17.3 Temperature Measurement and Modeling of Low Thermal Resistance GaN-on-Diamond Transistors

Replacing SiC substrates with the highest thermal conductivity material available, diamond (κ up to 2000 W/mK), will result in significantly lower thermal resistance AlGaN/GaN HEMTs. In this work we combine Raman thermography and thermal simulation to assess the thermal resistance of state-of-the-art GaN-ondiamond HEMTs. INTRODUCTION The RF output power density achievable for GaN-based high electron mobility transistors (HEMTs) is limited by thermal resistance, despite the relatively high thermal conductivity of the SiC substrates commonly used (κSiC=420-450 W/mK). New thermal management solutions are needed to fully realize the potential of AlGaN/GaN HEMTs in high-power RF applications, particularly in the near-junction region where Joule heating occurs. Replacing SiC substrates with the highest thermal conductivity material available, diamond (κ up to 2000 W/mK) [1], will result in significantly lower thermal resistance AlGaN/GaN HEMTs. Although the potential thermal resistance benefit of GaN-ondiamond is clear, and excellent RF electrical performance has already been demonstrated [2], experimental assessment of the thermal resistance of GaN-on-diamond transistors is still needed, to demonstrate the low thermal resistance already achieved and to validate the thermal models that will be used to optimize device design. In this work we combine Raman thermography [3] and thermal simulation to assess the thermal resistance of state-of-the-art GaN-on-diamond HEMTs. RESULTS AND DISCUSSION In order to demonstrate the benefit of current GaN-ondiamond we compare peak channel temperatures in GaN-ondiamond and GaN-on-SiC transistors with identical layouts (Fig. 1). Peak channel temperatures, which are extrapolated from Raman measurements with the aid of thermal modeling, were found to be 40% lower in GaN-on-diamond with respect to GaN-on-SiC for device geometry measured. A low thermal resistance of 10 °C mm/W is found for the GaN-on-diamond transistor at a power dissipation of 15 W/mm, for the device geometry considered. GaN-ondiamond thermal resistance will be further reduced by identifying and optimizing any thermal resistance limitations in the heat path between device channel and heat sink. In previous Raman thermography measurements on ungated transistors, diamond thermal conductivity values of up to 1200 W/mK have been measured close to the GaN/diamond interface in Raman measurements and a GaN/diamond interfacial thermal resistance of 2.7±0.3 ×10 -8 m 2 °C/W was obtained [4], having a similar magnitude to measured GaN/SiC interface thermal resistances. The GaN layer, forming the active component of the measured GaN-on-diamond wafer, originates from GaN-onFigure 1: Peak channel temperatures obtained for 2×100 μm, 35 μm gate pitch AlGaN/GaN HEMTs, comparing GaN-on-diamond versus GaN-on-SiC. The GaN-on-diamond transistor exhibits a 40% lower thermal resistance for the measured device geometry. Peak channel temperature was determined from Raman thermography measurements with the aid of thermal modeling. 0 5 10 15 0 50 100 150 200 250 300 T e m p e ra tu re c h a n n e l tr e m p e ra tu re r is e [ o C ] Power density [W/mm] GaN-on-SiC