+THE LOST HISTORY OF THE
How,TRANSISTOR 50 years ago,Texas Instruments and Bell Labs pushed electronics into the silicon age BY MICHAEL RIORDAN
44 IEEE Spectrum | May 2004 | NA he speaker’s words were at once laconic and electrifying. Teal replied, pulling several out of his pocket to the general amaze- “Contrary to what my colleagues have told you about the ment and envy of the crowd. Then, in a bit of quaint but effec- T bleak prospects for silicon transistors,” he proclaimed in tive razzle-dazzle, he cranked up a record player, which began his matter-of-fact voice, “I happen to have a few of them here blaring out the swinging sounds of Artie Shaw’s big-band hit, in my pocket.” “Summit Ridge Drive.” Amplified by germanium transistors, the Silicon transistors? Did he say silicon transistors? music died out instantly as Teal dunked one into a beaker of Yes—among the few in the world at that moment. It was hot oil. But when he repeated his demonstration immersing a sil- 10 May 1954. icon transistor instead, the music played on without faltering. A long and till-then uneventful session on silicon devices had As his talk ended, Teal mentioned that copies of his paper on been winding down at the Institute of Radio Engineers (IRE) the subject, innocuously titled “Some Recent Developments in National Conference on Airborne Electronics, in Dayton, Ohio. Silicon and Germanium Materials and Devices,” were available There, a parade of engineers and scientists were lamenting the near the rear door. A crowd stampeded back to get them, leaving sobering challenges of developing and eventually manufactur- the final speaker of the session without an audience. Minutes later, ing silicon transistors. Amid the torpor, scattered attendees were a Raytheon engineer was overheard in the lobby shouting into a stifling yawns, glancing at watches, and nodding off. But that was telephone: “They’ve got the silicon transistor down in Texas!” before Gordon Teal of Texas Instruments Inc. made his surpris- ing announcement—and jaws dropped in disbelief. IN THE BEGINNING: Gordon Teal [left] directed the development of the silicon transistor at “Did you say you have silicon transistors in production?” Texas Instruments. William Shockley [middle] led the team at Bell Telephone Laboratories that asked a stupefied listener about 10 rows back in the audience, developed the very first transistor, which was made of germanium. TI’s silicon device with its three long leads became famous, making the Texas upstart the sole supplier of silicon transistors which now began to perk up noticeably. for several years in the 1950s. Morris Tanenbaum [right] at Bell Labs actually made the first “Yes, we have three types of silicon transistors in production,” silicon transistor, but he felt “it didn’t look attractive” from a manufacturing point of view.
May 2004 | IEEE Spectrum | NA 45 At the time, the silicon transistor seemed to be one of the first February 1951, publishing the results a year later. He added specific major breakthroughs in transistor development not to occur at Bell impurity atoms to the molten silicon to alter the electrical prop- Telephone Laboratories in Murray Hill, N.J., where physicists John erties of crystals drawn from it. Elements from the fifth column Bardeen and Walter Brattain had invented the transistor in December of the periodic table—arsenic or antimony, for example—create an 1947. Their device featured two closely spaced metal points jabbed excess of electrons in the tetrahedral crystal structure, yielding delicately into a germanium surface—hence its name, the “point- n-type silicon. Elements from the third column, such as boron or contact” transistor. They called one point the “emitter” and the other gallium, create a deficit of electrons (usually regarded as an excess point the “collector,” while a third contact, known as the “base,” of holes), yielding p-type silicon. By adding first one kind of impu- was applied to the back side of the germanium sliver. A positive elec- rity and then the other to the molten silicon from which they slowly trical bias on the emitter enhanced the conductivity of the germa- withdrew the crystal, Teal and Buehler formed transition regions nium just beneath the collector point, amplifying the output current called pn junctions between the two types of silicon. Small bars cut that flowed to it from the base. across these junctions act as diodes when Bell Labs achieved a long string of firsts a potential is applied across them through in the years following that momentous electrical contacts on the two ends. invention, which it announced six months Meanwhile, Calvin Fuller was beginning later at a 30 June 1948 press conference in “They’ve got experiments in an adjacent lab on diffus- New York City. Among its major advances ing impurity atoms from hot gases into the was the so-called junction transistor, first the silicon germanium or silicon surface—one of the conceived the previous January by William major technology milestones on the road to Shockley, who led the group that