# Junction Temperature Optical Sensing Techniques for Power Switching Semiconductors: A Review
## 3 Junction Temperature Optical Sensing Techniques
### TSOP Sensing Technique  
The TSOP sensing technique uses the electroluminescence (EL) phenomenon to measure the junction temperature of Si power switching devices. As shown in Figure 6a, the radiation emitted (blue-like visible light around SiC chip) as a result of external stimulation, such as an electric field or photon excitation, is known as luminescence [78; 79], and EL, in particular, is the emission that occurs as a result of excitation through the recombination of electrons and holes across a _p-n_ junction, or in other words, via bias voltage [80]. The peak energy of luminescence depends on the temperature and the spatial resolution of the EL [15], while the spatial resolution is dependent on the area of the _p-n_ junction that is producing the light signal [79; 81; 82; 83]. The EL process includes simultaneous current and temperature effects. A controlled source is used to generate current in order to control the conductivity of the body diode of the SiC MOSFET. The light emitted from the SiC is then exported to an optical grating spectrometer for spectral analysis via a quartz optical cable fixed to the MOSFET chip. The heat controller positioned beneath the SiC accounts for the junction temperature difference at various forward current settings to decouple the effect of the temperature and forward current on the emitted photons. The spectral features exhibited in this process can be \begin{table}
\begin{tabular}{c c c} \hline \hline  & **Key Benefits** & **Challenges** \\ \hline \(\bullet\) & Temperature changes can be easily sensed at some distance. & \(\bullet\) & Junction temperature sensing requires the removal of the power semiconductor package for radiation detection. \\ \(\bullet\) & Suitable for offline thermal extraction of power semiconductor devices. & \(\bullet\) & To obtain an accurate measurement, even emissivity of the surface is required across the region. \\ \(\bullet\) & Modern IRC sensors have high spatial resolution and, thus, more accurate results. & \(\bullet\) & Even though thermal imaging can be detected at a distance, it requires clear sight of the object for accurate results. \\ \(\bullet\) & IRC renders attractive temperature mapping with temperature range bar. & \(\bullet\) & Thermal IRC sensors are external to the system and difficult to embed in the power switches. \\ \hline \hline \end{tabular}
\end{table} 
Table 3: Summary of key benefits and underlying challenges of IRC sensing technique. adapted to estimate the junction temperature of other semiconductor devices such as LED and Si MOSFET [84; 85]. Figure 6b shows a generic schematic of the TSOP technique for junction temperature measurement. The optical path is typically a low-loss optical fiber sensor for transmitting EL to the spectrometer for spectral analysis. The fiber sensor tip is often fixed onto the Si chip die for EL extraction via optical power coupling to the fiber. Generally, every forward-biased \(p\)-\(n\) junction semiconductor device can emit light, and this luminescence will be strong for direct bandgap semiconductors. Unfortunately, power switching devices, such as SiC MOSFETs and Si IGETs, are indirect bandgap \(p\)-\(n\) junctions; thus, the radiative recombination process is very weak [80; 86]. However, they can act as parasitic light-emitting diodes while they are forward-biased, thus emitting weak EL, which makes it possible to detect and hence it can be utilized as a viable sensing parameter for Si/SiC power switching devices. Moreover, for EL to be strong, SiC MOSFET and Si IGETs are operated in the third quadrant window where the forward-biased condition can be achieved [87]. The measurement of photoemission bandgap carried out in [88] has shown that 4H SiC, the most-used SiC polytype in power electronics due to its thermal and mechanical properties, has a junction emission in the ultraviolet (UV) spectrum with a \(-3.62\) eV energy band.