2N389 Silicon Diffused Transistor
Making a 2N389 Silicon Diffused Transistor
For the full history of Texas Instruments on this site please return to The Early History of Texas Instruments Semiconductors.
The 2N389 was the first silicon diffused transistor to be commercialised anywhere. The following steps in the process were provided in 2011 by Elmer Wolff who led the development on this transistor. The drawings and some additional information are from his patent 2952896.
The key steps are summarised in his lab book with entries for May and June 1957 (scan courtesy Elmer Wolff)
1. Start with N-type silicon wafer and dice up into chips.
2. Load 100 - 200 chips into a small quartz tube along with the diffusion source. The diffusion source consists of powdered silicon loaded with both N-type impurity and a P-type impurity. Tube is evacuated and sealed, and put into a furnace for simultaneous diffusion. Both the P-type base diffusions and the N-type emitter diffusions occur at the same time. The diffusion may take about 8 hours at 1200 C. This method exploited the fact that in silicon the diffusion rate of P-Type impurities is up to 100 times faster than N-Type impurities. Thus with the right choice of impurities and their concentrations a P-Type subsurface layer could be formed because:
The N-Type impurity had lower diffusion but the higher surface concentration and
The P-Type impurity diffused in faster but had the lower surface concentration.
3. Chips now have a NPNPN structure where the surface layer is approximately 0.3 mils. The chips are fastened face down with black wax on a glass microscope slide, 10 or 20 at a time, and the backside NP layer is etched off.
4. An aluminum base ring made from soft aluminium wire is now placed on the center of the emitter side and furnace alloyed into the top of the chip.
5. The chips are electroless nickel plated all over: top, sides, and bottom in order to form the emitter and collector connections.
6. Chips are waxed back down on a glass slide with the collector side down.
7. Paint a round dot of etch resist inside the aluminum base ring on the emitter area, but leaving the emitter-base junction (a ring around the inside of the aluminum wire) bare.
8. Remove the exposed nickel.
9. Using the small brush and the red etch resist, the center of the device is painted over out to the middle of the aluminum base ring. That is, the outer half of the wire ring is bare, as seen from the top, and the inner half painted, including all the way into the center of the nickel-plated emitter region.
Etch to remove the edge portions of the chip.
10. Strip the resist off and do final clean up etch (creates moat around base and emitter contacts). Take the chips off the slide, and they're done.
11. Solder the chip in the header on the nickel backside of the collector, fasten a base connection to the aluminum wire ring, and solder the emitter wire connection to the nickel-plated emitter.
Photos of 1958 vintage 2N389 courtesy Joe Knight:
Close up of the chip:
