Science Matters

by Robert M. Hazen

  A similar situation can arise if you replace a few silicon atoms with aluminum, an element with one fewer electron in its outer shell than silicon has. Now instead of a few extra electrons with negative charges, there are a few missing electrons—holes—with positive charge. A semiconductor doped with holes is called a “p-type” (for positive charge carriers).

  Microelectronics

  Microelectronics lie at the heart of all your electronic gear—TVs and radios, car ignitions and home security systems, dishwashers and pocket calculators. All of these useful things rely on semiconductor devices. In technical jargon a device is any composite of two or more n-and p-type semiconductors that does something useful. The simplest semiconductor device is a diode formed from two semiconductor layers, one n-type and the other p-type. When such a device is first made, electrons and holes diffuse across the boundary. If a free electron encounters a hole, it falls in and goes back to being an ordinary electron forming a covalent bond. In the process, both the hole and the free electron “disappear” in the sense that they no longer are available to move electric charge. The impurity atoms (aluminum and phosphorus in our example) for a certain space on both sides of the boundary are left without the original electrons or holes to balance their charge. The result: a layer of charged ions lines up on both sides of the boundary—positive ions on the n side and negative ions on the p side.

  This double layer of charge is locked into the atomic structure of the semiconductor; once it has been formed, the charges stay where they are forever. Attracted toward the positive layer and repelled by the negative, free electrons in the material are thus accelerated across the junction. The charge layer establishes a “right” way across the junction, since it is easy for electrons to move from n to p, but hard for them to go the other way. As trivial as the junction seems to be, most of the modern microelectronics industry is based on this microscopic charge layer.

  Many diodes are used as rectifiers—they convert alternating current of the type found in household wires to a current that flows in only one direction. While the alternating current flows in the “right” direction, the junction allows electrons to pass. When the current reverses, the electrons cannot force their way through the charge layer, so the current coming out of the device flows only one way and no longer alternates. If you look inside your home computer or TV set, chances are that the first thing encountered by alternating current from the wall is a more sophisticated version of this sort of diode. Its function is to convert the AC from the generator to DC, the type of current necessary to run the electronic gear farther downstream.

  Diodes form the key ingredient in some solar energy systems, which may be important in future energy production. If sunlight strikes a thin layer of n-type material, it can add enough energy to jar loose some electrons. These electrons will then be accelerated across the boundary by the stationary charges that, in essence, act like an atomic-scale battery. If this so-called photovoltaic cell is connected to an electrical circuit, it produces a continuous supply of current so long as the sun shines. Satellites in space routinely use solar energy to run their instruments, and large-scale solar installations have been tested to see if they can substitute for conventional coal or nuclear plants. Solar power is not yet economically competitive in many markets, but many scientists and engineers are convinced that solar energy will play an ever-increasing role in the twenty-first century.

  Perhaps the most important semiconductor device ever in vented is the transistor. The simplest transistors are sandwiches formed from three slices of semiconductor—either pnp or npn—with wire leads to each of the three segments. At each boundary in the sandwich, a double layer of charge forms, just as it does in the diode. Transistors serve as fast and reliable amplifiers, detectors, or switches. They are useful because they require only a small external voltage to overcome the effects of the charge layer, thus allowing electrons to flow in the “wrong” direction across the boundary. Turn the applied voltage off and the charge layer stops the flow. In this way, a small voltage can turn a large current off and on and the transistor can act as a fast switch, a vital part of every computer. A somewhat different arrangement of applied voltages allows the transistor to act as an amplifier, taking a small current (the one from the antenna on your car radio, for example) and converting it into a large current (like the one that drives your speakers). There are hundreds of uses for the transistor, one of the most versatile electronic tools ever devised.

  Microchips, the heart of the modern microelectronics revolution, consist of large collections of devices like the diode and transistor connected on a single piece (chip) of silicon. Many advanced techniques can build up the necessary layers of n-and p-type domains. A common procedure is to expose the chip to a vapor containing the desired silicon-based mix and allow these materials to condense onto the surface. The so-called integrated circuits built in this way contain a complex juxtaposition of numerous n and p regions, equivalent to many transistors. Each integrated circuit is a module designed to perform a specific function, such as voltage regulation, mathematical logic functions, or timekeeping. A postage-stamp-sized chip rests at the heart of your hand calculator and home computer.

  Computers

  Of all the microelectronic devices that engineers have produced, the computer has the greatest potential impact on society. Future historians will record that the last decades of the twentieth century marked the early stages of a revolutionary social change marked by the widespread availability of computers. Computers are everywhere—they make our airline reservations, control our traffic, ring up our supermarket purchases, and even oversee the workings of our car engines. At the heart of every computer, be it the PC in your office or a giant supercomputer in a research laboratory, lies a microchip containing an arrangement of transistors that operate as switches—they are either on (allowing current to flow) or off (blocking the flow of current). The computer works by switching transistors from off to on and back again, millions of times per second.

