## 4 Conclusions  
In this study, we introduce a sub-micron high-resolution, high-speed thermal imaging technique capable of simultaneously measuring the temperature of a substrate and a channel. To simultaneously measure the temperature of the substrate and the channel and to achieve a high resolution, we used Raman spectroscopy using visible light. To overcome the slow speed of Raman spectroscopy, we designed a modified Raman spectrometer for fast measurement at the expense of spectral resolution. Instead of the commonly used beam splitter to minimize optical loss, a 42\({}^{\circ}\) tilted bandpass filter was used. Additionally, pin hole and aperture were not used. In addition, a cylindrical lens with a short focal length was used in front of the CMOS detector for fast speed and a high signal-to-noise ratio. Through this, we built a measurement system with a measurement speed approximately 20 times faster than that of a commercial Raman spectrometer. Through measurements of SOI reference patterns, we confirmed that patterns with 1 \(\upmu\)m spacing can be clearly distinguished and measured, confirming that this measuring system has a sub-micron spatial resolution. In addition, we confirmed that the Raman signals of the Si substrate and GaN channel were separated, confirming that the temperatures of the substrate and channel can be measured simultaneously. Additionally, the temperature distribution during the operation of an AlGaN/GaN on Si HEMT was measured in low- and high-current conditions. As a result, it was confirmed that there was almost no self-heating due to current in the low current situation, and it was confirmed that the temperature increased by self-heating to 250 \({}^{\circ}\)C for the Si substrate and 350 \({}^{\circ}\)C for GaN at the high current. In addition, hotspots measuring 1-3 \(\upmu\)m in size were found in the GaN channel, which were 100 \({}^{\circ}\)C higher than the surroundings, while these were not found in the Si substrate. The fact that hotspots were not observed on the substrate may suggest that hotspots were formed when the heat generated in the channel was prevented from escaping to the substrate by micro voids or cracks. Hotspots are reported to be a major factor in the deterioration of the performance and lifetime of devices and in the deterioration of reliability due to sudden failures, and directly measuring them can be of great help in early defect inspection and device improvement research. In addition, the fact that these hotspots are not visible on the substrate is a new result that has not been reported before, and it is expected to play an important role in identifying the cause of hotspots and in improving and resolving them. Validation, Y.M. ; Formal analysis, G.E.C. ; Investigation, J.-k.P. ; Resources, H.-Y.C., C.-H.K. and J.L. ; Writing--original draft, J.K. and S.L. ; Writing--review & editing, J.-H.S. All authors have read and agreed to the published version of the manuscript. Figure 8: Temperature map of AlGaN/GaN HEMT on Si at a high-current driving condition (1.2 A). (**a**) shows temperature map of Si substrate, (**b**) shows temperature map of GaN channel. **Funding:** This work was supported by the Korea Evaluation Institute of Industrial Technology (KEIT) grant funded by the Ministry of Trade, Industry and Energy, Republic of Korea (20017406) and this work was conducted with the support of the [Gyeonggi-do Testbed Utilization Semiconductor Technology Department Project] managed by the Advanced Institute of Convergence Technology with financial resources from Gyeonggi-do (AICT-05-TB1). **Institutional Review Board Statement:** Not applicable. **Informed Consent Statement:** Not applicable. **Data Availability Statement:** The data presented in this study are available on request from the corresponding author. **Conflicts of Interest:** Author [PERSON] was employed by the company AccuOptotec Co., Authors [PERSON], [PERSON], [PERSON] were employed by the company ChipsK Co., Author [PERSON] was employed by the company UJL Co.