Making a 2N389
Silicon Diffused Transistor
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:
impurity had lower diffusion but the higher surface concentration and
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
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
8. Remove the
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.
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: