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Chapter9   The Model 2T7 Transistor

The Model 2T7 Transistor

  • The children's electronics exhibition at the Mitsukoshi Department Store in Nihombashi, Tokyo.
In keeping with Ibuka's ideal of "having youngsters come in contact with science as early as possible," Sony today still takes an active part in science-related exhibitions aimed at children. These activities have gained Sony much public acclaim. One such event was the "Children's Electronics Exhibit" held at the main branch of the Mitsukoshi Department Store in Nihombashi, Tokyo.

Mitsukoshi's PR manager Shigeru Okada offered Sony free use of the main store's large exhibit and roof areas for one week. Okada suggested that Sony display some of its unique technology that would be of interest to children, as the exhibition period coincided with the holiday week which included Children's Day.

After much consideration, it was decided to display a transistor and a transistor radio production line complete with fifteen female workers who peered into microscopes and assembled parts. These assembly lines, along with a solar-powered helicopter and airplane, Japan's first VTR, and a fully automated driverless car, were big hits among children and adults as well.

The exhibit was a great success, easily topping 200,000 visitors over the week-long period, making it the largest drawing card in Mitsukoshi's history. In fact, the crowds were so large that ropes had to be drawn along all the staircases to control the long waiting lines. The temperature in the hall was 3c higher than the rest of the building.

By that time, many Japanese companies had begun to produce transistors. Their manufacturing processes were carefully guarded secrets, however, and the very idea of putting them on public display was unthinkable. Only Sony had the courage and resolve to expose its technology to public view.

Then in June, Sony announced the successful testing of the Esaki Diode, also known as the tunnel diode, which Sony researcher Leona Esaki invented.

Two years earlier, Iwama, then manager of the Semiconductor Department, had been trying to improve the poor yield percentage of the new 2T7 grown junction transistor. Tetsuo Tsukamoto of the Semiconductor Department had developed it to replace the earlier 2T5 model.

During the six months he was bedridden while recovering from the side effects of penicillin, Tsukamoto had constantly considered replacements for the 2T5, which had a poor production yield and performance. High frequency transistors require the creation of a thin base layer with a large amount of impurities. An emitter layer, also with many impurities, has to be deposited on top of that. The higher the effective impurity concentration in the emitter, the greater the transistor's amplification. The problem was determining the upper limit of impurity concentration. Tsukamoto hit upon the idea of using phosphorous instead of the conventional antimony as the impurity in the emitter. Immediately after his recovery, Tsukamoto conducted experiments to corroborate his theory. The results were unprecedented, and frequency characteristics five times higher than those of the 2T5 model were obtained. Beyond certain levels, antimony impedes the formation of germanium monocrystals and thus cannot be used in large amounts. Phosphorous, on the other hand, can be mixed with germanium in unlimited volumes, and the higher the concentration, the better the crystals. And because phosphorous does not diffuse as much as antimony in germanium, the thickness of the base can easily be made to design specifications.

Iwama was overjoyed with the new transistor, called the 2T7. As noted in the guinea pig article((See Part I, Chapter 8), the transistor field was no longer Sony's unchallenged domain. In fact, the transistor makers were already involved in a fierce price war. Sony felt that since its products were more expensive, it would have to win out with new products and was forced to expand into shortwave and FM radio. Thanks to the 2T7, however, it appeared as though the problem of developing a high frequency transistor for use in these new products had been resolved in a single stroke.

The Esaki Diode

  • Nobel Prize winner Dr. Leona Esaki at Sony.
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.

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.

Ibuka's Dream

  • Reailzation of a long-held dream: Ibuka inspecting a portable TV production line.
Scientists outside of Japan were the first to recognize the significance of the Esaki diode. In June 1958, Esaki, together with two other Japanese scientists, went to the International Conference on Solid State Physics in Brussels to deliver an address on tunnel diodes entitled "Solid State Physics in Electronics." In his opening address, Dr. William Shockley, one of the discoverers of the transistor and chairman of the conference, gave general introductions of the papers to be read. As he was only one of 500 presenters, Esaki had only expected an introduction of 1/500 proportion. Although he could not understand all that was being said, he did realize that Shockley was mentioning his name time and time again. Here, before the world's most eminent scientists, Shockley was lavishing praise on Esaki's diode as a promising new high frequency device. This rocketed the Esaki diode to fame.

Because the diode realized negative resistance and the tunneling effect was an extremely rapid phenomenon, the Esaki diode could be used in high frequency oscillation, amplification, and circuit switching devices. It gained even more attention and popularity for its capacity to increase the operating speed of electronic computers, which was something that American scientists had been waiting for.

The significance of Esaki's discovery was even greater considering that it was made at Sony, which was neither a large corporation nor a government-funded transistor research laboratory.