included transistor the integrated circuit. By December 1953 Bardeen and Brattain. He figured that much Fuller was so successful that Shockley better transistor performance and reliabil- down in Texas!” started building a new research team to ity could be realized by eliminating the frag- attempt to fabricate silicon transistors using ile point contacts and instead forming the the technique. And early in 1954, Fuller and emitter, base, and collector as a single semi- Gerald Pearson formed pn junctions by dif- conductor sandwich with three different layers [see sidebar, fusing a thin layer of boron atoms into a wafer of n-type silicon, mak- “Transistors 101: The Junction Transistor”]. Current flowing from ing a hole-rich p-layer on its surface. These large-area diodes gen- emitter to collector in Shockley’s device could be modulated by an erated substantial current when sunlight fell on them. On 25 April, input signal on the base. Bell Labs trumpeted this achievement as the “solar battery,” the Teal (then working at Bell Labs) and his fellow physical chemist first photovoltaic cell operating at efficiencies near 10 percent. Morgan Sparks successfully fabricated the first working junc- tion transistor from a germanium crystal in April 1950. But—partly + + + because the frequency response of early junction transistors was inferior to that of point-contact devices—Bell Labs held off By then TI had made its first silicon transistor—under Teal’s announcing this achievement for over a year, until 4 July 1951. Five general direction. Back at Bell Labs, he had become homesick for his years later, Bardeen, Brattain, and Shockley shared the Nobel Prize native Texas, where he had grown up a devout Baptist in South Dallas for inventing this revolutionary solid-state amplifier. and pursued his undergraduate studies in mathematics and chem- Their brilliant pioneering work has overshadowed much of the istry at Baylor University, in Waco. Restless in Murray Hill, N.J., and subsequent development years of the transistor, including the looking for more responsibility, Teal responded to an ad in The crucial change from germanium to silicon in the mid-1950s. That New York Times for a research director at TI. He met with TI vice shift in semiconductor material proved essential to the device’s president Pat Haggerty, who offered him the position. He began there glorious future as the fundamental building block of virtually all on 1 January 1953, bringing with him his vast expertise in growing of today’s integrated circuits. For germanium, to put it simply, was and doping semiconductor crystals. just not up to the task. Under Haggerty’s leadership, TI was moving aggressively into The material does have advantages: it is far less reactive than military electronics, then burgeoning with the Cold War in full silicon and much easier to work with because of its lower melting swing. The Dallas company had been founded during the 1930s as temperature. And current carriers—electrons and holes—flow through Geophysical Services Inc., developing and producing reflection germanium more rapidly than through silicon, which leads to higher seismographs for the oil industry. During World War II, it snagged frequency response. But germanium also has serious limitations. For a U.S. Navy contract to supply airborne submarine-detection example, it has a low band gap (0.67 electron volts versus 1.12 eV for equipment; afterward it continued to expand its activities in mil- silicon), the energy required to knock electrons out of atoms into the itary electronics, reorganizing itself as Texas Instruments Inc. in conduction band. So transistors made of this silvery element have 1951. By the time Teal arrived, the firm had almost 1800 employ- much higher leakage currents: as the temperature increases, their del- ees and was generating about US $25 million in annual sales. icately balanced junctions become literally drowned in a swarming The company was also beginning to manufacture what were called sea of free electrons. Above about 75 °C, germanium transistors quit grown-junction germanium transistors under the direction of engi- working altogether. These limitations proved bothersome to radio neer Mark Shepherd. He had attended a 1951 Bell Labs symposium on manufacturers and especially the armed services, which needed sta- transistor technology with Haggerty, where he listened to a Teal work- ble, reliable equipment that would perform in extreme conditions. shop on growing semiconductor crystals. In early 1952, after much Nowhere were these concerns appreciated more than at Bell Labs, wheedling and cajoling by Haggerty, TI purchased a patent license which led the way into silicon semiconductor research during the to produce transistors from Western Electric Co., AT&T’s manu- early 1950s. Working in its chemical physics department with tech- facturing arm, for $25 000. By the end of that year, it was already man- nician Ernie Buehler, Teal grew single crystals of silicon and “doped” ufacturing and selling them under Shepherd’s leadership.
