Those elements having properties in between those of metals and nonmetals are lightly shaded in the identical periodic Tables IV (in Chapter 13) and B.2 (in Appendix B). Among these semiconductors, also called semimetals, are silicon and germanium. The atoms of these solid elements, like those in most solids, tend to stack themselves over distances of millions of atoms (or more) in neat, even, three-dimensional crystalline arrays.

Quantum mechanics was utilized in the late 1940s and early 1950s (by Nobel Prize–winning physicists William Shockley and John Bardeen) to provide an understanding of semiconductors and the operation of semiconductor devices like the diode and the transistor. These basic devices are now present by the billions in the integrated circuits of chips that are at the operational core of most electronic devices. I describe next just how semiconductors work.

ELECTRICAL CONDUCTION IN UNDOPED SEMICONDUCTORS

Semiconductors were defined for the quantum view in Chapter 17 as solids in which the electrons either: (a) nearly fill the valence band below a band gap, (b) just fill a valence band below a relatively small band gap, or (c) just start to fill the states of the conduction band above the band gap. For each of these situations, electrical conduction can occur but is limited.

For semiconductors of type (a), the proximity of the Fermi level to the gap leaves relatively few higher-energy states above the Fermi level at the top of the valence band into which the electrons can be selectively shifted (to those states having electron motion primarily in one direction), as pushed by a voltage source, like a battery, for electrical conduction. For semiconductors of type (c), few states at the bottom of the conduction band above the gap are occupied to begin with, so there are few electrons to be pushed by a voltage, and conduction is limited. For semiconductors of type (b), the situation is more complicated.

For semiconductors of type (b), the thermal agitation of the lattice can excite a relatively small fraction of the electrons out of the valence band, across the relatively small band gap, and into the conduction band of states above the gap where they have higher energies, are not tightly bound, and (thinking classically now) are free to roam around the solid. This leaves holes of local net positive charge in the valence band since the number of electrons locally surrounding an atom is now less than the number of protons in that atom's nucleus.

A hole can effectively move from one atom to its neighboring atom if an electron of a neighboring atom hops over to fill the hole in the first, thereby creating a hole of its own. The holes can thus wander around from atom to atom, and they can move like positively charged particles under the influence of a voltage that may attract them to a negative terminal and repel them from a positive terminal. (What is really happening is that electrons are attracted to hop toward the positive terminal, leaving the hole to move in the opposite direction.) At the same time, the electron that escaped to roam around in the conduction band can also move toward the positive terminal. So both the electrons in the conduction band and the hopping electrons that move the holes in the valence band move toward the positive terminal. Having negative charges moving to the positive terminal is the same as having positive charges flowing from the positive terminal, which is the way that current is defined.

Unlike the situation in metals, for this (b) situation the electrical resistance to conduction in these semiconductors goes up as the semiconductor is cooled. That is because the thermal excitation that creates the conduction electrons and holes is reduced or removed as temperature is lowered. It is this property that has been used to make temperature-measuring instruments called thermisters.

The stacking of atoms in solids can occur in various patterns. And in silicon and germanium, as in the diamond form of carbon discussed in Chapter 15, the atoms tend to stack themselves in a basic tetrahedral geometry that extends throughout the crystal. The two s-state outer electrons and two p-state outer electrons in the atoms of these elements combine in a hybrid arrangement, extending out to bond with neighboring atoms at what can be visualized as the corners of a tetrahedron. In the diamond form of carbon, the band-gap energy difference between the valence band and the conduction band is much too high for excitation from the one to the other to take place at normal temperatures. And so diamond has no conduction electrons and no holes and is a perfect insulator. But for silicon and germanium, the bonding is not as strong, the valence band is at a higher level, and the band gap is smaller, permitting the semiconductor behavior described above.

DOPING, AND THE CONSTRUCTION OF CHIPS CONTAINING BILLIONS OF TRANSISTORS

Conduction electrons can also be created by substituting some of the silicon or germanium atoms with atoms, like phosphorus, that have five rather than four outer electrons. This doping produces a so-called n-type material with extra (negatively charged) conduction electrons in the conduction band beyond the band gap. Or these elements can be doped by substituting some of their atoms with the atoms of elements, like gallium, that have three rather than four outer electrons, to produce p-type semiconductors that have a starting population of holes (we say positive, because of the electron deficiency), vacant electron states in the valence band below the band gap. Special properties and applications then result when sandwiches of n-type and p-type semiconductors are put together—as in diodes, which let the flow of electrons occur across the boundary between the two types in only one direction, and in transistors, three-terminal devices that may have three layers n-p-n or p-n-p materials in which the control of a small current entering one terminal (layer) regulates the large current flow across the other two terminals (layers), so that electrical signals can be amplified.¹ Or information can be stored and retrieved by controlling the flow to be either on or off, representing 0 or 1 binary states, for information storage in billions of tiny, tiny transistors in the memory or processor chips of computers.

