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  1. Gallium nitride (GaN) high electron mobility transistors (HEMTs) are key components enabling today’s wireless communication systems. However, overheating concerns hinder today’s commercial GaN HEMTs from reaching their full potential. Therefore, it is necessary to characterize the respective thermally resistive components that comprise the device’s thermal resistance and determine their contributions to the channel temperature rise. In this work, the thermal conductivity of the GaN channel/buffer layer and the effective thermal boundary resistance (TBR) of the GaN/substrate interface of a GaN-on-SiC wafer were measured using a frequency-domain thermoreflectance technique. The results were validated by both experiments and modeling of a transmission line measurement (TLM) structure fabricated on the GaN-on-SiC wafer. The limiting GaN/substrate thermal boundary conductance (TBC) beyond which there is no influence on the device temperature rise was then quantified for different device configurations. It was determined that this limiting TBC is a function of the substrate material, the direction in which heat primarily flows, and the channel temperature. The outcomes of this work provide device engineers with guidance in the design of epitaxial GaN wafers that will help minimize the device’s thermal resistance. 
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    Free, publicly-accessible full text available August 24, 2024
  2. Featuring broadband operation and high efficiency, gallium nitride (GaN)-based radio frequency (RF) power amplifiers are key components to realize the next generation mobile network. However, to fully implement GaN high electron mobility transistors (HEMT) for such applications, it is necessary to overcome thermal reliability concerns stemming from localized extreme temperature gradients that form under high voltage and power operation. In this work, we developed a deep-ultraviolet thermoreflectance thermal imaging capability, which can potentially offer the highest spatial resolution among diffraction-limited far-field optical thermography techniques. Experiments were performed to compare device channel temperatures obtained from near-ultraviolet and deep-ultraviolet wavelength illumination sources for the proof of concept of the new characterization method. Deep-ultraviolet thermoreflectance imaging will facilitate the study of device self-heating within transistors based on GaN and emerging ultra-wide bandgap semiconductors (e.g., β-Ga2O3, AlxGa1-xN, and diamond) subjected to simultaneous extreme electric field and heat flux conditions. 
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