“Substrateless” Millimeter- and Submillimeter-Wave Circuits

Monolithic integrated circuits (in particular, Schottky-diode-based frequency multipliers) that operate at frequencies as high as a few terahertz are being developed in a program that utilizes the recent advances in methods of computer-aided design and micro-fabrication. In the approach followed in this program, the active semiconductor devices (GaAs-based Schottky diodes) in a frequency-multiplier circuit are integrated with passive devices (planar metal transmission lines). To reduce radio-frequency losses associated with dielectric layers in the passive circuitry, the semiconductor substrate under the transmission lines is etched away, leaving metal conductors insulated by air and held only by their edges on a semi-conductor frame. The monolithic integration makes the integrated circuit larger (in comparison with discrete circuit components that one would otherwise have to assemble), thereby making the circuit more robust and easier to handle in fabrication and mounting. Metallic beam-leads are used extensively, serving as (1) mechanical handles that facilitate handling and mounting, (2) current paths for dc grounding and biasing of the diodes, and (3) thermal conductors. Moreover, this approach enables the precise positioning of the diodes with respect to the rest of the circuitry and facilitates scaling for operation at higher frequencies.