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Advanced Thermal Dissipation in GaN-on-Diamond Transistors
Keywords: GaN-on-diamond, Raman spectroscopy, Thermal management
Transistor engineers have struggled for years to surpass the thermal barriers that stand in the way of achieving the intrinsic performance potential of gallium nitride (GaN) semiconductors. A recent solution to this challenge is replacing GaN’s entire host substrate—such as silicon (Si) or silicon carbide (SiC)—with a synthetic diamond substrate. Tests prove that GaN-on-diamond wafers actually enable GaN to perform at levels that have never been seen before, delivering three times more areal power density. GaN-on-diamond has emerged as a RFPA technology, enabling technological innovations in radar and other microwave defense applications, satellite communications and cellular base stations. In this paper, we demonstrate greatly enhanced thermal dissipation performance in GaN devices made from GaN epi integrated on a diamond substrate with bulk thermal conductivity about four times more than copper and SiC. Specifically, the GaN-on-diamond devices consist of 2.5/20/800 nm GaN/AlGaN/GaN layers on a 100 um chemical-vapor-deposition (CVD) diamond substrate with a 33 nm interfacial layer in between. Accurately measuring thermal dissipation in such devices is challenging, and several different techniques have been developed. In this work we employ scanning confocal Raman spectroscopy with submicron resolution to determine the temperature distribution in the GaN channel and diamond substrate. The results of using this measurement technology are compared to the results obtained from other methodologies. Thanks to the sensitive phonon behavior to temperature variation in solids, Raman spectroscopy can be used to extract the local temperature rise induced by the Joule heating in active GaN transistors. Raman spectra of the GaN layer and diamond substrate are taken while different source-drain voltages (Vds) are applied. Two Raman modes, E2 and A1, are observed in GaN layers. As Vds increases in the channel, both of the Raman peaks soften. Through the relative shift of A1 peak, which is insensitive to stress, we extract the channel temperature rise of ~140 K under a power density of 16 W/mm, which is more than 35% lower than that of GaN devices on widely used SiC substrate at this similar power level. The depth temperature profile in the diamond substrate could also be determined through 3D Raman thermography, showing a reduction of more than 70% peak temperature rise in diamond compared to that in SiC substrate at the same power level. By comparing the measured temperature with the simulated temperature distribution using 3D finite element method, the thermal conductivity of the diamond and thermal boundary resistance (TBR) at the diamond/GaN interface are determined to be 1550 W/mK and 2.5x10-8 m2K/W, respectively. Such high diamond thermal conductivity and modest TBR lead to the significantly lowered channel temperatures. The paper also explains why analysis using the A1 instead of E2 peak is critical to obtain accurate local temperatures, even though A1 peak requires much longer integration times. In conclusion, using this accurate method of Raman spectroscopy, the authors have quantitatively characterized the temperature distribution of GaN-on-diamond devices and have found a more than 35% reduction of peak temperature in these devices when compared to GaN-on-SiC devices.
Rusen Yan,
University of Notre Dame
Notre Dame, Indiana

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