The main objective of this comprehensive text is to introduce the students the physics and the operational principles as well as the characteristics, and applications of the microwave semiconductor devices. These devices are making a revolutionary change in the field of communication and radars. As a result of the accelerating rate of growth of microwave technology in research and industry, students, engineers and scientists need to understand the theoretical and experimental design and analysis of these devices. The book also deals with higher frequency microwaves called millimeter waves, which are finding wide applications in ground and satellite communication, radars and missile guidance. Millimeter wave system development is one of the most advanced technologies in radio science, especially in view of the ever increasing demand of communication and saturation of microwave frequency range with increasing number of channels. The book discusses in greater detail about the semiconductor devices such as IMPATT diodes, Gunn diodes, HEMT diodes and FET diodes. It emphasizes on various two and three terminal devices in the microwave and millimeter wave field based on silicon and Groups III-V compound semiconductors. The book is intended to serve as a textbook for undergraduate electronics and electrical engineering students and postgraduate students of physics. It would also be a valuable reference book for professional engineers and physicists