The integrated circuits of billions of transistors and other elements in these chips are created using a process of photolithography in which dozens of chips are produced at once on up to twelve-inch-diameter wafers of silicon. As many as fifty processing steps are used, repeating a sequence of coating the wafer with a photo-resist that is etched away only in areas exposed to particular ultraviolet light and then treating the regions thus revealed. (The photo-resist is a uniform covering that prevents the deposition of new material or other operations, except where it is exposed to light and etched away, exposing the surface to be treated beneath.) The light is projected or shines through a mask to expose a “blueprint” of minute transistor and circuit geometries on microscopic and nearly sub-optically-microscopic scales, so that the exposed features can be variously treated. In one subsequent step, they may be injected with a particular doping element (dopant). In another, after differently masking, exposing, and etching again, another dopant may be added in other regions. Many steps later on, after still differently masking, exposing, and etching again, copper may be evaporated onto the wafer and then selectively removed by coating, exposing, and etching, to make connections between transistors and other circuit elements and electrical “leads” to the edges of the chip for its later connection. Finally, the chips may be cut, sealed, individually encased, and tested for use in computers or other devices.

CHARGE-COUPLED DEVICES (CCDS)

Charge-coupled devices (as nicely described by authors Bruce Rosenblum and Fred Kuttner in Quantum Enigma), “have greatly expanded personal photography, revolutionized astronomy and are steadily improving diagnostic medicine. A typical digital camera has a semiconductor chip with millions of CCDs.”2 In something related to the photoelectric effect (identified as such by Einstein) described in Chapter 2 and Figure 2.3, photons excite a cluster of electrons in silicon states, which can then be moved by an electric field giving the location where they were created and converted to measure the intensity of light at that position. As a result of his work, in 2009 Willard S. Boyle was awarded the Nobel Prize in Physics “for the invention of an imaging semiconductor circuit—the CCD sensor.”

APPLICATIONS

Most of what we broadly call “electronics” and almost everything that we use in electrical controls and communications involves semiconductors formed to make diodes or transistors that are integrated by the thousands or millions or billions in the chips that are assembled with other types of circuit elements to make devices. The uses for semiconductors and the inventions and products made from them are too numerous to describe here. These include, to name just a few: computers; smartphones; hearing aids; radios; TVs; DVD players; hi-fi systems; modern telephones; printers; scanners; fax machines; the computerized nerve centers in robots, automobiles, airplanes, rocket ships and satellites; GPSs; cell phones; motor controls; lock-in amplifiers and other scientific instrumentation; exercise equipment; radar and sonar; fish finders and depth sounders; automatic pilots; numerically controlled machine-shop equipment; digital cameras; and, with the basic laser element, the multitude of laser applications that are described in Chapter 4, including the barcode reader.

And, of course, we have semiconductor solar panels for the direct conversion of sunlight into electrical power. The utility of this intermittent or highly variable power source may rely on technologies for energy storage through any of a number of means, including one that involves the electrolysis of water to produce hydrogen, as described in Chapter 22 in relation to carbon nanotubes as a storage material.

NEW DEVELOPMENTS

The rate of advancement in the development of semiconductor components and applications is astounding. I'll mention first just a few of the recent developments in chip components.

I have already described in Chapter 22 the development of miniature supercapacitors. Similar developments are reported by Charles Q. Choi in his article “Nitrogen Supercharges Super-Capacitors.”³ In the same News section of Spectrum, February 2016, less resistive interconnects within chips is described in “Rise of the Nanowire Transistor” by Richard Stevenson,⁴ and the integration of 70 million transistors and 850 optical components into a silicon processor is described in “Linking Chips with Light” by Neil Savage.⁵ “Survival in the Battery Business” in MIT Technology Review of July/August 2015⁶ describes a company built around the development of a solid-state battery. In the same issue, the company highlighted in the article, SAKTI3, was cited among “The 50 Smartest Companies”; along with Imprint Energy, which has developed ultrathin flexible rechargeable batteries that can be printed cheaply in commonly used industrial screen printers; and along with SolarCity, which is planning to become the Western Hemisphere's largest manufacturer of silicon solar panels. An updated report, “SolarCity's ($750 million) Gigafactory” is reported in the March/April issue of the same magazine.⁷

Month by month, new or improved products are brought to market. The Internet has become a driving force. But in computers there appears to be a classical limit as transistors and other circuit elements begin to approach the size of the atoms of which they are composed. This would seem to signify the end of advancement in the capacity of chips. (See “Transistors Could Stop Shrinking in 2021,” by Rachel Courtland.⁸)

Meanwhile, the prospect of operating with circuit elements exhibiting quantum behavior has resulted in the early-stage development for some applications of superpowerful quantum computers. Projects underway in various parts of the world may revolutionize the industry for those applications, as described in Chapter 8 and Appendix C.