High-density low-effective-mass plasmas in semiconductors allow plasma and cyclotron frequencies in the ultramicrowave range. It is proposed that the birefringence of these magnetoplasmas be utilised for gyratory devices such as circulators, isolators etc. at millimetre wavelengths. The conditions for the feasibility of these devices are discussed and are shown to be satisfied in practical cases. Experiments are described that demonstrate the existence of the circularly polarised semiconductor-plasma waves necessary for these devices.