An MMIC-compatible tightly coupled line structure using embedded microstrip

The authors present a highly manufacturable coupled line structure for MMICs (monolithic microwave integrated circuits) which uses embedded microstrip to achieve tight coupling and only uses the process steps necessary to make MIM (metal-insulator-metal) capacitors. Passive circuits demonstrated using this technique include a single-section 5-21-GHz broadband 3-dB coupler and a 6-15-GHz 90 degrees Schiffman section. The coupler and the Schiffman section use tight (2-dB) to extremely tight (0.7-dB) coupling factors, respectively. This is the first time that a coupler has achieved such a wide bandwidth on a 125- mu m-thick GaAs substrate.>