This book provides information on the classification of materials according to the magnitude and temperature dependence of electrical conductivity; the temperature dependence of metal conductivity; the electron theory of electrical conductivity in solids; characteristic parameters of semiconductors including electron drift velocity, mean free path, and electron mobility; weak and strong electric fields and relaxation time; crystal lattices and translation periods; crystalline systems and Bravais lattices; model representations of semiconductor conductivity and the concept of holes; intrinsic conductivity in semiconductors; impurity conductivity in semiconductors; electron and hole concentrations and the Fermi level; density of states and the number of quantum states; the Schrödinger equation for crystals; adiabatic approximation and the Born-Oppenheimer approximation; temperature dependence of the Fermi level in intrinsic semiconductors; and related topics.