Transistors

Transistors are used in almost every electronic device now made. A transistor is an electrically controlled resistor that has three terminals: two for the end-to-end flow of electrical current and one for the electrical signal that controls its end-to-end resistance. John Bardeen (1908–1991), Walter H. Brattain (1902–1987), and William B. Shockley (1910–1989) invented the transistor in 1947. Billions of transistors have been made since then, many of them inside the integrated circuits that make up the processors and memory modules of modern computers. The transistor is such an important device, and its invention was such a scientific breakthrough, that its three inventors were awarded the Nobel Prize in 1956.

Impact of the Transistor

Since early in the twentieth century, vacuum tubes had been used in electronic circuits, like amplifiers, to make electronic equipment, like radios. Even the first computer, built before the invention of the transistor, was made with vacuum tubes. But vacuum tubes were large, they consumed and dissipated a lot of energy, and they had a short lifetime before they burned out.

In the 1950s, only shortly after the transistor's invention, battery-powered portable radios were introduced. They were called "transistor radios," or simply "transistors," and were extremely popular. By the end of the 1950s, transistors were used regularly in digital circuits. Then it became possible to put entire circuits on a single chip, making what are called "integrated circuits." Digital electronics then became significantly faster and cheaper, leading to the advent of personal calculators and computers.

Hand-held calculators were introduced in the late 1960s. The market competition on price and functionality was so fierce that consumers were almost afraid to buy one, worried that their purchase would become obsolete quickly. Personal computers were introduced in the late 1970s, and there was a similar explosion of low price and higher functionality in the PC market.

Integration of circuits also made electronics lighter, so complex electronics could fit inside satellites. As crystal growing and photolithography improved during the last decades of the twentieth century, it has become easier to increase the number of transistors on a single chip, and they have become cheaper to produce and much faster.

Transistor Basics

Transistors are made from crystals of semiconductor material, typically silicon . With 4 valence , silicon lies near the center of the periodic table of the chemical elements. The elements on its left, the metals with valence 1 and 2, are good conductors (low resistors) of electricity and those on its right, the non-metals with valences 0 through -2, are poor conductors (high resistors). The elements in the center can be good or poor conductors, depending on their chemical composition and physical structure; so they are called "semiconductors."

When silicon forms a crystal, each atom's four outer electrons are tied to surrounding atoms in covalent bonds. Since these electrons are not very free to move, the crystal is a poor conductor. However, if the crystal is not pure, and some of the atoms in the crystal, called "impurities," have valence 5 or 3, then any extra electron or any "hole," where an electron could fit in the crystal, can conduct electricity.

Impurities with 5 valence are called "donors" because they donate an extra electron to the crystal. Impurities with 3 valence are called "acceptors" because they contribute a hole in which an extra electron could be accepted into the crystal. A crystal with a majority of donor impurities is called an "N-type" semiconductor because electricity conducts by negative electrons. A crystal with a majority of acceptor impurities is called a "P-type" semiconductor because electricity conducts by positive holes (actually, electrons hop from hole to hole in the opposite direction).

The effect of donor impurities (extra electrons) is canceled by the effect of acceptor impurities (holes where electrons could go). So, a silicon crystal's ability to conduct electricity can be controlled by varying the type and density of impurities when making the crystal—or by electrically controlling the impurities' effect, as in a silicon transistor.

How Are Transistors Made?

Although a single transistor resides on a tiny "chip" of crystalline silicon, transistors are manufactured in batches. The process begins with a thin circular "wafer," about the size and shape of a CD-ROM (compact disc-read only memory), that is sliced off a large cylinder of pure crystalline silicon. Imagine inscribing an imaginary square inside the circle on the surface of the wafer and partitioning the square into an N-by-N array containing N² imaginary cells. A process called "photolithography" allows the creation of N² transistors, one inside each cell, simultaneously.

The wafer is coated with a substance, called "photo-resist," and then exposed to a black-and-white pattern as if the pattern were being photographed and the coated wafer were the film in the camera. The white areas of the pattern correspond to the upper surfaces of the end regions (called the emitter and collector) of all N² transistors. Light hits the wafer in these white areas of the pattern and chemically alters the photo-resist there. The wafer is dipped in a solvent that dissolves away the chemically altered photo-resist, where the pattern had been white, but not the unaltered parts, where the pattern had been black.

The wafer is then heated in an air-tight oven, filled with a gas of donor impurities. Although the wafer is not heated enough to melt the silicon, it is hot enough that some of the gas atoms diffuse from the surface into the body of the material. Donor impurities fix themselves into the crystal structure, but only under the open places in the photo-resist. The wafer is cooled and removed from the oven. The emitter and collector regions of N² separate transistors have been embedded in the wafer.

The patterned photo-resist is washed away and the wafer is given a second fresh coat of photo-resist. Again, the wafer is exposed to a black-and-white pattern, but this time the white areas of the pattern correspond to the upper surfaces of the control regions (called the base) of all N² transistors. After similar chemical processing, the wafer is heated again in an air-tight oven, filled this time with a gas of acceptor impurities. They fix themselves into the crystal structure, but again only under the open places in the photo-resist. The wafer is cooled and removed from the oven. The base regions of N² separate transistors have been embedded in the wafer, in between and touching the respective emitter and collector regions previously made. The N² complete transistors are all disconnected from each other, but they are also disconnected from any wires. So, photolithography is performed a third time on the wafer.

