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edfas.org ELECTRONIC DEVICE FAILURE ANALYSIS | VOLUME 21 NO. 2 12 wavelength, it is possible to sense a near surface zone and subsequently determine the temperature near the channel. Figure 2b shows the GaN E 2 h pho- non temperature calibration results for epitaxial layers on different substrates such as sapphire, Si(111), and SiC. Observation shows that the calibration curves are strongly substrate depen- dent. For the same substrate, they can also varywidely depending on themater- ials constituting the hetero-structure. Consequently, the calibration must be performed for each device epitaxy. RESULTS IN THE LITERATURE Micro-Raman thermography is able to achieve temperature and spatial res- olution as high as ±5 C and 0.5–0.7 µm, respectively. [14] It is possible to build a thermal map with a 0.1-µm step size by using a computer-controlled XYZ table. Through simulations, it appears from the front face perspective that the hot spot is located between the gate and the drain near the gate contact. [14] Similar results were obtained by the back surface through the substrate. [15] By using a confocal instrument, it is also possible to probe the vertical structure of a component and then determine the temperature along the vertical direction. This technique has been applied in operando to monitor the temperature during an electrical stress. [16] Figure 3 presents the evolution of the self-heating at dif- ferent locations named spot 1, 2, and 3 along a gate of a GaN HEMT before and after an electrical stress. The hottest point is located at the center of the gate. Due to the decrease of the drain current during the electrical stress, the temperature is lower after the stress. Figure 4 clearly shows the correlation between the decrease of the drain current and the temperature during the stress. Fig. 2 (a) A typical Raman spectrum of GaN-on-SiC, with the GaN E 2 h and A 1 (LO) phonon modes labeled. Phonon modes related to the SiC substrate are also observable. (b) E 2 hphonon frequency versus temperature for GaNandAlGaN on different substrates. Fig. 3 Evolution of self-heating at different locations along aGaNHEMTgatebeforeandafter anelectrical stress. [16] (a) (b) Fig. 4 Evolution of drain current and self-heating temper- ature during an electrical stress. [16]