Semiconductors
Recall from the discussion of electricity that some materials are able to support electrons moving through them, while others are not. Metal allows current to flow through it, and is thus termed a conductor. The electrons in the outer orbital band, the valence electrons may be dislodged from one atom and transferred to another fairly easily. The outer orbit is also called the valence band. The valence band acts as a conduction level, a kind of "no man's land" between atoms, from which they can be dislodged, even at normal temperatures. (Think of a river flowing over stones. The top layer of stones may be moved fairly easily by the river's current, while stones at the lower layers tend to stay put.) This is why electrical wire is made of metal. Other materials, such as plastic, rubber, or styrofoam, do not allow current to flow through them, and thus they are termed insulators. At pbs.org this is explained with a handy cinematic analogy:

Different kinds of atoms have different numbers of electrons swarming around them. These electrons can only sit in specific places around the atom. It's sort of like rows of seats in a theater-in-the-round: a few electrons get to sit in the first row around the stage, and when that's filled the next electrons sit in the next row and so on. Electrons in a filled row stay put -- just as in a theater it's harder to get out when you've got people sitting on each side of you. In an insulator, every row is completely filled. Consequently the electrons rarely move. No moving electrons means no electricity can pass through.

But if you're sitting in the back row of a movie theater and the seats aren't full, you could easily get up, switch seats, maybe even decide to check out a different movie in the next theater. In a metal, the last row isn't filled with electrons. The outer electrons have little loyalty to the atom they're with and readily wander off in search of other atoms. This translates to many moving electrons, which means metals can easily conduct electricity.

Conductors and insulators represent two opposite extremes of electrical capability. In between there are a variety of materials that can allow current to flow, but more selectively, under certain conditions. They are neither conductors or insulators, but something in the middle, and are thus called semiconductors. Semiconducting elements do not have the number of "available" valence electrons that are found in metals, but the electrons are not as tightly packed as those of insulators either. Applying small amounts of energy to these elements causes lower-level electrons to "jump" up to the conduction level, and cause the element to become a conductor.

Two particularly useful examples of this phenomenon are the elements germanium and silicon. These each possess four valence electrons. As it turns out, a more stable arrangement for atoms (as dictated by Nature) is to have eight valence electrons. These two elements, then, in their pure forms configure themselves into a crystal lattice structure. Imagine country-western dance where the women are encircled by four men. As the dance progresses, the women in the center remain stationary, while the men swap places in adjoining circles, circling different women. The germanium and silicon atoms form "partnerships" with adjoining atoms, sharing electrons, so that electrons travel first around one nucleus, then another. The result is an interlocking configuration in which each nucleus "feels like" it has eight valence electrons. This is a very stable configuration, rendering these elements resistant to any kind of current flow.

The breakthrough came when scientists realized that this stability could be disrupted by adding impurities to the elements (also called doping the element). This refers to a process of adding a small number of other elements to the crystal structure. For example, the element arsenic has five outer ring electrons. Injecting a small number (about one part in a million) of arsenic atoms to a germanium crystal throws off the balance. Now there is an atom with an extra dancer, throwing off the precise synchronization of the lattice dance. When an electromagnetic field is applied to the structure, the structure acts as a conductor and the extra electrons may move from atom to atom, establishing a current. Doping a crystal structure in this way, adding electrons to the structure, is known as N-doping, since it adds negatively charged elements to the structure. (NOTE: the doping itself does not create a negatively charged area. What it does is disrupt the crystal structure with its tight sharing arrangement, leaving extra electrons that may be transferred from one place to another when energy is applied.) Another method of doping is to introduce an element with three valence electrons, such as gallium. Now there is a missing dancer in the configuration. Should a negative charge be introduced, a repelled electron may jump into the missing dancer's place, leaving a missing dancer in its place. This place for the missing dancer is sometimes termed a "hole."

Click here for an animation showing electron movement in pure and doped crystal structures.

Solid State Devices
Electronic devices are able to use doped crystal elements in place of vacuum tubes to control current. This is a tremendous advantage. These crystal structures are easily produced in laboratories, are much smaller than tubes, plus they do not generate the heat levels or require the frequent replacement that are a fact of life for vacuum tube devices. To distinguish these components from their vacuum tube equivalents, these components are termed solid state.

Solid State Diodes
A solid state diode can be constructed by taking an N-type and a P-type semiconductor and sticking them together. In the region where the two are joined, the junction, the extra dancers in the N section move into the holes in the P section, creating a depletion zone, where there are no extra (or empty) dance places. The depletion zone acts as a barrier between the two sections, preventing any dancers from moving from one side to another. The other electrons may hear the music on the other side of the junction, but they are unable to cross it to join the dance.

Recall how the vacuum tube diode capitalized on the Edison Effect and allowed current to flow through it in one direction only, as described in the page on the Fleming valve. The same thing can be accomplished with crystals to create a solid state diode. If a power source, such as a battery, is connected to opposite sections of the semiconductor, current will flow if the negative battery terminal is connected to the N material (the cathode) and the positive battery terminal is connected to the P material (the anode).

