The junction transistor, often now called the bipolar junction transistor or BJT, depends upon well-understood bulk semiconductor properties within its volume for amplification, rather than the mysterious behaviour around point-contacts. The first junction transistors were of the grown-junction type, in which a single crystal of germanium was grown and doped at the same time. The crystal was pulled slowly but continuously from a melt containing initially N-type impurities, to which P-type impurities were later added and left for a short while, then more N-type was added again. The result was an N-doped crystal with a thin P-type layer in it. The crystal was then cut into small blocks each forming a single NPN transistor.

These early transistors were primitive and very limited in capability. An obvious difficulty was locating the thin base layer and attaching a lead to it. Different methods of fabrication played an important part in improving them. The grown-junction process was soon superseded by the alloy diffusion technique, in which two beads of the element indium, the 'impurity' used to create P-type germanium, were fused onto a thin N-type germanium block, one on either side. Some indium diffused into the germanium, creating a PNP structure. This process was easier to control and reproduce than the grown-junction approach, yielding a thinner base layer, which gave a better high-frequency response.

Further specialised manufacturing techniques were developed, such as the surface-barrier transistor and the mesa transistor.




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