William L. Bishop

GaAs Schottky diodes for THz mixing applications

The operation of GaAs Schottky barrier diodes, the critical mixer element used in heterodyne receivers for a variety of scientific applications in the terahertz frequency range, is reviewed. The constraints that the receiver system places on the diodes are considered, and the fundamental guidelines for device optimization are presented. The status of ongoing research, both experimental and theoretical, is examined. Emphasis is placed on investigations of the various effects that can limit diode performance at these high frequencies. Investigations of planar diode technology are summarized, and the potential replacement of whisker-contacted devices with planar structures is considered. >

A Novel Whiskerless Schottky Diode for Millimeter and Submillimeter Wave Application

A novel whiskerless Schottky diode has been developed in which shunt capacitance is minimized by means of an etched surface channel. This structure is easily fabricated and the DC I-V characteristics areas good as those of the best available whisker-contacted devices. Preliminary RF characterization in an unoptimized mount at 110 GHz has yielded room temperature SSB mixer noise temperature of 950 K and SSB conversion loss of 6.4 dB. The diode is robust and can be operated at cryogenic temperatures. Potential applications include waveguide and planar mixers, planar arrays, multipliers, varactor tuners, and microwave integrated circuits.

Effects of self-heating on planar heterostructure barrier varactor diodes

The conversion efficiency for planar Al/sub 0.7/GaAs-GaAs heterostructure barrier varactor triplers is shown to be reduced from a theoretical efficiency of 10% to 3% due to self-heating. The reduction is in accordance with measurements on planar Al/sub 0.7/GaAs-GaAs heterostructure barrier varactor (HBV) triplers to 261 GHz at room temperature and with low temperature tripler measurements to 255 GHz. The delivered maximum output power at 261 GHz is 2.0 mW. Future HBV designs should carefully consider and reduce the device thermal resistance and parasitic series resistance. Optimization of the RF circuit for a 10 /spl mu/m diameter device yielded a delivered output power of 3.6 mW (2.5% conversion efficiency) at 234 GHz.

A micron-thickness, planar Schottky diode chip for terahertz applications with theoretical minimum parasitic capacitance

The design and fabrication of a novel planar Schottky diode with greatly reduced shunt capacitance for millimeter- and submillimeter-wave applications is described. The dominant pad-to-pad shunt capacitance is minimized by replacing the substrate GaAs with a low-dielectric substitute. This replacement substrate can be easily removed by the user after the device is soldered into the mixer circuit. This will yield the minimum possible pad-to-pad shunt capacitance.<<ETX>>

A planar wideband 80-200 GHz subharmonic receiver

A wideband planar subharmonic mixer has been designed for millimeter-wave operation. The receiver consists of a back-to-back Schottky-diode pair integrated at the base of a wideband log-periodic antenna and placed on a silicon lens. The wideband planar receiver results in state-of-the art-performance at 90 GHz (and 182 GHz) with a double-sideband conversion loss and noise temperature of 6.7 dB (and 8.5 dB) and 1080 K (and 1820 K), respectively. These results are about 3 dB higher than the results for best tuned waveguide subharmonic mixers using planar diodes. The design is well suited for higher frequencies (up to 1 THz) and for the inclusion of biased back-to-back planar diodes to ease the LO power requirements. The planar subharmonic approach results in an inexpensive wideband receiver, and the design can be easily extended to receiver arrays. >

Integrated GaAs Schottky mixers by spin-on-dielectric wafer bonding

A novel wafer bonding process has been used to integrate high quality GaAs devices on quartz substrates. The method of adhesion by spin-on-dielectric temperature enhanced reflow (MASTER) uses a spin-on-dielectric as a bonding agent to achieve a robust bond that in no way degrades either high frequency performance or reliability. A 585 GHz integrated mixer fabricated using this process has achieved record double-sideband mixer noise temperatures of 1,150 K at room temperature and 880 K at 77 K. Furthermore, the integrated mixers require no mechanical tuning, are easy to assemble, and repeatable. Precise control of the circuit geometry, coupled with the reduction of parasitic elements, allows greater accuracy of computer simulations and will therefore lead to better high frequency performance and bandwidth. This new technology is easily extended to other circuit designs and will allow the development of a new generation of submillimeter-wave integrated circuits.