them with tiny impurities to make solid-state diodes in Early the next year, Teal was back in Dallas organizing TI’s PREVIOUS SPREAD: TEXAS INSTRUMENTS (TEAL, SHEPHERD, AND TRANSISTOR); MORRIS TANENBAUM
46 IEEE Spectrum | May 2004 | NA TRANSISTORS 101: THE JUNCTION TRANSISTOR To understand how a transistor works, first consider the lowly have far to go before reaching the collector) and using low dop- diode. It is a simple union of the two most fundamental kinds of ing (electrons cannot easily find vacant holes to fill). The voltage semiconductor, known as n-type and p-type. Both conduct across the base-emitter junction provides the electric field that current, but the n-type does it with electrons, while the p-type drives electrons from the base into the collector. depends on electron deficiencies, better known as holes. With the emitter-base junction forward-biased, a varying volt- Joining these two types of semiconductors forms what is age put on top of it—an input signal—varies the depletion region, known as a pn junction at their boundary. This is the core of a which in turn varies a relatively large current flowing through the semiconductor diode, which conducts current in one direction. device. Add a load resistor in the collector circuit, and that small Connect a battery’s positive terminal to the n-type material varying input produces a much larger varying collector voltage: [figure A, top] and electrons are attracted to that terminal, while the transistor amplifies the signal at the base. Depending on the holes in the p-type material move toward the negative terminal. In circuit, the result will be current, voltage, or power amplification. other words, charge carriers stream away from the junction, Although bipolar junction transistors have been surpassed expanding a barren volume, aptly called the depletion region. The for many applications by various forms of field-effect transistors, diode is said to be reverse-biased, and hardly any current flows. bipolars remain popular for applications involving high- Now reverse the battery connections [figure A, bottom]. frequency signals. They’re found in countless modern electronic Electrons in the n-type material move toward the junction and are devices, including broadband Internet modems, set-top constantly replenished by the battery. Meanwhile, holes in the boxes, DVD players, and CD-ROMs. —Alfred Rosenblatt p-type material stream toward the junction, repelled by the posi- tive battery terminal. The depletion region shrinks tremendously + - FIGURE A as holes and electrons combine at the junction, neutralizing one another, as more stream in on either side from the battery. The Wide depletion region diode is said to be forward-biased; current flows easily. Thus, a diode can control the direction of current, but not how large it is. n material p A transistor, on the other hand, can control how much current material goes through it and also act as an amplifier. The simplest transis- tor has three parts: an emitter, a base, and a collector. Think of Holes the transistor as a sandwich of two pn junctions back to back [figure B] in either npn or pnp order; they operate similarly. Reverse-biased pn In an npn transistor, for example, the n-type emitter has junction Electrons many extra electrons, the relatively thin p-type base has a small Very low current flow number of holes, and the n-type collector has a moderate num- ber of electrons. (Junction transistors are also known as bipolar devices because, in the emitter, holes and electrons flow in - + opposite directions.) A transistor amplifier takes a small, varying voltage—an input signal—between the base and the emitter, and Narrow depletion region n material uses it to control a larger current flowing from the emitter to the collector. That’s the output. The key agents in this amplification are the depletion regions. With two pn junctions in the device, there are two depletion regions: one between the emitter and the base, the other between the base and the collector. p material First, the emitter-base diode is forward-biased by a voltage Forward-biased source [left in figure B]. Electrons flow from the emitter into the pn junction base. The base-collector diode, on the other hand, is reverse- biased, so that holes will not flow into the base, which would intercept any electrons coming across from the emitter and C FIGURE B therefore block current from flowing through the device. C With this setup, the current through the transistor, from emit- + l ter to collector, is controlled by the depletion region around the n material emitter-base junction. When it is thick, the current is choked off; V B lB Collector when it is thin, lots of current flows through the device. But hold p material on—when it is thin, and electrons shoot across the emitter-base - B l junction, aren’t they blocked by the fat depletion region around + Base the base-collector junction? No—the base is narrow, so the Hole flow Electron V + momentum of the electrons pouring in from the emitter brings n material flow Hole them close to that junction. From there, the positive voltage at the V - flow Electron junction then sweeps most of the electrons into the collector. flow V Emitter JOHN MACNEILL JOHN l Only a few are lost in the base as they move into the vacant holes. E - 1 The transistor is designed so that the flow of electrons from E l emitter to collector is very sensitive to the current into the base. E This is done by making the base very thin (so electrons don’t E
