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Adiffused junction transistor is atransistor formed by diffusingdopants into asemiconductorsubstrate. The diffusion process was developed later than thealloy-junction andgrown junction processes for making bipolar junction transistors (BJTs).
Bell Labs developed the first prototype diffused junction bipolar transistors in 1954.[1]
The earliest diffused junction transistors werediffused-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 likePhilco'smicro-alloy diffused transistors the substrate was the bulk of the base.
At Bell LabsCalvin 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:[2]
Texas Instruments made the first grown-junction silicon transistors in 1954.[3] The diffused siliconmesa transistor was developed atBell Labs in 1955 and made commercially available byFairchild Semiconductor in 1958.[4]
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 contamination, thus requiringhermetic seals orpassivation to prevent degradation of the transistor's characteristics over time.[5]
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Theplanar transistor was developed by Dr.Jean Hoerni[6] atFairchild Semiconductor in 1959. Theplanar process used to make these transistors made mass-produced monolithicintegrated circuits possible.
Planar transistors have a silicapassivation 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 a switching speed much lower thanalloy 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.