Planar multibarrier 80/240-GHz heterostructure barrier varactor triplers

Prototype planar four barrier GaAs/Al/sub 0.7/Ga/sub 0.3/As heterostructure barrier varactors (HBVs) for frequency tripling from 80 to 240 GHz have been fabricated using a process in which the device surface channel is etched prior to the formation of the contact pad-to-anode air-bridge finger. Formation of the device air-bridge finger after etching the surface channel is facilitated by a trench planarization technique and yields a device with minimal parasitic capacitances. Planar four-barrier HBV triplers with nominal 10-/spl mu/m diameter anodes have been tested in a crossed-waveguide tripler block; as much as 2 mW of power has been generated at 252 GHz with a flange to-flange tripling efficiency of 25%. These devices are the first planar or multibarrier HBV triplers reported and their output powers are nearly double that of previous whisker-contacted single-barrier HBVs.

Measurements on a 215-GHz subharmonically pumped waveguide mixer using planar back-to-back air-bridge Schottky diodes

This paper presents design and performance data for a 215-GHz subharmonically pumped waveguide mixer using an antiparallel-pair of planar air-bridge-type GaAs Schottky-barrier diodes. The waveguide design is a prototype for a 640-GHz system and uses split-block rectangular waveguide with a 2:1 width-to-height ratio throughout. The measured mixer noise and conversion loss are below that of the best reported whisker contacted or planar-diode mixers using the subharmonic-pump configuration at this frequency. In addition, the required local oscillator power is as low as 3 mW for the unbiased diode pair, and greater than 34 dB of LO noise suppression is observed. Separate sideband calibration, using a Fabry-Perot filter, indicates that the mixer can be tuned for true double sideband response at an intermediate frequency of 1.5 GHz. Microwave scale model measurements of the waveguide mount impedances are combined with a mixer nonlinear analysis computer program to predict the mixer performance as a function of anode diameter, anode finger inductance, and pad-to-pad fringing capacitance. The computed results are in qualitative agreement with measurements, and indicate that careful optimization of all three diode parameters is necessary to significantly improve the mixer performance. >

An 86-106 GHz quasi-integrated low noise Schottky receiver

An integrated planar receiver has been developed and tested over the 82-112 GHz bandwidth. The quasi-integrated antenna used in the receiver has a high gain, a high Gaussian coupling efficiency and a wide bandwidth. The mixer design consists of a planar GaAs Schottky diode placed at the feed of a dipole probe suspended inside an integrated horn antenna. The diode uses an etched surface channel and a planar air bridge for reduced parasitic capacitance. At 92 GHz, the room temperature antenna-mixer exhibits a double sideband (DBS) conversion loss and noise temperature of 5.5+or-0.5 Db and 770 K+or-50 K, respectively. The measured DSB conversion loss, and noise temperature over a 20-GHz bandwidth (86-106 GHz) remain less than 6.2+or-0.5 dB and 1000 K+or-50 K, respectively. The low cost of fabrication and simplicity of the design makes it ideal for millimeter- and submillimeter-wave receivers. >

Improved 240-GHz subharmonically pumped planar Schottky diode mixers for space-borne applications

Low-noise broad intermediate frequency (IF) band 240-GHz subharmonically pumped planar Schottky diode mixers for space-borne radiometers have been developed and characterized. The planar GaAs Schottky diodes are fully integrated with the RF/IF filter circuitry via the quartz-substrate upside-down integrated device (QUID) process resulting in a robust and easily handled package. A best double-sideband-mixer noise temperature of 490 K was achieved with 3 mW of local-oscillator power at 2-GHz IF. Over an IF band of 1.5-10 GHz, the noise temperature is below 1000 K. This state-of-the-art performance is attributed to lower parasitic capacitance devices and a low-loss waveguide circuit. Device fabrication technology and the resulting RF mixer performance obtained in the 200-250-GHz frequency range will be described.