New Waveguides for Microwave Monolithic Integrated Circuits Using Distributed Parameter Effects of Semiconductor Carriers

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The integration level of monolithic microwave integrated circuits (MMICs)is presently limited owing to large substrate area requirements for passive circuitry in spite of the advanced miniaturization of active semiconductor devices with fine-line lithography.In the present study, MIS (metal-insulator-semiconductor) and Schottky coplanar waveguides for application to MMICs are investigated theoretically and experimentally. They are formed on semi-insulating compound semiconductor substrates ( GaAs and InP) with epitaxial surface layers, and show remarkable slow-wave propagation due to distributed parameter effects of semiconductor carriers. Such waveguides can reduce the size of distributed parameter passive circuits. Additionally, their transmission properties can be altered by electrical bias, opening up the possibility of electrical tuning and of interesting non-linear interactions.Basic design principles of MIS and Schottky coplanar waveguides were established and an theoretical analysis of transmission properties was made, using an equivalent circuit approach.Analytical expressions on frequency and bias dependences of transmission properties were derived.Molecular beam epitaxy (MBE) was used to form GaAs and InGaAs surface semiconducting layers on GaAs and InP semi-insulating substrates. The GaAs Schottky-type coplanar waveguides showed slow-wave propagation, and their transmission properties including frequency and bias dependences, were found to be in good agreement with the theoretical prediction.Control of Fermi level pinning is obviously extremely important for practical exploitation of MIS coplanar waveguides with bias tuning capability. A novel passivation technique using an ultra-thin MBE silicon layer was developed, and it resulted in a very promising InGaAs MIS structures on InP without any indication of Fermi level pinning.