Philco's micro-alloy diffused transistors the substrate was the bulk of the base.

At Bell Labs Calvin Souther Fuller produced basic physical understanding of a means of directly forming the emitter, base, and collector by double diffusion. The method was summarized in a history of science at Bell:

Texas Instruments made the first grown-junction silicon transistors in 1954. The diffused silicon mesa transistor was developed at Bell Labs in 1955 and made commercially available by Fairchild Semiconductor in 1958.

These transistors were the first to have both diffused bases and diffused emitters. Unfortunately, like all earlier transistors, the edge of the collector–base junction was exposed, making it sensitive to leakage through surface contaminats, thus requiring hermetic seals or passivation to prevent degradation of the transistor's characteristics over time.

The planar transistor was developed by Dr. Jean Hoerni at Fairchild Semiconductor in 1959. The planar process used to make these transistors made mass-produced monolithic integrated circuits possible.

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