Diffusion transistor
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A diffusion transistor is any transistor formed by diffusing dopants into a semiconductor substrate. A diffusion transistor can be either a bipolar junction transistor or a field effect transistor.
Bell Labs developed the first prototype diffusion transistors in 1954[1].
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[edit] Diffused-base transistor
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.
[edit] Mesa transistor
The mesa transistor was developed at Texas Instruments in 1957. 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 contamination, thus requiring hermetic seals to prevent degradation of the transistor's characteristics over time.
[edit] Planar transistor
The planar transistor was developed by Dr. Jean Hoerni[2] at Fairchild in 1959. The planar process used to make these transistors made mass produced monolithic integrated circuits possible.
These transistors have a silica passivation layer to protect the junction edges from contamination, making inexpensive plastic packaging possible without risking degradation of the transistor's characteristics over time.
The first planar transistors had much worse characteristics than alloy junction transistors of the period, but as they could be mass produced and alloy junction transistors could not, they cost much less and the characteristics of planar transistors improved very rapidly, quickly exceeding those of all earlier transistors and making earlier transistors obsolete.
