Coaxial Transistor
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The Coaxial Transistor

 With Permission, Bell Laboratories RECORD April 1949 This Reprinted form SMEC 'Vintage Electrics Vol. #2 1990



Shortly after the discovery of the Transistor, J. N. Shive of these Laboratories observed that amplification could be obtained with a germanium wedge when the emitter and collector points were placed on opposite sides of the wedge. In this construction, the germanium wedge is narrowed down to a sharp edge and the point contacts are placed on opposite sides at a point where the wedge is only a few thousandths of an inch thick.

Investigation of the wedge device seemed to indicate that the current passing between emitter and collector was actually passing through the semi-conductor and not around the wedge surface. Thus the effect was apparently not a surface phenomenon but rather a current amplification process occurring within the semi-conductor itself. If this were true, transistor action should also be possible by reproducing the wedge geometry in circularly symmetrical form, thereby providing complete shielding between emitter and collector points.

A disc of germanium 1/8 inch in diameter and 20 mils thick was cut from a thin slab of germanium by means of a hole saw. Two dish-shaped depressions were ground and lapped into the faces of the disc by means of a spherical tool. This wafer was placed in a mount and held by spring pressure.

The germanium disc, normally grounded electrically, is seen to provide an electrostatic shield between the emitter and collector points, and all three parts - emitter point, collector point, and germanium disc - are coaxial.

With the highly polished surfaces desirable for operation at higher current, it had been found with the Type A Transistor that points which were not perpendicular to the surface often had a sufficient tangential component of force to cause them to slip as they were pressed against the surface. In the coaxial construction, the two contact points are on opposite sides and thus can be exactly perpendicular to their contacting surfaces.

Characteristics are comparable with those of the Type A Transistor, which has both points on the same side of the disc.

Advantages which might be attributed to the coaxial construction are: improved stability of the points, especially on highly polished surfaces; electrostatic shielding between input and output circuits; and the avoidance of construction problems involving the placing of two spring contacts within a few thousandths of an inch of one another. Close tolerances are not completely avoided by this design, however, since the two points should be accurately in line on opposite sides of the germanium for most satisfactory operation.

MUSEUM  UPDATE

Although these transistors were never produced in any large quantity, we have two of them here at the museum!

The coaxial point contact transistor, though being an excellent study in principles, really did not increase the frequency handling ability of point contact transistors. The coaxial design was to be used later when device technology allowed the exploration of higher frequencies once the alloy and diffusion technologies were invented.

 

W. E. Kock and R. L. Wallace, Jr. with a large model of the coaxial transistor which they described in a paper at the A.I.E.E. Winter Convention (Add photo here)
 

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