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Arguably the greatest invention of the 20th century, the transistor established the groundwork for a technological revolution that has had enormous impacts on the world. The broad range of practical applications that the transistor has served and its use in virtually every piece of technology on the market today is a testament to its unprecedented impact on society. In the early half of the 20th century, every electrical appliance relied on vacuum tubes for power. The two primary functions of the vacuum tube were conversion of alternate current to direct current and the amplification of electric signals. Although vacuum tubes were essential for powering everyday appliances, they were bulky, easily broken, and because of their inefficiency, tended to overheat quickly. Telephone companies were perhaps the ones most adversely affected by the limitations of vacuum tubes. They relied on call relaying and found themselves unable to cope with the unreliability of the vacuum tube. Bell Labs took the initiative to find an alternative. Bell Labs’ Vice President at the time, Mervin Kelly, created a special Research and Development team to investigate the possibility of quantum based alternatives. The research team was led by William Shockley and included the famous quantum theorists Walter Brattain and John Bardeen. Shockley selected Bell Lab’s Walter Brattain as an experimental physicist and hired John Bardeen from the University of Minnesota as a theoretical physicist. The team worked well together and quickly got off on the right foot. In 1945, Shockley began experimenting with what he called the field effect. He reasoned that a strong electrical field would create a current in a nearby conductor and set out to make a device that would do just that. Shockley and his team knew that they needed a slab of germanium and two gold point contacts just fractions of a millimeter apart. Brattain placed a ribbon of gold foil around a plastic triangle, and sliced it through one of the points. When the point of the triangle was placed onto the germanium, the signal came in through one gold contact and increased as it raced out the other: it was the first point-contact transistor. In 1956, Shockley, Brattain, and Bardeen won the Nobel Prize in Physics for creating a solid state device capable of not only amplification but also switching. Figure 1 What began as a life-long pursuit for a viable alternative to the bulky and inefficient vacuum tube culminated in one of the greatest scientific discoveries of all time. The birth of the point-contact transistor on December 16, 1947 and its subsequent unveiling on June 30, 1948, signaled the end of the vacuum tube’s reign and the beginning of a glorious new age. Compared to the vacuum tube, the point-contact transistor was smaller, cheaper, more reliable, dissipated less power, capable of controlling large currents, non-microphonic, didn’t require an internal heater, and most importantly required less power. The birth of the transistor had not come about without controversy, however. While Shockley was investigating the transistor effect, Bardeen and Brattain were nestled away in Bell Labs developing a rudimentary transistor. The point-contact transistor that they developed had been made from strips of gold foil on a plastic triangle, pushed down into contact with a slab of germanium5. When William Shockley discovered that the transistor had been developed ‘behind his back’ he resolved to make a better one. Shockley’s competitive spirit inspired him to build an improved version of the Point-Contact Transistor called a Junction Transistor which relies on three layers, either NPN or PNP, instead of merely two.
Part II: Application of the Nobelist’s Contributions For such a useful invention, the scientific principles that govern the transistor are remarkably simple. Transistors are based on solid-state technology and rely on two fundamental concepts: crystal layering and doping. There are four basic types of transistors: the Point-Contact transistor, the Bipolar Junction transistor, the Junction Field Effect transistor, and finally the Metal-Oxide Semiconductor Field Effect Transistor. The Point-Contact Transistor that Shockley and his colleagues developed was designed to serve two functions: signal amplification and switching. The Point-Contact Transistor relies on the sandwiching of semiconductor layers. Semiconductors are any of a class of solids, for instance Germanium or Silicon, whose amount of conducted electricity is between that of a conductor and that of an insulator. What that means is that they will exhibit characteristics of metals at high temperatures and essentially become insulators at low temperatures6. Figure 3 There are two types of semiconductors: Intrinsic and extrinsic semiconductors. Intrinsic semiconductors, which include elements from Group IV of the periodic table, absorb thermal energy from their surroundings and liberate some of their electrons. This electron liberation leads to the formation of electron-deficient positively charged spaces called ‘holes’. These holes in turn create current by forcing the movement of holes and electrons in the opposite direction. The current generated by intrinsic semiconductors however is extremely weak. Extrinsic semiconductors generate much greater current by subjecting the semiconductor to a process called doping in which impurities are mixed into the crystal lattice structure of the semiconductor. This disruption results in a restructuring of the perfectly valent bonds and a freeing up of electrons, ultimately generating a reasonable current. For this reason, Shockley relied on extrinsic semiconductors rather than the comparatively weaker charge-capacity of the intrinsic semiconductor. There are two types of ‘doping’. The first type of doping is referred to as p-type in which the electron serves as an electron donor and the second method of doping is referred to as n-type doping in which electrons are accepted by the impurities and ‘holes’ are formed. The elements chosen as the dopant are often from Group