  The computer stores and manipulates information, which is represented by the on-off sequence of its transistors. The PC on which this book was written, for example, stored each letter as a sequence of eight on-offs, and distinguished one from another by changing the sequence. “On-on-on-on-on-on-on-on” might represent the letter a, “on-on-on-on-on-on-on-off” the letter b, and so on.

  Computer engineers measure information with a unit called the bit, which is the amount of information stored in a single switch (on or off). A byte is eight bits, and typical personal computers are capable of storing and manipulating millions or even many billions of bits of information. The letters in this book contain about 10 million bits of information.

  Superconductors

  In a normal conductor, moving electrons collide with atoms, transferring energy in the process. As a result, the conductor heats up; we say it exhibits resistance to the flow of current. Superconductivity, a phenomenon exhibited at low temperature by relatively few materials, is the conduction of electricity with no resistance at all. In the world of physics, the difference between low resistance and zero resistance is all the difference in the world. Superconductors are important scientifically because they represent a state of matter quite different from normal solids. But that scientific interest is amplified by the vast technological potential of superconductors in our electronic world.

  Many commercial uses of superconductors arise because they can carry large currents without heating up and can therefore be used to make extremely powerful electromagnets. Magnets formed from traditional loops of copper wire would require literally tons of metal and rivers of cooling water to accomplish what modest-sized superconducting magnets do daily. Doctors employ these strong magnetic fields to probe the human body safely and without surgery, a technique called magnetic resonance imaging (MRI). Similar imaging equipment is used in factories and airports to test critical metal components for cracks and other defects. High-energy physicists depend on supercond
ucting magnetics to accelerate subatomic particles so their behavior can be studied.

  The so-called maglev (for magnetic levitation) trains are an exciting use of superconducting magnets. Such magnets in the train’s body induce currents in a metal track, lifting or levitating, the train off the ground. Conventional metal-wheeled trains cannot travel much faster than 150 miles per hour, but a train that floats on a magnet cushion can “fly” at jet speeds.

  FRONTIERS

  The basic types of bonding and the resultant variety of electrical behavior have been known for decades, but new materials with special conducting properties are discovered regularly. The search for new materials with novel electrical properties remains a high priority for many researchers, and every year it seems that an important new class of materials is discovered.

  New Superconductors

  Until 1987 all known superconductors worked only at temperatures near absolute zero. Commercial superconductors were alloys of the metal niobium. Expensive liquid helium provided the necessary extreme refrigeration. The worldwide business in superconducting medical, military, and research equipment was about a billion dollars per year.

  In 1987 and 1988 superconductivity made headlines when scientists discovered a new class of materials that became superconducting at much higher temperatures. New temperature records were set repeatedly, as the effect soared to a high of 125 Kelvin (about—150°C)—almost twice the temperature of cheap liquid nitrogen coolant.

  The search for new superconductors still occupies hundreds of researchers around the world, though not at the frantic pace of the 1980s. The Holy Grail of these workers is a superconductor that works at room temperature, though most physicists would probably bet against such a discovery any time soon.

  New Semiconductors

  Other materials scientists will focus on the synthesis of new semiconductors and the fabrication of ever more microscopic electronic devices. Silicon dominates the industry at present, but other materials such as gallium arsenide (a one-to-one mixture of the elements gallium and arsenic) may eventually provide even faster circuitry.

  Novel Conductors

  Given the importance of electricity in our lives, any discovery that promises to provide new ways of using it is sure to get a warm reception from industry. The advantage to be gained from a new material may lie in the ease with which it can be manufactured and used, or it may lie in the possibility it brings of establishing a new kind of control over conducting electrons.

  Organic conductors are made from long molecules with carbon backbones, supplemented by metal atoms. The metals turn plastics that ordinarily insulate into conductors. This introduces the possibility of building electrical circuits from plastic, an outcome that could revolutionize the electrical industry.

  In another new class of materials, the atoms are arranged so that electrons cannot flow through the entire bulk of the material, but are constrained to travel in two-dimensional planes or one-dimensional channels. For example, some of these materials act as if they were made of alternating sheets of paper and tinfoil, with the electrons moving only in the latter. Others have long chains of metal atoms locked into an otherwise insulating structure. Electrons move only through the channels and thus are confined to move in one dimension only. In both of these materials, we gain a much greater control of the electrical current than we would have with an ordinary copper wire. Engineers believe that these properties can be exploited to make whole new classes of electronic equipment.

