The past three decades have been a period where useful current and voltage instabilities in solids have progressed from exciting research problems to a wide variety of commercially available devices. Materials and electronics research has led to devices such as the tunnel (Esaki) diode, transferred electron (Gunn) diode, avalanche diodes, real-space transfer devices, and the like. These structures have proven to be very important in the generation, amplification, switching, and processing of microwave signals up to frequencies exceeding 100 GHz. In this treatise we focus on a detailed...