May 2004 | IEEE Spectrum | NA 47 research department. Haggerty had hired him to build a team of “The Origins of the pn Junction,” IEEE Spectrum, June 1997.] The scientists and engineers that could generate enough ideas and tech- main problem was the extinction of so-called minority carriers nologies to keep the firm poised at the leading edge of the explod- (electrons in p-type or holes in n-type layers) due to impurities in ing semiconductor industry. Teal was up to the challenge. He was the base layer. Electrons will easily “recombine” with holes at any introverted and difficult to work with, but also smart and stubborn. impurity centers within the base. Consequently, too few of these These qualities had served him well at Bell Labs, where he pur- minority carriers could survive while crossing this daunting bridge sued his crystal-growing research in the late 1940s, working doggedly between emitter and collector to achieve sufficient current gain, or after hours with almost no support from management. Perhaps amplification, in silicon. The only solution to this problem, other most important, this pioneering research had made him a minor than struggling to purify the silicon, was to make the base layer celebrity in the fledgling industry, which would prove crucial in hir- extremely thin so that the minority carriers would have some chance ing bright young people for a group he had to create from scratch. of making it from one side to the other. “We could never have attracted the stable of people that we did Adcock, Teal, and their team wrestled with these problems for without him,” Shepherd admitted in a 1993 interview. “And we got over a year. Then, in April 1954, using a special, high-purity silicon some really outstanding scientists in those days.” purchased from DuPont at $500 a pound, they managed to grow a Among his new hires was Willis Adcock, like Teal a physical suitable npn structure with an emitter region carefully doped to chemist with a Ph.D. from Brown University, in Providence, R.I. He enhance current gain and a p-type base layer about 1-mil (25 micro- had been working for a natural gas company meters) thick. Cutting a half-inch (1.27-centi- in Oklahoma and joined TI early in 1953. meter) bar from this crystal and attaching Adcock began leading a small research group electrical contacts on the morning of focused on the task of fabricating “grown- 14 April, Adcock’s group prepared to test junction silicon single-crystal and small- Smart, stub- it. Soon Haggerty got an excited call from signal transistors that would meet military Teal urging him to come see a demonstra- environmental conditions,” according to Teal, born, and intro- tion. A few minutes later, “I was observing who viewed this as the principal short-term transistor action in that first grown-junction goal for his new research department. transistor,” Haggerty recalled at TI’s It was no easy task at the time. Because verted, Gordon 25th-anniversary celebrations in 1979. It was of a high melting temperature of 1415 °C a defining moment for the budding semi- and its great reactivity, the molten silicon Teal was up to conductor company. Realizing that another from which crystals are drawn interacts company might well achieve the same break- with just about any crucible that can con- the challenge through, Teal hurriedly wrote a paper for tain it. Even fused quartz slowly reacts presentation at the Dayton conference. And with the melt, contaminating it with oxy- held his breath after Bell Labs announced gen and other impurities that subse- the silicon solar battery later that month. quently find their way into the silicon crystal, degrading its elec- trical performance. And most of the silicon samples then + + + available from suppliers came with substantial impurities. Unlike germanium, which could be purified using zone-refining Another company, in fact, had already fabricated a working sili- techniques so that impurities could be reduced to about one part con transistor a few months earlier. In January 1954, Morris per billion, the purest silicon available in those days had much higher Tanenbaum made one while working as a member of Shockley’s levels. And while silicon pn junctions had been fabricated for more research group at Bell Labs. But the world’s dominant semi- than a decade, ever since Russell Ohl first achieved this feat at conductor company kept this achievement under wraps, while the Bell Labs in 1940, making a successful npn or pnp junction transis- Texas upstart rushed to announce it. tor from this semiconductor material was far more difficult. [See Tanenbaum had come to Bell Labs in June 1952 after earning degrees
TRANSISTOR FIRSTS: Bell Labs’ junction transistor, of germanium, was fabricated in 1950 [left]. Texas Instruments’ commercial silicon transistor came four years later. LEFT: LUCENT LEFT: TECHNOLOGIES INC./BELL LABS;TEXAS INSTRUMENTS RIGHT:
48 IEEE Spectrum | May 2004 | NA ating it at up to 500 megahertz. Tanenbaum spearheaded the effort to duplicate this device in silicon, succeeding on 17 March 1955, with an npn transistor that worked at up to 120 MHz. Thus, there was little enthusiasm for the rate-grown silicon transistors that he had developed, and Bell Labs made no effort to publicize the achievement. Tanenbaum presented his results at the IRE Solid-State Device Research Conference in June 1954. During the question-and-answer session afterward, he recalls, Teal men- tioned similar work that had been done at TI—but was cagey about specifics. Later that year Tanenbaum submitted a paper about his research on rate-grown silicon transistors to the Journal of Applied Physics, where it was finally published in June 1955. By then the semiconductor industry was on the verge of yet another fundamental shift. At the 1955 Solid-State Device Research Conference held that same month, few people mentioned rate- grown transistors. Everyone