|| From the analysis of the spectrum of black-body radiation, the concept of quantization is introduced and the duality of matter and wave is also introduced. Combining the idea of quantization and the semi-classical hydrogen model of Bohr, the structure of atom is described. Solving the Schrodinger equation, the deduced wave function is related to the probability of finding the electron in space. The concept of atom is expanded to the molecular and the energy band theory of solid state physics. The energy band theory of solid state physics is applied to account the optical, electrical, and thermal properties of solid states, also the fundamental principles of semiconductors are introduced.