Section 4.2 presents the basic quantum concepts and wave mechanics that are essential for dealing with systems of atomic scale and for solving the electron wave functions and energy band structures in crystalline solids. Section 4.3 describes the basic constraints imposed on the electron wave functions that are attributed to the translational symmetry of the periodic crystal. For example, suitable electron wave functions in a crystal must obey the Bloch theorem. According to this theorem, the electron wave functions in a periodic crystal consist of a plane wave modulated by a Bloch function that has the same periodicity as the crystal potential. Section 4.4 depicts the Kronig-Penney model for the one-dimensional (1-D) periodic crystal lattice. Section 4.5 describes the nearly- free electron (NFE) approximation for a three-dimensional (3-D) crystal lattice. The NFE method can be used to find the electronic energy states for the outer-shell valence electrons in which the periodic potential of the crystal can be treated as a small perturbation. Section 4.6 presents the tight-binding approximation [or the linear- combination of atomic orbits (LCAO)]. The LCAO method may be employed to calculate the electronic states for the inner- shell core electrons in a crystalline solid. The solutions of Schrödinger equations and the density of states functions for low-dimensional systems (zero- D, 1-D, 2-D, quasi- 1-D, and quasi- 2-D) will also be depicted in this section. Section 4.7 describes the energy band structures for some elemental and compound semiconductors. In general, the calculations of energy band structures for semiconductors are carried out using more rigorous and sophisticated methods than those depicted in this chapter. The effective mass concept for electrons and holes in a semiconductor is depicted in Section 4.8.
4.2. BASIC QUANTUM CONCEPTS AND WAVE MECHANICS
In this section several important historical experimental observations dealing with the blackbody radiation, optical spectra emitted by atoms, and the wave-like nature of particles that could not be explained by the classical mechanics, and the success of the quantum mechanics in describing the behavior of systems with atomic dimensions will be depicted.