Diffusion enabled the drift transistor: where the doping profile across the base could be graduated to improve the rate of carrier migration and hence frequency response. This was solved by moving from alloy doping to gas phase (diffusion) doping which enabled better control of transistor geometry. But it suffered from poor performance in the RF range. The alloy junction transistor was better suited to mass production and became the method of choice early on. Rate growing and the later variants gave significantly improved performance but were extraordinarily expensive to produce and only viable for military use.Īn alternative approach started with a wafer of P or N-type semiconductor and diffusing in dopants in a controlled manner to create an NPN or PNP transistor. The early grown junction transistors had very poor frequency response due to the width of the base and base-collector capacitance. Since this approach created large single crystals, transistors had to be cut from the crystal in the form of a small bar that had the base layer at its centre. The early junction transistors were variants of the grown junction types in which single crystals of germanium (originally) and later silicon were pulled from a melt and manipulated to create P-N junctions. They were rapidly relegated by the junction transistor invented by William Shockely. From an evolutionary point of view these were dead ends.
The first transistor, the point contact transistor developed at Bell Laboratories and the later surface barrier transistor developed by Philco are examples of metal-semiconductor types. P-N Semiconductor junction transistors from 1950 Metal-Semiconductor transistors from 1947Ģ. A taxonomy of transistor types begins with the junction type:ġ.
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