A solid state plasma monolithic microwave integrated circuit within a parallel plate structure having single or multiple elemental devices with at least three terminals operating within the microwave, millimetre wave or terahertz bands, comprising: a semiconductor dielectric substrate 3; parallel plates 1,2 which comprise an upper conducting parallel plate 1 and a lower conducting parallel plate 2 and which parallel plates 1,2 are used to guide an electromagnetic wave; an isolating trench which is between the parallel plates 1,2, and which is used to contain a solid state plasma; a distinct p-doped region and a distinct n-doped region which are within a first semiconductor region defined by the isolating trench below the upper conducting parallel plate 1, and which are connected to two electrical bias terminals, where at least one electrical bias terminal forms a radio frequency short to the upper parallel plate 1; and a p or n doped region within a second semiconductor regiondefined by the isolating trench a
