Semiconductors

The most prevalent use today of semiconductors is forming them into transistors, the basic electronic building blocks of all “solid-state electronics” and computer microchips. Semiconductors are the most important materials in the burgeoning revolution in computers and other electronic devices today. Such applications also arose, beginning in the 1930s, from the quantum mechanics of the band structure of solids.

The most common semiconductors are made of silicon or germanium, elements 14 and 32 on the periodic table. It was known that they form very stable crystal structures that should be insulators but are in fact weak conductors of electricity. For both of these elements, it was found that the numbers of electrons are just enough to fill up to the top of an energy band in each case. This is why they should be insulators, and, in fact, at near absolute zero, 0 K, they are insulators (not superconductors).

At very low temperature, the lattice vibrations in silicon and germanium are minimal, and the electrons at the top of the valence band are not able to obtain enough energy from the lattice vibrations to enable them to jump the energy gap to the next band and become conducting. However, the gap to the next band is very small, only 0.7 eV for germanium and only 1.1 eV for silicon. Because these gaps are so small, at temperatures somewhat above absolute zero the electrons can pick up enough energy from the vibrations of the crystal lattice to jump the gap and land in the empty conduction band. So at room temperature these metals, which by their structure should be insulators, are actually weak conductors.

semiconductors 2As failed insulators and poor conductors, silicon and germanium were not much use in electronics until the 1950s when advances were made in. But the real use of these two plentiful metals took off during the 1980s with the introduction of mass-production methods for super-thin, microscopically structured layers of silicon and (to a lesser extent) germanium crystals, which, when properly arranged can act as transistors. Today, wafer-thin layers of silicon, when made into micro-transistors through the introduction of impurities and broken up into “chips,” are the basis of the trillion-dollar-per-year electronics and computer industries, and there is as yet no end in sight to the revolution they have unleashed. And it all depends on the narrow energy gaps in these crystals.

Since germanium and silicon are so sensitive to impurities, their large escale use as semiconductors did not occur until methods were developed for producing ultra-pure graphite for nuclear reactors and ultra-pure germanium for electronic circuits during World War II. Pure germanium was also used at first in photoelectric cells. A photon from the outside can strike an electron in the valence band of germanium (and later silicon), providing the electron with enough energy to reach the conduction band in a type of internal photoelectric effect. For this to occur, the energy of the photon must be at least 0.7 eV for germanium and 1.1 eV for silicon. From the relationship for the energy of a photon, E = hf, these energies correspond to photons possessing frequencies in the infrared range of electromagnetic waves. Any waves (or photons) with frequencies in the infrared or higher range, which includes visible light, will cause electrons to jump into the conduction band and form a current. This is one reason these crystals are known as semiconductors, since they are good conductors only when the band gap can be overcome.

This type of light-induced conductivity can be used in a photoelectric cell, or photocell for short—a cell that produces electricity when light shines on it. You can imagine some of the many possible applications of a photocell. It can be used, for instance, for motion detectors. A light beam shining on a photocell in a circuit will generate a steady current. If somebody walks through the beam, this will interrupt the current, which could either set off an alarm or open an automatic door for the person to exit or enter. Automatic controls on night lights use the same principle. Since photocells are sensitive even to infrared rays, as long as there is sufficient daylight the cell will produce a current. When the sun goes down, the current stops, which signals a circuit to switch on the lights.

References:

Asimov, I. (1993) New Guide To Science Penguin Press Science

Cassidy, D. et al (2002) Understanding Physics Springer Verlag New York

ChemWiki. UC Davis

Author: César Tomé López is a science writer and the editor of Mapping Ignorance.

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