The Point-contact Transistor
This page ©2012 Andrew Wylie all rights reserved
The first transistor was a point-contact transistor. This device used a wafer of N-type germanium as the base block, into which were pushed two phosphor-bronze wires, similar to the 'cats whisker' of a radio crystal set. Brief high-current pulses were used to fuse the wires to the germanium, a technique called 'electrical forming'. This caused some phosphorus to diffuse from the wires into the germanium, creating P-type regions around the points. If forming was done correctly, a PNP structure with a narrow N region was created: the conditions needed for transistor action.
Point-contact transistors were only manufactured for a few years before being superseded by the junction transistor. They are rarely mentioned in modern electronics books, and if they are, they are usually dismissed as some inferior early aberration. This is unfair to a device which was never fully understood theoretically, and which possessed its own unique solid- and surface-state physics.
It is true that the forming process was difficult and unreliable, a major failing. However, it is a common misconception that point-contact devices were fragile. This is certainly untrue, as contemporary accounts show. Devices were tested up to 10,000g acceleration without problems.
Point-contact transistors exhibited characteristics which reveal how different they were from junction types. In particular, they had a common-base current gain ('alpha') well in excess of one, and they also exhibited negative resistance, useful in oscillators and switches. Junction transistors always have alpha less than one.
Of course, point-contact transistors had no standardised numbering scheme and came in all sorts of weird shapes. Two are shown at the top of this page. In my opinion, they are the most collectable of all semiconductor devices.