Making a Germanium Single Diffused Mesa Transistor
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Germanium mesa single diffused transistors we developed by Lee at Bell Laboratories. As a licensee TI had access to this information and developed similar transistors: initially the 2N623 and then the high performing 2N1142 series.
These transistors were developed by Elmer Wolff who has kindly provided these details:
1 Make a P type Germanium wafer typically 1 inch square from the largest crystal that could be grown at the time. Multiple transistors were made on a single wafer.
2 Polish one side of the Germanium wafer.
3 Diffuse an N type layer and lap the unpolished back of the wafer.
4 Metal coat the back of the wafer.
5 Evaporate the P type aluminum emitter which produced a P type germanium re-growth after micro alloying to wafer.
6 Evaporate the Gold Base contact The gold was doped with N type material which produced the base contact to the transistor. Both evaporations were accomplished at an elevated temperature which achieved the micro alloying process.
7 Mask an area that covers both the emitter stripe and the base stripe keeping this area as small as possible without exposing either the emitter or base stripes. The area of the potential mesa defined the capacitance of the transistor which defined the maximum frequency capability of the device.
8 Etch the device to open the base to collector junction forming the mesa.
9 Clean the structure.
10 Dice the wafer into individual transistors.
11 Alloy the die to a header.
12 Bond an aluminum wire from the emitter and base contacts to the respective header contacts. Weld on the can.
13 Some devices were sealed with cans that had a copper tube into the top of the can permitting a vacuum to be drawn on the finished device. The copper tube was sealed by cold welding while under vacuum. These devices were used in the transatlantic cable repeaters, therefore the devices needed to last a very long time!
Courtesy Elmer A Wolff 6/13/2011