The study of solid-state physics and its applications in sensing technologies requires a rigorous foundation that bridges fundamental quantum mechanics with the physics of real materials. This textbook is designed to provide such a pathway, guiding the reader from the essential principles of quantum theory to the specific mechanisms underlying semiconductor devices and, in particular, metal oxide (MOX) gas sensors and their applications. By integrating rigorous theoretical formulations with physical interpretation, the book is intended to function both as a reference resource for researchers and as an effective didactic support for graduate-level education.