Diffusion Transistor - Double Diffusion

Double Diffusion

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 1983 in a history of science at Bell:

Fuller had shown that acceptors of low atomic weight diffuse more rapidly than donors, which made possible n–p–n structures by simultaneous diffusion of donors and acceptors of appropriately different surface concentrations. The first n–layer (the emitter) was formed because of the greater surface concentration of the donor (for example, antimony). The base formed beyond it because of the more rapid diffusion of the acceptor (for example, aluminum). The inner (collector) boundary of the base appeared where the diffused aluminum no longer over-compensated the n–type background doping of the original silicon. The base layers of the resulting transistors were 4 μm thick. ... Resulting transitors had a cut-off frequency of 120 MHz.