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Graphene Heat Spreaders for High-Power GaN Transistors
Keywords: GaN, Transistors, Graphene Heat Spreader
Self-heating presents a severe limitation to the performance of the high-power GaN electronics. A possible method for improving heat removal from GaN/AlGaN heterostructure field-effect transistors (HFETs) is introduction of an additional heat-escape channel utilizing materials with high thermal conductivity. Graphene and few-layer graphene (FLG) are promising candidates for such heat spreader applications. Some of us discovered that graphene’s intrinsic thermal conductivity is extremely high, and exceeds that of bulk graphite K=2000 W/mK [1-4]. For comparison, the room-temperature thermal conductivity of GaN films is in the range from 80 to 200 W/mK, depending on the quality, i.e. concentration of point defects, impurities and dislocation lines [5-6]. The overall thermal resistance of the devices structure is increased further due to alloying in AlGaN and presence of interfaces [7]. Here, we show that an addition of an extra heat escape channel made of FLG on top of the GaN device structure helps to reduce the temperature rise in GaN/AlGaN HFETs. The schematic of the lateral graphene heat spreader is shown in Fig. 1. For the proof-of-concept demonstration we used mechanically exfoliated FLG transferred on top of the device structure. Possible industrial applications can utilize CVD grown graphene. The temperature rise was monitored in-situ with micro-Raman spectroscopy. The Raman measurements were performed under the 488-nm laser excitation with the electrically biased GaN/AlGaN HFETs. The temperature was determined from the spectral position of the E2 phonon peak of GaN. The laser spot size was ~0.5 μm, which allowed us for spatial resolution within the device structure. From comparison of the Raman spectra for GaN devices with and without graphene heat spreaders, we demonstrated that graphene heat spreader reduced the hotspot temperature by ~20 K for the given power density (Fig. 2). The current characteristics of GaN transistors with graphene heat spreaders revealed substantial improvement owing to a smaller temperature rise compared to the reference GaN devices [8]. Our results for graphene heat spreaders can lead to a breakthrough in thermal management of high-power density electronics. This work was supported by ONR award N00014-10-1-0224 on Graphene Heat Spreaders.
Zhong Yan, Student
University of California - Riverside
Riverside, CA

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