Indeed, this project embodied Sony's essence. The determination and scientist-like scrupulousness of Sony engineers combined to create this unprecedented product. In this instance, Sony engineers resolved to meet the company's demand for a higher frequency transistor. They later identified the cause of the trouble which arose during production with the fastidiousness of scientists.

The intensity with which Sony engineers pursued new challenges was truly awesome. They next devoted themselves to satisfying public demand for a transistorized television, the next logical step after the transistor radio. In doing so, they pursued research around the clock, day in and day out. To the Semiconductor Department staff, however, the very difficulties involved made the reseach and development of TV transistors all the more rewarding.

In a magazine interview in early 1959, Ibuka said that his dream for the year was the realization of a transistorized television. The TV8-301 television, the first non-projection type transistorized television in history, completed by the end of that year, was the realization of his dream.

Turning dreams into reality is the essence of the Sony ideal. And the Sony engineers were not about to disappoint people who believed that Sony lived up to its ideals. This was, however, easier said than done.

Radio and telvision are fundamentally different, the main difference being the semiconductor chips used. Transistors and diodes are suitable for low voltage and low current circuits, but are not suited for high voltage and high current circuits. Television, which has many high voltage, low current circuits, required a complete redevelopment of the transistor. More specifically, television requires frequencies one hundred times higher than radio, about twenty times more current than radio, and transistors with ten times higher voltage capacity. In other words, television requires more highly sophisticated transistors.

In September 1958, four months before Ibuka's remark about his dream, Sony staff gathered for their first discussions on transistorized television. In reality, however, the Semiconductor Department had begun developing a device for television much earlier.

Sony's "Frail Little Baby"

  • World's first direct-view portable TV "TV8-301"
By 1957, the TR-63 transistor radio had finally made inroads in the market. Secure in this knowledge, Ibuka turned his attention to longrange planning.

Ibuka's prediction that "This will be the age of silicon" led the perceptive semiconductor staff to realize that he intended to work on television. In January of 1958, Iwama ordered Tsukamoto to begin research on a silicon transistor for television-use. At Sony, transistor research and development always take the final product into account, and this project was no exception.

Television transistors used in deflection and image output in cathode-ray tubes consume a large amount of electricity. As a result, the surrounding air heats up. Thus television requires transistors that are stable under high temperatures. This is where silicon came in. Silicon, however, was not without its drawbacks.

R&D efforts ran into complications from the outset. High quality silicon crystal was hard to produce. Silicon has a much higher melting point than germanium and is extremely active chemically at high temperatures. This makes it difficult to obtain crystals which have a high level of purity. In addition, material for crucibles (silicon corrodes quartz crucibles), temperature control apparatus, and devices for pulling the monocrystals posed many more problems than those confronted during the process of creating germanium crystals.

In August, Tsukamoto's staff began research on circuits to try to determaine what type of semiconductor should be used. With vacuum-tube TVs, engineers had a rough idea of their performance capabilities once they were designed. Designing a television transistor on paper alone is not of much use, however. The crux of the design is running actual experiments over and over again. Moreover, in contrast to radio, which leaves some room for error since it involves audio alone, television is like a measurement instrument---it leaves no room for error, since all imperfections appear brightly and clearly on the screen. Extremely demanding conditions had to be met. At any rate, the fragile semiconductors were cracked and crushed into little pieces during circuit experimentation. Desktops and drawers of the staff involved in the experiments were literally full of broken semiconductor pieces. Tsukamoto's staff was preoccupied alternately with growing and depositing the semiconductors on the one hand and developing circuits and cathode-ray tubes on the other.

After a year of experimentation, an acceptable transistor was born. The semiconductors and circuits were still far from ideal, however, as was obvious in the prototype TV set that was built using the newly developed devices. The team's continued research and development paid off in November, though, when they completed a high frequency germanium transistor which was capable of receiving channels four through twelve. This was just one month before the announcement that Sony had developed the TV8-301 television.

The TV8-301 employed 23 silicon and germanium transistors, 19 diodes, and 2 high-voltage diodes. In addition, Tsukamoto and his staff developed 9 new transistor devices, including a silicon mesa transistor for horizontal deflection, a germanium mesa transistor for high frequency, and a grown silicon transistor for image output.

This achievement gained Sony worldwide recognition for its excellence in transistorized television technology and transistor technology in general. It did not, however, help to sell the new televisions. When the sets went on sale in 1960, television was still considered a luxury commodity for the average family. For the price, most people considered a large console set more practical than a portable model. In fact, most of the early Sony TV owners were either very rich or eccentric. And, to be truthful, the TV8-301 broke down often. Having gone through such a difficult delivery, it was known as Sony's "frail little baby."
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