## 3 Results  
In this study, the temperature distribution during the operation of an AlGaN/GaN HEMT was measured using the Raman thermography measurement system introduced above. Two measurements were performed to compare low-current and high-current conditions. The gate voltage was applied at the same 6 V, and the source-to-drain voltage was applied so that the drain current flowed at 50 mA and 1.2 A, respectively. The mapping speed was 50 ms/point, and the mapping interval was 0.5 \(\upmu\)m to measure the area between the gate and drain. The time taken for measurements was about 40 s.  
Figure 6a,b show the temperature maps of the Si substrate and GaN channel, respectively, measured under low-current conditions. For the Si substrate, the average temperature was measured to be about 80 \({}^{\circ}\)C, which is higher than the average temperature of the GaN channel at 18 \({}^{\circ}\)C. This is because the temperature of the Si was measured to be high due to laser absorption. This phenomenon occurs because Si absorbs a 532 nm laser at 1.12 eV, while GaN does not absorb a 532 nm laser at 3.4 eV. As can be seen from these results, there was almost no self-heating effect due to the current under low-current conditions. The increase in Si temperature due to laser heating raises concerns about the accuracy of the temperature measurements. One method to mitigate this effect is to calibrate the temperature while accounting for laser-induced heating; however, this approach is cumbersome, as it requires material-specific calibration. Alternatively, reducing the laser power can minimize the heating effect. To determine the appropriate laser power level, we measured the temperature variation in Si and GaN as a function of laser power, as shown in Figure 7. The temperature of GaN remained relatively stable across the power range, whereas Si exhibited a linear increase in temperature with increasing laser power. A very low error was observed in the linear fitting. The fitting parameters and associated errors are provided in Figure 7, and repeated measurements yielded virtually identical results. To sufficiently suppress laser-induced heating, the laser power must be reduced below 5 mW. Figure 6: Temperature map of AlGaN/GaN HEMT on Si at a low-current driving condition (50 mA). (**a**) shows temperature map of Si substrate, (**b**) shows temperature map of GaN channel. However, this significantly increases the measurement time by approximately 30 times compared with using a 150 mW laser. Furthermore, the anti-Stokes peak of GaN was not observed at power levels below 40 mW, indicating the need for further investigation to overcome these limitations. Figure 8a,b show the temperature maps of the Si substrate and GaN channel, respectively, measured under high-current conditions. The average temperature of the GaN channel was measured to be 350 \({}^{\circ}\)C, which is higher than the Si substrate's temperature of 250 \({}^{\circ}\)C. This indicates that the temperature increase is due to the self-heating effect caused by the current, rather than the heat generation from the laser. Additionally, while the Si substrate exhibited a relatively even temperature distribution, a hotspot measuring 1-3 \(\upmu\)m in size and 100 \({}^{\circ}\)C higher than the surroundings was observed in the GaN channel. The presence of such hotspots has been reported in previous studies, and they are known to significantly impact the performance, lifetime, and reliability of the device [22; 24; 26]. The fact that these hotspots are not observed on the Si substrate indicates that the heat generated at the hotspots is not sufficiently dissipated through the Si substrate. This result is a new finding that has not been reported in previous studies and was discovered because the temperatures of the Si substrate and the GaN channel were measured simultaneously. These results suggest that the hotspots may have been caused by micro-sized cracks or voids between the Si substrate and the GaN channel.