Planar diodes for millimeter and sub-millimeter wave oscillators.

Abstract

This thesis presents the results of an effort carried out at the University of Michigan directed toward the design, fabrication and evaluation of millimeter wave two terminal oscillators. We have proposed two novel devices as candidates for millimeter wave power generation. Flat doping profile p$\sp+$n homojunction InGaAs TUNNEl Transit-Time (TUNNETT) diodes have the advantage of a simple doping profile and low ohmic contact resistance, and are shown to be appropriate for frequencies above 150 GHz. Break down characteristics of InGaAs p$\sp+$n junctions are presented at n-doping levels higher than has been reported in the literature. A second device, the homopolar n$\sp+$n TUNNETT device, which uses the conduction band discontinuity at an InGaAs/InAlAs heterojunction for carrier injection is proposed to further reduce ohmic contact resistance, for better high frequency performance. We have developed a new diode mounting technology which overcomes two shortcomings of the standard integral heat sink diode process: (1) bonding of the fragile InGaAs based TUNNETT devices, and (2) scaling quartz stand-off packaging to submillimeter wave frequencies is expected to be very difficult. The new mounting technology is a modification of the Schottky flip-chip technology successfully demonstrated at the University of Michigan. The modifications resulted in improved series resistance and in thermal conductivities compatible with oscillator diodes. A theoretical analysis of the thermal conductivity was carried out, and the results are verified experimentally with 20 $\mu$m Gunn diodes operating at the designed power density of 150 kW/cm$\sp2.$ Both the homo- and heterojunction TUNNETT devices were demonstrated to be compatible with the technology. We have developed two oscillator circuits compatible with the planar oscillator diode structure. The first is a micromachined analog of the Sharpless wafer mount. This structure has the advantages of providing a good thermal heat sink for the diode and of having very little dielectric in the waveguide. The second structure developed is a microstrip circuit fabricated on a dielectric post. This structure has the advantages of simple construction and being very similar to mixer diode mounts currently in use. Extensive theoretical studies were carried out on the structures and are presented. In order to test the new structure, 90 GHz Gunn diodes fabricated using the planar diode technology were tested in the new oscillator circuits. The oscillation characteristics of both circuits are presented. The RF power produced was lower than expected, and the frequency of oscillation was nearly independent of tuning efforts. The characteristics of the RF oscillation are similar for both circuits, indicating a difficulty with either the Gunn diode epilayers or the planar oscillator diode structure.