Bibliography

A. F. Ioffe, Electronic Semiconductors (from the series “Problems of Modern Physics”), GTTI, 1933.

The modern quantum theory of the solid state is the key to understanding all those processes that occur both in conductors and in semiconductors and dielectrics. The sharp boundary between these two groups of bodies has been erased. The difference comes down to how the energy bands are arranged in a given body, and how the electrons are situated on the levels of such a band: whether they fill it completely or only partially. The binding of electrons in the lattice is determined by the character of the lattice, i.e., by the transparency of those potential barriers which separate one atom from another in the crystal lattice. But it is not the degree of binding in this literal sense of the word that distinguishes a conductor from a dielectric. Sometimes, under the action of an electric field, even very weakly bound electrons, easily leaking from one atom to another, do not have the right to move. This right is granted by the Pauli principle, which ultimately determines everything. Therefore, in all those cases where an energy band, representing an aggregate of discrete levels, is filled with electrons only halfway (as occurs for the elements of the first group), we have a metal before us. If, however, all the levels in a band are occupied, then the difference between a conductor and a semiconductor (or dielectric) is determined by whether or not this filled band is contiguous with the next empty band. On the other hand, the distinction between a semiconductor and a dielectric is determined by how narrow or wide the forbidden region between these two bands is. In this sense the electronic model of a semiconductor is perhaps the most typical one for a solid body. Conductors, on the one hand, and dielectrics, on the other, may be regarded as deviations in opposite directions from this intermediate case.

All these conceptions, very simple in their idea, have unfortunately nowhere yet been presented with sufficient accessibility. There are excellent review articles on these questions (for example, Sommerfeld–Bethe in the last volume of the Handbuch der Physik), but to turn to them without knowledge of quantum mechanics is pointless.

In this respect Ioffe’s little book is especially valuable. The modern model of the solid body is presented here without abuse of the mathematical apparatus of quantum mechanics, i.e., in the form most agreeable to the experimentalist. The question of the splitting of energy levels in the crystal lattice, the idea of bands, and the electronic model of a semiconductor, to which the first paragraphs are devoted, are set forth in such a way that the physical aspect of the matter becomes clear to anyone, even one encountering these questions for the first time.

Although the book is devoted to semiconductors, the ideas developed in it will be useful also to all those concerned with dielectrics. Physicists began to study semiconductors closely comparatively recently. Nevertheless there already exists a great many works on this subject. Ioffe’s book is not an exhaustive survey of all these works. It is rather a beautifully written essay acquainting the reader with the present state of affairs in this field.

At the beginning, as already indicated, the basic ideas of the electronic model of a semiconductor are set forth; then the conditions of thermal equilibrium of electrons between the filled band and the conduction band are analyzed, as are the dark conductivity of semiconductors, conductivity under the action of light, diffusion of electrons in a semiconductor, phenomena in boundary layers (rectifying properties), and, finally, very briefly, the external photoelectric effect.

The book reflects mainly the work of the Leningrad Physico-Technical Institute. Without dwelling here on individual points, one may say that, on the whole, the book is filled with very interesting and fresh material. This makes it interesting for the theorist, although it does not constitute any complete experimental survey. On the other hand, it is also of interest to the experimenter, since it acquaints him with the basic concepts of the electronic model of the solid body—admittedly somewhat schematically and predominantly from the qualitative side, but nevertheless sufficiently for clarifying the physical mechanism of the processes occurring in a semiconductor.

F. Vol’kenshtein