  Quantum Wires

  This same sort of control can be achieved in semiconductors by devices called quantum wires. These materials consist of thin layers of one kind of doped semiconductor sandwiched between layers of another kind. An n-type layer doped with phosphorus might, for example, be situated between two layers of n-type semiconductor doped with arsenic. These materials tend to trap electrons in the middle layer (which can be as little as one atom thick!). The sandwich is then sliced into fine strands to make the wires. At the moment this is a purely experimental technique, but it illustrates the magic of modern semiconductor technology.

  CHAPTER SEVEN

  Atomic Architecture

  THINK AGAIN about the following list of things:

  an elephant

  panty hose

  the Empire State Building

  sand

  your left ear

  the Pacific Ocean

  air

  tofu

  Jupiter

  beer

  this book

  If they’re all made of just a few different kinds of atoms, how come they’re so different?

  The answer can be found as close as the “lead” in your pencil and the diamond ring on your finger. It’s hard to imagine two solids more different than pencil lead (also known as graphite) and diamond. One is black, the other colorless. One is so soft it leaves a streak on paper, the other so hard it scratches anything. One has a flat, dull surface, the other shines with a brilliant luster. Graphite costs pennies a pound, diamonds can cost millions. Yet both graphite and diamond are pure forms of the element carbon. The only difference between them is the way nature packed the carbon atoms together. Studies of atomic architecture in graphite, diamond, and tens of thousands of other materials reveal that:

  The way a material behaves depends on how

  its atoms are arranged.

  Everything you touch or feel, all the objects of our world with their extraordinary range of appearances and properties, consist of different arrangements of atoms. There are only a few dozen different kinds of common atoms, but there are infinite ways to organize them into gases, liquids, and solids.

  THE STATES OF MATTER

  Most of our experience with atoms has to do with atoms in combination, rather than taken singly. A clump of atoms bound together is called a molecule, and molecules form our familiar world. What you wear, what you drive, even what you eat and breathe, all are made from them. Depending on how the atoms are organized, large-scale collections of atoms or molecules may appear quite different from each other, even though they contain the same set of atoms. Scientists call each style of organization a state of matter, and the three most familiar are gases, liquids, and solids.

  Gases

  Gases fill balloons, propel bullets, and form Earth’s atmosphere. Many gases like the air you breathe are invisible, but you know something’s there when the wind blows. The distinctive feature of all gases is their ability to expand, filling whatever volume is available. This behavior reflects the atomic structure of gas.

  Each gas particle is an individual atom, like the gaseous elements neon or helium, or a molecule with two or more linked atoms, such as oxygen (O2), carbon dioxide (CO2), or methane (CH4). If we could magnify gas atoms and molecules a hundred million times they would look something like the wildly flying Ping-Pong balls of state lottery games. Gas particles do not stick to one another, but rather careen off walls and collide with other particles. The gas pressure that fills your car’s tires is a consequence of the impact forces of countless trillions of these speeding projectiles, bouncing off each other and the inside of their confining vessel. If you crowd more particles into a fixed space like your pressure cooker, or if you reduce the volume as when you squeeze on a balloon, then pressure must increase. Heating gas, which causes molecules to move faster and collide with more force, also increases the pressure. That’s how a pressure cooker works.

  The energy of chemical explosions results when dense solids or liquids are quickly converted into hot expanding gas. Bullets, cannonballs, and rockets accelerate because of this force, which is nothing more than the cumulative effect of individual atomic collisions.

  At extremely high temperatures, like those in the sun, gas takes on a different character. In this gas-like state of matter, called plasma, electrons are stripped off atoms. At low temperatures, only a few electrons per atom may be separated, but at very high temperatures—typically above 100,000 degrees—electrons are completely stripped fro
m gas molecules, yielding a complete plasma. Plasmas exhibit unusual properties not encountered in typical gases. For example, plasmas conduct electricity and can be confined in magnetic fields called magnetic bottles.

  Even though you’ve never actually felt a plasma, they are by far the commonest state of matter in the universe. Every star, the sun included, is composed primarily of dense hydrogen-and helium-rich plasma, while tenuous plasma-like gas occurs in the outer atmospheres of several planets, including Earth. There is even plasma (though one in which only a few electrons have been stripped from each atom) in your fluorescent lightbulb.

  Liquids

  Like gases, liquids have no fixed shape, but they differ from gases in having a fixed volume. At the atomic level liquids behave something like a bowl full of marbles. Like marbles, the liquid molecules slide over each other, easily shifting while retaining their available volume. Individual molecules do not stick tightly together, however, so the entire mass is free to change shape (or spill on the floor).