Abstract: A non-hermetic, three-dimensional, microwave semiconductor device carrier with integral waveguide couplers is disclosed. A molded plastic substrate having a suitable dielectric constant and varying thicknesses comprises plated conductors and locations for receiving GaAs MMIC's. MMIC's are mounted to a metal backplate and die bonded to the plated conductors. The waveguide couplers are integrally molded as part of the carrier substrate, and comprise plated through cylindrical members. Signals from a waveguide cavity are coupled to the MMIC's by inserting the waveguide couplers into a waveguide port. The carrier and integral waveguide coupler together with a plated molded cover forms a non-hermetic package providing pseudo-shielding cavities about the resident multiple semiconductor GaAs die. Transmission line impedance control is enhanced varying the substrate thickness on a per conductor basis. Frequency of operation exceeds 12 gigahertz.