The patterned photo-resist is washed away again and the wafer is given a third fresh coat of photo-resist. This time, the wafer is exposed to a black-and-white pattern, where the white areas of the pattern correspond to small openings on the upper surfaces of all three regions of all N² transistors. After similar chemical processing, the wafer is "sputtered" (like being spray-painted) with a metal. The metal forms a small blob that adheres to the wafer's surface, but only in the open places in the photo-resist. When the photo-resist is washed off, the metal on top of the photo-resist washes away with it, leaving the small blobs. These blobs are the electrical contacts on the three regions of each transistor.

The wafer is then sliced and diced into its N² chips. For each chip, wires are attached to the transistor's three metal blobs and the chip is encapsulated, with only the three wires sticking out. Transistors can be made extremely small, or many transistors can be constructed on the same chip. Transistors on the same chip can even be connected together, by thin P-type or N-type regions, in complex "integrated" circuits, like amplifiers, digital circuits, or memories.

How Does a Transistor Work?

Suppose a silicon chip has an N-type region on one side, a P-type region on the other side, and a distinct junction in the middle where the two regions touch each other. Any loose charges—electrons on the N-side and holes on the P-side—naturally wander around a little; this is called "diffusion." Any electrons from the N-side that cross the junction into the P-side, combine with holes there, and any holes from the P-side that cross the junction into the N-side, combine with electrons there.

There are two effects. First, the region around the junction becomes depleted of any free carriers of charge. Second, the negative charge accumulating on the P side of the junction and the positive charge accumulating on the N-side of the junction act to repel further diffusion.

Now, suppose a battery is connected to this chip. Connecting the battery's positive terminal to the N-side and its negative terminal to the P-side, called a "reverse bias," attracts the carriers in each side away from the junction, reinforcing the depletion region near the junction. So, very little current flows.

Connecting the battery's positive terminal to the P-side and its negative terminal to the N-side, called a "forward bias," overcomes the charge barrier at the junction and pushes the appropriate carriers toward the junction. Since this forces continuing electron-hole combinations, the chip is a good conductor (low resistor) in this direction. An electronic device, like this PN semiconductor chip, that allows current to flow in one direction but not the other, is called a "diode."

There are several kinds of transistors; the two most popular are the Field Effect Transistor (FET) and the Bipolar Junction Transistor (BJT). This article describes only the BJT.

The BJT

A BJT has three regions in series: the emitter, the base, and the collector. Each region has a connecting wire. The two outer regions, the emitter and collector, have the same kind of impurity and the base in the middle has the opposite kind of impurity. A "PNP" transistor has P-type silicon in its emitter and collector and N-type silicon in its base region. An "NPN" transistor is the opposite.

A BJT consists of two "back-to-back" diodes. If the collector-base diode is reverse-biased, one expects little current to flow through the device, from emitter to collector. If the emitter-base diode is forward-biased, carriers move through the base region to its connecting wire. But, if the base region is made extremely thin, perhaps only 5 percent of the carriers in the base reach the wire and 95 percent reach the collector, even through the reverse-biased diode. One sees that the emitter-to-collector current is .95/.05 19 times greater than the emitter-to-base current and we can control the end-to-end emitter-to-collector flow by controlling a smaller flow in the emitter-to-base. The FET is a little different, but it has the same effect.

A transistor can increase the intensity of, or the amplification of, an electrical signal. The electrical signal received from a microphone has insufficient intensity to make an audible sound in a speaker. The electrical signal received from the head of a CD-ROM drive has insufficient intensity to be processed by digital electronics. However, a series of transistors can amplify an electrical signal so that the output signal from the last "stage" of a multistage is sufficient to create the desired effect.

The basic building blocks of digital design can be implemented as simple transistor circuits. Since circuits like these can be integrated, making it possible to fit many of them on a single chip, one begins to appreciate how this technology has had such a huge impact on computing.

Conclusion

The ease with which transistors regenerate digital signals is probably the single most important factor that underlies the success of today's digital electronics, digital transmission, and digital computing. Computers were once large and expensive machines that only large corporations, universities, and government agencies could own. This is, of course, no longer the case. More than any other technology, the transistor is responsible for making computers so small and fast and inexpensive that they are now relatively common household appliances used by people of all ages for work, education, entertainment, and communication.

see also Generations, Computers; Integrated Circuits; Vacuum Tubes.

Richard A. Thompson

Bibliography

Amos, Stanley W., and Michael R. James. Principles of Transistor Circuits: Introduction to the Design of Amplifiers, Receivers and Digital Circuits. Boston: Newnes, 2000.

Riordan, Michael, and Lilliam Hoddeson. Crystal Fire: The Invention of the Transistor and the Birth of the Information Age. New York: W. W. Norton, 1998.