Thermal Characterization of Quasi-Vertical GaAs Schottky Diodes Integrated on Silicon Using Thermoreflectance and Electrical Transient Measurements

Abstract

Thermal management and design have been understood, for many years, as critical factors in the implementation of submillimeter-wave Schottky-diode-based circuits and instruments. Removal of heat is particularly important for frequency multipliers, as these circuits generally exhibit low-to-modest conversion efficiencies, and are usually driven with high-power sources to achieve usable output power in the submillimeter-wave region of the spectrum. Elevated diode junction temperature due to inadequate heat sinking is known to degrade performance, accelerate aging effects (for example, due to electromigration, ohmic contact deterioration, or thermally-induced stress), and can ultimately lead to device failure. The relatively-low thermal conductivity of GaAs (the predominant material technology for submillimeter-wave diodes), coupled with restrictions on diode anode size and geometry needed to minimize parasitics and achieve the device impedances required for high-frequency operation, present significant challenges and trade-offs between electrical and thermal designs of these devices. Recognition that heating is a major factor limiting efficiency and output power has prompted a number of approaches to mitigate excessive temperature rise in the junction of planar Schottky diodes, including the use of AlN or diamond as low-loss substrates that act as heat spreaders. A new diode topology, based on a quasi-vertical geometry that is realized through heterogeneous integration of GaAs with high-resistivity silicon, was recently developed at the University of Virginia for submillimeter-wave applications. Unlike planar diodes, the device structure of the quasi-vertical diode consists of a metal contact that underlies the diode's anode and epitaxial mesa, thus providing a large-area ohmic cathode contact that also serves as an integrated heat sink. Measurement of high-efficiency multipliers based on this technology suggest the quasi-vertical architecture provides an effective approach for heat removal and thermal management in Schottky diodes. This paper presents the first results reporting thermal performances of terahertz quasi-vertical GaAs Schottky diodes integrated on silicon. The devices are characterized using a thermoreflectance measurement technique, a method based on the change in refractive index, and therefore surface reflectivity, with changes in temperature. Heating and cooling temperature profiles and 2-D temperature maps are obtained for 3.5 micron and 5.5 micron diameter diodes. From these measurements, the device thermal resistances, junction temperatures, and thermal time-constants are extracted. Equivalent thermal circuit and finite element models are developed to study the device geometry, and extract material thermal parameters. The devices are also characterized using an electrical transient method, and the temperature and cooling transients found from this technique are found to be comparable to those obtained from thermoreflectance measurements. The quasi-vertical diodes studied in this work are shown to demonstrate a faster transient thermal response compared to flip-chip bonded terahertz diodes reported in the literature.