Casos de Uso del Sistema (CUS)

4.4.1 The invention of the transistor

In 1947, John Bardeen and Walter Brattain,³³both research scientists at Bell Tele-phone Laboratories, while trying to understand the nature of the electrons at the interface between a metal and a semiconductor, made a startling discovery.

They found that by making two point contacts very close to one another, they could make a three-terminal device—the first “point contact” transistor³⁴ (see Fig. 4.2). They made the two point contacts at the bottom of a triangular quartz crystal from two strips of gold foil separated by about 50 mm, and pressed it

30ibid., p. 12.

31ibid.

32ibid., pp. 13, 14.

33J. Bardeen and W.H. Brattain, “The transistor, a semiconductor triode,” Phys. Rev. 71, 230 (1948).

34The Nobel Foundation, http://nobelprize.org/educational games/physics/transistor/history/

index.html (2007).

onto a semiconductor surface of germanium. With one gold contact biased forward (i.e., positive voltage with respect to the third terminal) and the other reverse biased, they observed transistor action. In other words, they observed the amplifi-cation of the input signal.³⁵

The researchers quickly made a few of these transistors and connected them with some other components to make an audio amplifier. Showing this audio amplifier to executives at Bell Telephone Company, who were very impressed that it didn’t need time to “warm up” (like the heaters in vacuum tube circuits), they quickly realized the power of this new technology.³⁶It should be mentioned that the original motivation for this invention was to find a way to amplify voices in telephones, a key product sold by the Bell Telephone Company—a goal that the inventors ably met.

This invention ignited a huge research effort in solid state electronics across the world. Bardeen and Brattain received the Nobel Prize in physics in 1956,³⁷together with Shockley, “for their researches on semiconductors and their discovery of the transistor effect.” Shockley had developed a so-called junction transistor in 1949,³⁸ which was built on thin slices of different types of semiconductor materials pressed together. The junction transistor was easier to understand theoretically, could be manufactured more reliably, and became the preferred semiconductor device, which ushered in the modern electronic era.³⁹

Transistors eventually made their way into portable radios and other electronic devices, and are most prominently used today as building blocks of integrated cir-cuits. Remarkably, the invention of the point-contact and p-n junction transistors, Figure 4.2 The first point-contact transistor. It was invented at Bell Laboratories on December 23, 1947. (Reprinted with permission from Alcatel-Lucent.)

35G.S. May and S.M. Sze, Fundamentals of Semiconductor Fabrication, Chapter 1, John Wiley &

Sons, Hoboken, NJ (2004).

36The Nobel Foundation, http://nobelprize.org/nobel prizes/physics/laureates/1956/index.html (2007).

37ibid.

38The Nobel Foundation, http://nobelprize.org/educational games/physics/transistor/history/

index.html (2007).

39ibid.

followed by Shockley’s classic paper⁴⁰ on p-n junctions and bipolar transistors, also in 1949, spawned the microelectronics revolution that has had profound impact on our technological society over the latter half of the twentieth century and into the twenty-first century in such diverse fields as automobiles, computers and communications, medicine, energy, and home entertainment, to mention but a few.

What, then, really is a transistor? The transistor is a three-terminal, solid state electronic device. It consists of a three-layer structure that comprises an n-type semiconductor layer sandwiched between p-type layers ( p-n-p configuration) or a p-type layer between n-type layers (n-p-n configuration) (see Chapter 16).

In such a three-terminal device, electric current or voltage between two of the terminals can be controlled by applying an electric current or voltage to the third terminal. In this way, a transistor regulates current and voltage flow and acts as a switch or gate for electronic signals. A small change in the current or voltage of the inner semiconductor layer (which acts as the control electrode) can produce a large, rapid change in the current passing through the entire component.⁴¹

The three-terminal character of the transistor makes it possible for one to use it as an amplifier for electrical signals, such as the one in a radio. With the three-terminal transistor, one can also make an electric switch that can be con-trolled by another electrical switch. By cascading these series of switches (i.e., switches that control switches, which in turn control other switches, etc.), one can build up very complicated logic circuits,⁴² resulting in switching times that are extremely short, on the order of nanoseconds, today. Such logic chips are at the heart of the personal computer and many other electronic gadgets in use today.⁴³

4.4.2 Limits of discrete transistors

Following the invention of the transistor, for many years, they were made as indi-vidual, discrete electronic components and were connected to other electronic com-ponents (resistors, capacitors, inductors, diodes, etc.) on printed circuit boards to make an electronic circuit. The transistor’s small size and low power consump-tion made it an ideal candidate to replace the bulky vacuum tubes then used to amplify electrical signals and switch electrical currents. These beneficial attributes of transistors made it possible for them to be used in making ever more complex electronic circuits in place of vacuum tubes. However, it did not take long before the limits of this approach of building circuits were reached. Circuits

40W. Shockley, “The theory of p n junction in semiconductors and p n junction transistors,” Bell Syst.

Tech. J. 28, 435 (1949).

41Nobel Foundation, http://nobelprize.org/educational games/physics/transistor/history/index.

html (2007).

42ibid.

43ibid.

based on many individual, discrete transistors became too large, too cumbersome, and too difficult to assemble and produce for the simple reason that they contained too many electronic components. In addition, because the transistor circuits were faster than vacuum tube circuits, noticeable problems due to time delays for electric signals to propagate a long distance in these large circuits were observed. To make the circuits even faster, one needed to pack the transistors closer and closer together, a task that became all the more daunting, if not outright impossible, to accomplish.⁴⁴