In this configuration (sometimes termed a forward bias), current from the negative battery terminal floods the N material with extra electrons. At the same time, the electrons in the P section will flow out to the positive terminal of the battery. The influx of electrons on the N side, plus the dearth of electrons on the P side, allow the newly arrived electrons on the N side to "push" those electrons near the junction over the barrier, where, in turn, these electrons cause those already there to flow out to the battery. Thus, current flows through the diode. If the battery's terminals are reversed (a reverse bias configuration), however, no current will flow. A solid state diode, then, can perform the same function as a vacuum tube diode and convert alternating current to direct current.

The Transistor

Early transistors Photo courtesy of the Transistor Museum

In 1947, researchers at Bell discovered what could happen if two of these diodes were attached in a kind of sandwich. The result was named the transistor (transfer resistor) by engineer Jonathan R. Pierce, who declared "Nature abhors the vacuum tube." A transistor is two diodes; three sections -- two of the same type on each end (the collector and the emitter) sandwiching a section of the opposite type in the middle (the base). Notice in the drawing that the collector's connection to its battery is reverse biased (N to positive terminal). No current flows between the two. However, when a smaller battery is added to the base and emitter, the connection is forward biased (N to negative terminal). Therefore, current flows in the base-emitter circuit. The base is a good deal smaller than both the emitter and collector. When electrons flow into the base from the emitter, there is an over capacity of them, enough so that they overflow into the collector, from which they are attracted to the positive terminal. Thus, the small current from the battery in the emitter circuit can cause a large current to flow in the collector circuit. Therefore, the transistor can function as an amplifier of a larger current, just as does a triode vacuum tube. In actual applications, the power supplied to the emitter is a signal from a telephone or antenna, which is amplified by the collector.

The first applications of transistors were in telephone equipment in the early 1950s. Transistors also made the hearing aid possible. (In honor of Alexander Graham Bell, who had been an advocate for the hearing impaired, Bell Labs waived royalties for this invention.) In 1954, IBM began to construct computers that used transistors in place of vacuum tubes. Transistors reached popular culture that same year with the invention of the transistor radio, which became the fastest selling retail item of all time. "Transistorized" became a buzzword, meaning "the latest and greatest of high tech!"

In 1956, Shockley, Bardeen, and Brattain were awarded the Nobel Prize in physics for their work on the transistor. Bardeen had left Bell in 1951 to join the faculty of the University of Illinois, where he researched superconductivity.

Semiconductors had utilized germanium, as it was easy to create in pure form. However, Shockley theorized that semiconductors made with silicon would be able to operate at higher temperatures. Shockley left Bell Labs after winning the Nobel Prize to capitalize on the invention. He returned to the area south of San Francisco, where he had been brought up, and founded Shockley Semiconductor Laboratory with the goal of producing silicon transistors.

Shockley was not easy to work for, carrying a competitive nature to the point of delusional paranoia. He began to suspect staff members of plotting to undermine the project, viewing trivial mishaps as malicious, and insisting on finding scapegoats. He set employees against each other, and had their work second-guessed by former colleagues from Bell Labs. When an office worker suffered a minor injury on the job, Shockley decided that all employees would be subject to polygraph tests to determine who was responsible. Incidents such as these did exactly speed progress on the work at hand. When some of the employees went to the financier to suggest that Shockley be removed from management, they found themselves out of a job. They started the Fairchild Camera & Instrument Corporation, which ultimately spawned Intel and nearly all of the semiconductor companies in the area. The area has since become known as Silicon Valley.

While transistors greatly improved the devices into which they were installed, producing them was exacting work. The spacing of its tiny components had to be precise and needed to be carried out by hand. Each cost between $5 and $45 to construct. The next breakthrough came in 1959 with the integrated circuit, which was a system of transistors that were wired together on a single silicon wafer. It was invented independently by Texas Instruments and Fairchild, although it was Fairchild that secured the patent. Thanks to techniques such as photolithography (which allows the production of materials from a photograph) and computer-aided designs, complex systems of thousands of transistors could be assembled on smaller and smaller surfaces, at lower and lower prices.

In 1963, Shockley accepted an appointment at Stanford in electrical engineering. While his former colleague John Bardeen was to become another co-recipient of the Nobel Prize in 1971 for work in superconductivity, Shockley veered into speculation on genetics and intelligence. Extrapolating on the US Army's pre-induction IQ tests, he saw the future of humanity being threatened by people with lower IQs procreating at higher rater than people with higher IQs. He was particularly concerned at the population growth of African-Americans, whom he viewed as inherently less intelligent than Caucasians. Not surprisingly, this was not a terribly popular view in the era of civil rights, and many academics sought to distance themselves from him as a result. Undeterred, Shockley continued to publish articles and lecture, suggesting that remedial educational programs were a waste of time, that individuals with IQs below 100 be paid to undergo voluntary sterilization, and encouraging others to join him in making donations to a "Nobel sperm bank" to maintain the genes of geniuses. At his death, the semiconductor industry was bringing in billions of dollars, but he considered his "research" in genetics as far more significant. Shockley might be considered the all-time winner of the "It Takes All Kinds" contest.

SOURCES:
Kenn Amdahl, There Are No Electrons: Electronics for Earthlings, Clearwater Publishing Company, 1991.
Online Radio & Electronics Course
Lucent Technologies - Transistor History
TIME 100: Scientists & Thinkers: William B. Shockley