## 5 Simulation Results and Discussion
We observe that temperature distribution behaves non-uniformly along the gate length. The simulated temperatures distribution (gate finger width) in device center gate along finger for (8 \(\times\) 250) \(\upmu\)m AlGaN/GaN HEMT device is presented in Figure 6c,d illustrates the \(Y\)-axis view. The results show the non-uniform behavior of the temperature distribution with a significant temperature variation along the length of the gate finger and a maximum temperature at the HEMT center. We can observe clearly at the top of the HEMT (see Figure 6b), the centers are the hottest surface characterized by gates 4 and 5. Indeed, for an HEMT having multiple fingers, the outer fingers are the coolest and the fingers in the middle are the hottest. Therefore, self-heating is induced leading to thermal cross-talk (thermal coupling) between individual gate fingers. This effect becomes serious if it persists and leads to the degradation of device performance or an irreversible damage and this is compared with similar results in the literature using TCAD simulator tool according to Figure 7 and good agreement is achieved [44; 45; 46]. In the aim to study the impact of heat excitation modeling, for the active region represented by a HTL Hotspot Temperature Line, two types of heat sources located on the top of GaN layer are considered (Figure 8):  
* Nano-scale hotspot heat source considered as a pulsed heat source, under gate and described by a square 50 nm \(\times\) 50 nm. * Micro-scale hotspot heat source considered as a continuous wave heat source, between gate and drain and described by L\({}_{\text{gd}}\)\(\times\) 0.4 \(\upmu\)m section. Figure 7: Comparison between 3D-TLM and electro-thermal (TCAD) simulation. In Z-direction, four zones are chosen for the evaluation temperature distribution in this TLM study. Zone (1) is the active region; it is the position where electrons flow between the drain and the source through a channel. Zone (2) is a position corresponding to the middle of GaN buffer layer, Zone (3) is a position corresponding to the interface layer GaN/SiC, and Zone (4) is a position corresponding to the substrate SiC layer. Figure 9 shows the temperature distribution along the \(x\)-axis at 4 zones. Through different device layers, TLM simulation provides a thermal behavior and detailed information on heat management in each layer node by node. Besides, we observe along the vertical position (\(Z\)-direction) a change in slope that specify a dynamic heat transfer and each layer is specified by a color. The high temperature layer in the device is localized in the top one, where temperature is higher than the one founded in the GaN layer. In the bottom is the SiC layer (the green one) where the temperature is lower, which is the high thermal conductivity of the substrate that allows for the diffusing of the heat from the top to the heatsink, evacuating away the generated heat. 3D-TLM results along the vertical position (\(Z\)-direction) for the four zones are shown in Figure 10. This simulation is used to map the surface temperature distribution and predict the expected and risked hot spots in the device. The results by TLM-3D simulation show that temperatures measured on the substrate of SiC are lower than those of the GaN layer. The substrate (green one layer) is  
Figure 8: Illustration of Hotspot Temperature Line (HTL) heat source. Figure 9: Thermal profile along the vertical position (\(Z\)-direction) for single gate. effectively evacuating the generated heat. As we can notice, the thermal interaction between the individual gate finger and the so called thermal cross-talk causes an additional temperature rise above that of single isolated fingers. The maximum gate finger temperature occurs at the center of the device, leading to self-heating phenomena, and decreases towards the outer fingers. This is a characteristic of thermal cross-talk in multi-finger devices.