A description of field-theoretical methods for the design and analysis of planar waveguide structures and antennas. The principles and limitations of transit-time devices with different injection mechanisms are covered, as are aspects of fabrication and characterization. The physical properties of silicon Schottky contacts and diodes are treated in a separate chapter, while two whole chapters are devoted to silicon/germanium devices. The integration of devices in monolithic circuits is explained together with advanced technologies, such as the self-mixing oscillator operation, before concluding with sensor and system applications.