there was talking excitedly about the newest breakthrough: diffusion. And Shockley was getting ready to leave Bell Labs to start his own semiconductor company focused on silicon transistors. SILICON PRECURSOR: Gordon Teal (then at Bell Labs) [left] and fellow physical chemist Morgan Sparks successfully fabricated the first working junction transistor from a germanium crystal. + + + in chemistry and physical chemistry at Johns Hopkins University, It is hardly surprising that the silicon transistor was invented in Baltimore, and Princeton University, in New Jersey. He started out twice, in two seemingly independent achievements just months apart. in the chemical physics department, growing large single crystals of By 1954 the crucial underlying technologies of silicon purification and various semiconductors and testing their properties. In late 1953 crystal growth were at a point where the silicon transistor was per- Shockley invited him to join the team being formed to push toward haps inevitable, given the market demands—which were quite dif- silicon transistors. Tanenbaum continued working with Buehler, Teal’s ferent for the two companies. TI was focused on military markets for former technician, whom he describes as “a master craftsman in transistors as replacements for the bulkier and far more fragile vac- building apparatus and growing semiconductor crystals.” uum tubes. The U.S. armed services, among its biggest customers, Buehler was working on a technique known as rate growing. The were willing to pay a big premium for transistors that performed uni- rate at which impurity atoms (such as gallium and antimony) are formly and flawlessly over a wide range of conditions. Bell Labs’ largest incorporated from the melt into the crystal depends to a great extent “customer” was AT&T’s Bell Telephone System, which needed rugged, on the crystal’s growth rate—on how rapidly it is being pulled from long-lived semiconductor switches that were truly “off” when they the melt. Both impurities are present in the melt simultaneously, were supposed to be off. Because of high leakage currents, espe- but the rate at which either one crystallizes out depends on the cially at elevated temperatures, germanium transistors simply could pulling speed. This process enabled the team to make much nar- not satisfy either of these important customers. rower base layers, just 13 to 25 micrometers (µm) thick, which proved It is also obvious that the two achievements had common tech- to be crucial in limiting the extinction of minority carriers. nological roots reaching back to the pioneering crystal-growth Tanenbaum cut a half-inch bar from one high-purity silicon crys- research of Teal and Buehler at Bell Labs from 1949 to 1952. Teal tal that Buehler had grown using special samples from DuPont; then brought this expertise with him to TI, although perhaps not the he attached an aluminum lead to the narrow base layer and carefully rate-growing techniques developed a bit later by Buehler. The two reheated the silicon to restore the layer’s p-type behavior. On groups both benefited from the fact that DuPont saw a growing 26 January 1954, according to his logbook, he achieved sufficiently market for high-purity, “semiconductor-grade” silicon and was high electron current and hence amplification in an npn silicon tran- beginning to supply small samples of the stuff in 1954. In both sistor. “I believe these were the first silicon transistors ever fabri- cases, the road to the silicon transistor had to cross a narrow, high- cated,” says Tanenbaum, savoring the moment in an interview nearly purity bridge made of the element. half a century later. Amidst all else that was happening at Bell Labs in the early 1950s, “When we made these first [silicon] transistors,” he contin- the first silicon transistor may not have seemed important enough ues, “we thought about patenting the process but determined to merit the same public attention given earlier transistors and the for two reasons that it wasn’t worth the effort.” For one, others solar cell. At the time, the managers were likely looking ahead eagerly had developed and used similar techniques. And he really did not to what they considered the real breakthrough: transistors fabricated like the rate-growing process, which had already been patented using diffusion that operated at over 100 MHz. And overconfidence by General Electric Co. “It just wasn’t controllable,” he adds. “From may have played a role, too. Bell Labs had habitually kept mum for a manufacturing point of view, it just didn’t look attractive.” months after its earlier breakthroughs, thereby permitting its sci- At the time, Shockley’s group was concentrating on adapting entists and engineers to work out most of the patentable ramifica- the new diffusion process pioneered by Fuller to the fabrication tions before going public. of germanium and silicon transistors. Diffusion seemed much Whatever the case, the delay allowed fledgling Texas Instruments more promising—as indeed it proved to be—because it was sub- to leap forward and claim victory in this race. And it stood alone as
AT&TARCHIVES stantially more controllable and could yield much narrower base the first company to manufacture silicon transistors in volume. Thanks layers, just micrometers thick, and hence transistors that work at to its foresight and aggressiveness, TI had the silicon transistor mar- higher frequencies. In July 1954 Charles Lee made a successful ger- ket essentially to itself for the next few years—and started down the manium transistor at Bell Labs using diffusion techniques, oper- road to becoming the international giant we know today.
May 2004 | IEEE Spectrum | NA 49