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Iwama and his staff were considerably relieved by the positive results
of Tsukamoto's experiments, and preparations were immediately undertaken
for mass production of the 2T7 transistor. Little did the staff realize
that a major pitfall awaited them at the outset.
The problem occurred during the bonding of the lead wire to the base
of the transistor. The transistors displayed satisfactory characteristics
with heavy phosphorous doping after drawing and cutting, but suddenly
stopped functioning the moment the lead wire was bonded onto the base.
Yield percentage was less than 10%. The production team grew more and more
anxious that they would not meet radio production schedules.
For days, all Sony engineers were mobilized for emergency
brain-storming sessions, but production remained at a standstill.
Eventually it was decided to resort back to the 2T5 transistor, despite its
poor characteristics and yield percentage. A special team was set up to
continue probing the cause of the defect in the 2T7.
Tsukamoto's team began looking for the cause by checking the
characteristics of the defective emitter junction. They soon found that
doping the base with excessively high concentrations of phosphorous
apparently destroyed the PN junction during the bonding process.
Measurements on different concentration levels were performed to determine
the maximum acceptable phosphorous concentration. Leona Esaki was called in
from the Research Department to lend assistance, while Yuriko Kurose and
Takashi Suzuki, a college student and trainee at Sony respectively,
assisted in taking the measurements.
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 | | Nobel Prize winner Dr. Leona Esaki at Sony. |
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Approximately one month after starting to take measurements, Suzuki
noticed a strange phenomenon in the high concentration phosphorous
crystals. Generally, when voltage is applied to a PN junction diode,
current tends to flow forward, with virtually no flow in the reverse
direction. Upon plotting these results on a graph, however, Suzuki found
that the reverse bias displayed larger currents and a curve with an unusual
peak appeared in the forward bias. Skeptical, Suzuki reran the tests
several times, but the results remained the same. He reported this to Esaki.
At first Esaki, too, thought it was some mistake. Suzuki insisted that
this could be demonstrated visually, however, and under Esaki's direction,
he produced the figure on a cathode-ray tube. After running several tests
and double-checking the measuring circuits, they finally realized that this
was no mistake. With this knowledge, Esaki was on the threshold of
discovering the Esaki diode.
After discovering that the defective 2T7 could be corrected by
lowering the phosphorous concentration below certain levels, Sony was able
to produce a high quality transistor. Esaki's next task was to determine
the cause of the negative resistance which was represented by the peak in
the graph. Esaki speculated that this phenomenon might be the "forward
bias tunneling effect. " According to quantum mechanics, all matter can be
treated as waves. As such, energy is concentrated at the peak of these
waves. The "tunneling effect " refers to particles which tunnel through
these waves of energy. Until then, scientists had all been preoccupied with
the reverse bias tunneling phenomenon. Esaki was the first to realize the
significance of the forward bias tunnel effect.
After conducting numerous experiments and steadily accumulating data,
Esaki's team was finally able to produce a new type of diode with negative
resistance in which current diminished as voltage increased. (Resistance
results from the proportionate increase of voltage to current. Negative
resistance occurs when the directions of voltage increase and current
increase are opposite.)
In the autumn of 1957, Esaki and his staff reported this discovery at
the Physics Society. The next year these findings were published in an
American physics journal and announced at the International Conference on
Solid State Physics held in Brussels. Although this discovery was widely
acclaimed throughout the world, the initial response by Japanese scientific
and industrial circles was cool — they virtually disregarded it at the time.
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