“Substrateless” Millimeter- and Submillimeter-Wave Circuits

Radio-frequency losses are reduced by suspending conductors in air.

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 computeraided design and microfabrication. In the approach followed in this program, the active semiconductor devices (GaAsbased 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 semiconductor 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.

Following this approach, a frequency multiplier is designed in a three-stage process. In the first stage, one uses (1) a computer program that simulates nonlinear circuits and (2) a computer program that implements a mathematical model of a diode in conjunction with a harmonicbalance- based simulator computer program to optimize the dimensions, doping profile, and number of diodes to be used in the circuit. This stage yields the diode-junction characteristics and embedding impedances that give the best performance.

In the second stage, the input and output impedance-matching transmission line circuits are designed by use of finiteelement electromagnetic-simulator software. The numerical output of this software comprises scattering-parameter matrices referenced to diode and transmission- line ports. The matrices plus the embedding impedances computed by the nonlinear-circuit simulator software are then provided as input to linear-circuit simulator software, which is used to analyze the impedance-matching effectiveness of the input and output transmissionline circuits. The parasitics associated with the diode(s) are included in this analysis as part of the passive circuit.

To simplify and speed up the analysis, the passive circuitry is divided into small elements at electromagnetically appropriate points, giving rise to several S-parameter matrices. Ports are modeled as being attached to probes on each anode so that the individual embedding impedance for each diode can be calculated directly. The diodes are then modeled as being embedded into the resulting cascaded S-parameter matrix blocks to determine the total efficiency and the power performance of the multiplier. If these are unsatisfactory, relative to the intrinsic efficiency and performance of the diodes, the circuit design is iteratively modified to correct for the parasitics found in the simulation.

Standard processing techniques, including stepper lithography and reactive- ion etching, are used to fabricate the diode structures on the front side of a GaAs wafer. The diodes are located on an edge of that portion of the GaAs wafer that is destined to remain as a transmission-line-supporting frame. After front-side processing has been completed, a back-side procedure is used to remove the GaAs under the metal conductors of the input and output transmission lines, except for edge supports as described above.

Thus far, two types of frequency-doubler circuits, designed for output frequencies of 200 and 400 GHz, respectively, have been designed, fabricated, and tested (see figure). Notwithstanding a need for further iteration to optimize design, the results of the tests are encouraging: For example, in a test in which the input frequency ranged from 179 to 212 GHz, one of the 400-GHz units exhibited a peak efficiency and peak power of ≈15 percent and ≈ 6 mW, respectively, at an output frequency of 369 GHz at room temperature. This represents a new performance record from planar Schottky diode varactors at this frequency.

This work was done by Imran Mehdi, Suzanne Martin, Jean Bruston, Erich Schlecht, and R. P. Smith of Caltech for NASA’s Jet Propulsion Laboratory. For further information, access the Technical Support Package (TSP) free on-line at www. nasatech.com/tsp under the Electronic Compontents and Systems category. In accordance with Public Law 96-517, the contractor has elected to retain title to this invention. Inquiries concerning rights for its commercial use should be addressed to Intellectual Property group JPL Mail Stop 202-233 4800 Oak Grove Drive Pasadena, CA 91109 (818) 354-2240 Refer to NPO-21080, volume and number of this NASA Tech Briefs issue, and the page number.

This Brief includes a Technical Support Package (TSP).

"Subsrateless" Millimeter- and Submillimeter-Wave Circuits (reference NPO-21080) is currently available for download from the TSP library.

Download it now!

 

This Brief includes a Technical Support Package (TSP).

"Subsrateless" Millimeter- and Submillimeter-Wave Circuits (reference NPO-21080) is currently available for download from the TSP library.

Login first to download.