Solid-state Physics, the branch of physics that deals with the structure and properties of solids. Solid-state physicists do research in a variety of areas ranging from technologically important work on semiconductors and magnetic materials to work of academic interest only, such as in the field of superconductivity. (The term "solid state" as most generally used in common speech refers to electronic equipment employing transistors and other semiconductor devices; but work with semiconductors is only one phase of this very broad field.)

Solids consist of atoms or molecules packed closely together—frequently in a very orderly way. Most of the physical properties of solids depend upon the way in which the atoms or molecules are packed, or arranged; that is, on the internal structure of the solid. For this reason the study of the structure of solids, called crystallography, is of basic importance to all other investigations in solid-state physics.

The study of the attractive forces between atoms or molecules in solids is another basic aspect; it has helped greatly in the understanding of many of the properties of solids. For example, since the attractive forces are electrical in nature, they play a very important role in determining such electrical properties of solids as conductivity. These forces also determine to a large extent the optical and thermal properties of solids, such as refractive power (the ability to bend light waves) and the ability to conduct heat.

Solids are rarely, if ever, pure. (A pure substance consists of one chemical element or compound only; impurities are traces of other elements mixed in with the main element or compound.) The study of the influence of impurities upon the properties of solids is an important aspect of solid-state physics. For example, devices such as the solar battery and the transistor were made possible only after scientists had learned how to control impurities in silicon and similar substances.

Frequently an atom or molecule is unexpectedly absent in the packing arrangement of a solid. The gap left in the structure is called a vacancy. Vacancies may profoundly alter the mechanical properties of materials. For example, a piece of cold-worked metal has many more vacancies than a comparable piece of annealed metal, and is usually much harder and tougher. Thus the study of vacancies by solid-state physicists is of considerable interest to metallurgists.

The atoms or molecules at the surface of a solid are not packed in the same way as those in the interior; unlike interior atoms or molecules, they are not completely surrounded by neighboring atoms or molecules. Thus the surface behavior differs substantially from that of the rest of the solid. The study of solid surfaces and of films (that is, very thin solids consisting mostly of surface) has led to the use in electronics of thin films with unusual electrical properties and to ways of increasing the resistance of metal surfaces to corrosion.


Although early experiments with ceramics and processes for tempering steel may be considered the forerunners of solid-state physics, it was not until the beginning of the 19th century that the study of solids was approached in a scientific manner. It was during this period that Ren Just Hay, a French mineralogist, began speculating about the internal structure of crystalline materials and laid the foundations for crystallography. However, Hay's work and the work of several others who followed received little attention at the time.

The real beginning of crystallography occurred in 1912 when Max von Laue, assisted by Walter Friedrich and Paul Knipping, discovered that crystals diffract X rays. Immediately following this discovery, William Henry Bragg and his son William Lawrence Bragg used the diffraction technique to determine the structure of solids. Von Laue received the 1914 Nobel Prize in physics for his work and the Braggs received the 1915 prize for theirs.

The discovery of X-ray diffraction was the turning point in the development of solid-state physics, providing a foundation for a theoretical understanding of solids. Solid-state physics has grown rapidly ever since—particularly since World War II. A great technological triumph of solid-state physics occurred in 1948—the development of the transistor by William Shockley, John Bardeen, and Walter H. Brattain of the Bell Telephone Laboratories.