## 4 Experimental Measurements by Micro-Raman Spectroscopy  
Several papers over the last few years have discussed the critical impact of the self-heating evaluation for the AlGaN/GaN HEMTs. Since heat is generated only on the surrounding of the gate and because of the sub-micrometer gate-length dimension, accurate temperature measurement requires spatial resolution on a length scale of 1 \(\upmu\)m or less. This measurement can be achieved using a micro-Raman Spectroscopy that offers the opportunity to quantify channel temperature and mapping the temperature distribution in the devices with spatial resolution nearly in range [0.5-0.8] \(\upmu\)m. [PERSON] et al. were able to measure local temperature right at the channel with an approximately 1 \(\upmu\)m resolution without any effect of perturbation of the transistor during operation. The concept of temperature measurement of AlGaN/GaN HEMTs by micro-Raman Spectroscopy is based on the temperature dependence of the phonon frequencies of the GaN crystal. This measurement technique  
\begin{table}
\begin{tabular}{c c} \hline \hline
**Definitions of Geometrical Parameters** & **Value (\(\upmu\)m)** \\ \hline Length of MMIC die: Ldie & 5500 \\ Width of MMIC die: Wdie & 430 \\ Length of HEMT: LA & 360 \\ Width of HEMT: Wg & 250 \\ Drain, source lengths: Ld/s & 20 \\ Drain/source-gate spacing: Lgd/Lgs & 2.8/0.8 \\ Gate-to-gate spacing pitch: Lp & 25 \\ Gate length: Lg & 0.8 \\ Thickness of metallization layer: hAU & 2 \\ AlGaN buffer thickness: hAlGaNN & 0.03 \\ GaN layer thickness: hGaN & 2 \\ SiC layer thickness: hSiC & 350 \\ Die attach Sn-3.5 Ag thickness: hdie & 50 \\ \hline \hline \end{tabular}
\end{table}
Table 2: Geometrical parameters of MMIC and AlGaN/GaN HEMT.  
\begin{table}
\begin{tabular}{c c c c} \hline \hline
**Materials** & \begin{tabular}{c} **Thermal Conductivity** \\ **K\({}_{t}\) (W/m\(\cdot\)k)** \\ \end{tabular} & \
\begin{tabular}{c} **Heat Capacity** \\ **C\({}_{p}\) (J/kg\(\cdot\)K)** \\ \end{tabular} &
\begin{tabular}{c} **Mass Density \(\rho\)** \\ **(kg/m\({}^{3}\))** \\ \end{tabular} \\ \hline Au (Top) & 320 & 229.1 & 19,300 \\ AlGaN & 30 & 490 & 6070 \\ GaN & \(\text{K}_{t}\)(T) = 150 \(\times\) (300/T)\({}^{1.42}\) & 490 & 6150 \\ SiC & \(\text{K}_{t}\)(T) = 387 \(\times\) (300/T)\({}^{1.49}\) & 750 & 3100 \\ Die-Attach & 33 & 220 & 3379 \\ Baseplate & 180 & 722 & 2980 \\ \hline \hline \end{tabular}
\end{table}
Table 3: Material parameters used for AlGaN/GaN HEMT.  
uses a Renishaw micro-Raman system with 488 nm line of an Ar\({}^{+}\) ion laser as excitation source. More details about the use of micro-Raman Spectroscopy for active device temperature measurements can be found in References [18; 22; 23; 41]. As part of the aim of measuring the profile temperature distribution by micro-Raman spectroscopy, a Radio Frequency amplifier for satellite applications, operating in S-band (2-4 GHz), was taken as described in Figure 3b. The device is a multi-finger AlGaN/GaN HEMT on SiC substrate with 8 gates, operated under DC drain bias voltages Vds = 20 V, Ids = 625 mA at Vgs = 0 V. As part of the purpose of verifying the slope form and the thermal behavior of this device under test, we have reproduced the experience result curve point by point to better view and extract the maximum peak temperatures. The thermal profile is shown in Figure 5. The measured channel temperature was approximately equal to 225 \({}^{\circ}\)C.