![]() This actually proved counterproductive (at least in silicon transistors, which were soon to take over from germanium entirely) the planar process, in which the oxide is just left there, proved to be much more reliable as the oxide actually protects the junction from outside contaminants. In previous transistors, the oxide that formed during processing (due to heat and exposure to oxygen) was removed out of a concern that it could contaminate the transistor. The planar transistor was a major breakthrough, though not particularly due to its planar shape. This caused some reliability problems, which were largely solved by the invention of the planar transistor in 1959. The early double-diffused transistors were known as mesa transistors, because the transistor itself was formed on a raised part of the substrate² (Or possibly parts of the substrate were etched away after the transistor was formed I'm having trouble finding clear sources). This produced very good transistors for the time, and similar processes are still in use for certain things today. The double-diffused transistor was produced shortly after, when it was realized that the diffusion process could be done with two dopants at once of opposite types, relying on the different diffusion rates of dopants of different atomic mass to get things to line up properly internally. One of the two output terminals, either the emitter or collector, was still applied like an alloy-junction transistor, but the other was not. They started with a substrate of appropriately doped semiconductor and then exposed it to a hot gaseous dopant material in an otherwise evacuated chamber, creating a region of opposite type on the surface. ![]() ![]() This allowed much better performance than the grown-junction transistors, but it was still an inherently slow process as each transistor had to be made individually. These were produced by taking a piece of doped semiconductor and fusing beads of dopant metals of the opposite type onto it, where the alloyed areas would act as the opposite-type semiconductor to form a PNP or NPN structure. Later, alloy-junction transistors would be made. This works, but the transistors produced have poor characteristics by modern standards and they're quite expensive. ![]() The first transistors that would be recognised as bipolar junction transistors are the grown-junction transistors.¹ These were made by growing a germanium (later silicon) crystal while changing the composition of the melt it's grown out of by adding dopants. ![]()
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