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Enhanced Thermal Management Solutions for RF Power Amplifiers
Keywords: Thermal Management, Diamond heat spreaders , GaN-on-Diamond.
Abstract Synthetic diamond heat spreaders and GaN-on-Diamond wafers and the transistors that embed them have emerged as a leading RF PA technology for use in next generation radar and other microwave defense applications, satellite communications, and cellular base stations. This is due to diamond's excellent thermal management properties. The authors report development progress on the state-of-the-art of diamond heat spreaders and GaN-on-Diamond wafers. Introduction GaN-based transistors and their related RF Power Amplifiers (PAs) have emerged as the leading solid-state technology to replace traveling wave tubes in radar, EW systems, and satellite communications and to replace GaAs transistors in cellular base stations. However, significant thermal limitations prevent GaN PAs from reaching their intrinsic performance capability. Metallized synthetic diamond heat spreaders have recently been used to address this thermal management challenge, particularly in cellular base station applications. And GaN-on-Diamond has previously been introduced by the authors as a viable approach for enabling GaN such that the thermal limitation is significantly diminished. In the approach, the GaN epitaxy is transferred to diamond by first removing the host Si (111) and transition layers beneath the AlGaN/GaN epitaxy, depositing a 50 nm proprietary dielectric onto the exposed AlGaN/GaN, and finally growing a 100 mm thick CVD diamond onto the dielectric adhered to the epitaxial AlGaN/GaN films. Summary In this paper, a novel diamond-integrated packaging design is introduced and thermal modeling of this novel package design is compared to packages with and without singulated diamond heat spreaders. FEA models and experimental results confirm a greater than 10% increase power dissipation by integrating the diamond heat spreader directly into standard packages compared to the singulated heat spreader. The diamond package design provides a 15% reduction in the thermal resistance compared to the standard singulated diamond heat spreader design. Continued progress in the reduction of junction temperature demonstrates that an optimized adhesive layer is the primary key in controlling thermal boundary resistance and heat transfer in high power devices. This is demonstrated in GaN-on-Diamond transistors and through thermal models. Additionally, coupled thermal-stress FEA models have been used to evaluate the introduction of features on the diamond surface to decrease CTE stresses in GaAs devices. Diamond heat spreader integration into GaAs devices has always been a challenge due to the CTE mismatch, but with recent design optimization using FEA models, alternative solutions have been discovered. And transistor performance recently reported by leading GaN groups – using the authors’ GaN-on-Diamond wafers – are reviewed, compared, and analyzed. The authors also present state-of-the-art data on their development efforts to mature GaN-on-Diamond wafers into a production line. The status on 4” wafers are discussed as well.
Bruce Bolliger, Semiconductor Business Manager
Element Six Technologies
Santa Clara, CA

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