V and III due to their extra valence electron or lack thereof depending on whether the impurity is a Donor or Acceptor. Figure 4 For n-type doping, the dopant is generally chosen from Group V of the periodic table and hence contains 5 valence electrons. Common dopants from this column include Arsenic(As), Phosphorus (P), and Antimony (Sb). The impurity atom then forms 4 covalent bonds with the neighboring atoms in the semiconductor crystal. Consequently, the fifth electron is set free and it possesses enough thermal energy not to be attracted to the nucleus of impurity atom. This results in an increase in the density of electrons, which act as majority charge carriers, thereby creating an electric current. P-type doping involves elements from Group III of the periodic table that contain only three valence electrons. Dopants from this column include Aluminium (Al), Boron (B), and Indium (In). The impurity atom then forms 3 complete covalent bonds and a fourth incomplete covalent bond with the neighbouring atoms in the semiconductor crystal. Due to the lack of an electron in the fourth covalent bond, an electron deficient region or 'hole' is created. The movement of these holes creates an electric current. Hence the 'holes' act as positive charge carriers. Note that even in extrinsic semiconductors, a very small number of holes and free electrons are formed due to absorption of thermal energy from surroundings. Extrinsic semiconductors are often referred to as minority charge carriers. Figure
5 The N-type and P-type layers are then combined to form either a NPN or PNP transistor. While the NPN transistor is the one more commonly marketed, both transistors achieve the same effect: the amplification of an input signal and on-off switch2. The two diagrams below illustrate the design of the point-contact transistor. There are several factors that make transistors so useful. They are extremely durable, incredibly small, highly resistant to physical shock, and are often developed with inexpensive materials that make them economy-friendly. The first advantages of the transistor were relatively low power consumption at low voltage levels which made large scale production of transistor-based appliances possible4. Indeed, it is their very usefulness which makes transistors so prevalent in society. Without transistors, there would be no phones and without phones, there would be no phone lines, which ultimately means no Internet, e-mail or faxes. The lack of signal receivers in phones means that the cell phones we covet so dearly would not even play a role in the world. GPS Technology and other guidance systems would most likely not exist and the age of computers would have been mere fantasy. Figure
6 In order to understand why transistors are used in varied electronic applications, it is necessary to examine the two principal functions of a transistor mentioned earlier: signal amplification and switching. Oddly enough, these two functions are closely connected. A transistor does not really amplify an electric current, due to the conservation of energy in nature. It instead allows us to control a large current with a small one. This is precisely what makes transistors so useful in computers which rely on Boolean logic to perform functions. By controlling the flow of current in a circuit, a transistor effectively turns the switch on and off and thus enables the computer to make piles of 0’s and 1’s that translate into instructions. The transistor has made the seemingly impossible possible. From hearing aids to space missions on the moon, the transistor has become mankind’s greatest ally in the campaign for improved standards of living. No one anticipated the impact that transistors would have on the world but it has nevertheless come to define itself as the single most influential invention of the 20th century aside from maybe…sliced toast. Part III: Implications of the Nobelist’s Contributions for the FutureIn 1965, Gordon Moore, co-founder of Intel, predicted that the number of transistors per square inch on integrated circuits would double every year. Moore predicted that this trend would continue for the foreseeable future. In subsequent years, the rate slowed down a bit, but data density has doubled approximately every 18 months6. This prediction came to be known as Moore's Law. Although we would all like to believe that transistors can be made ever smaller, there are unfortunately concrete physical limitations. Most experts agree that the transistor will reach its physical limit in another two decades when the size of the transistor will become so small that it will only let one electron through at a time. Any transistor smaller than this physical limit would prevent the flow of electrons and would therefore defeat the purpose of the transistor altogether. Another interesting consequence of single electron transistors is that current would not be allowed to fluctuate due to the single electron. Therefore, after the transistor gets to this size, it can only be used as a switch, to turn current "on" or "off". Evidence of the transistor’s limitations is already surfacing in the high-demand computer industry. According to an article in the San Francisco Chronicle3, researchers at UC Santa Barbara have begun to experiment with "Spintronics", a field of science that aims to control the random nature of electron motion. By controlling the direction of the electron’s spin, spin-based quantum computers could simulate the binary code of conventional switches but at much greater speeds. Although the transistor will inevitably be rendered obsolete, it will always be credited with ushering in an age of technological brilliance and social advancement. The development of the transistor has catalyzed a chain reaction of innovative breakthroughs and improvements in vitally important areas such as drug research, hospital safety, trade and commerce, communications and list goes on. William Shockley’s pioneering vision led to a technological and social revolution that has had one of the greatest impacts on mankind in recorded history. Works Cited |
Physics, 2nd Place
