# Junction Temperature Optical Sensing Techniques for Power Switching Semiconductors: A Review
## 2 Power Semiconductor Devices  
This section discusses the basic operational features, internal structure, and thermal behavior of power switching semiconductor devices. Typical semiconductor devices in power \begin{table}
\begin{tabular}{c c c c} \hline \hline
**Characteristics** & **Optical** & **Electrical** & **Physical** \\ \hline Measurand & Light Signal & Electrical Parameters & Physical Quantities \\ EMI Immunity & Yes & No & Yes \\ Physical Contact & Maybe & Yes & Yes \\ Invasive & No & Yes & Yes \\ Accuracy & High & Moderate & High \\ Complexity & No & Maybe & No \\ In-situ Sensing & Yes & Yes & No \\ Response Time & Moderate & Fast & Poor \\ Cost & Low & High & Low \\ \hline \hline \end{tabular}
\end{table} 
Table 1: Comparison of junction temperature sensing techniques for power switching semiconductor devices. electronics include; thyristors, Silicon (Si)-controlled rectifiers (SCR), IGBT, and SiC MOSFET. While Si MOSFET operates at a high frequency and a low power range, Si IGBT is used in high power and low power to moderate frequency applications [22; 23; 24]. Recently, composite SiC MOSFET has been shown to exhibit low switching loss compared to Si MOSFET, combining the features and benefits of both Si IGBT and Si MOSFET, thus strengthening the potential of SiC MOSFET for high-frequency and high-power applications [25; 26; 27; 28]. A comparison of all three popular semiconductor devices in terms of operating power and frequency of operation is shown in Figure 2. Moreover, the fact that Si IGBTs and SiC MOSFETs are the most frequently used power-switching semiconductor devices in several applications such as data centers, automotive systems, and power grids/systems, this review exclusively concentrates on both of these devices while illustrating their schematic diagram in Figure 3.  
One of the fundamental lifecycle evaluation factors of power switching semiconductor devices is their junction temperature and its fluctuation, since this affects their lifetime, which may cause device failure [29]. Junction temperature refers to the mean surface temperature on the SiC MOSFET chip or the absolute maximum temperature of the emitter metallization on the Si IGBT chip. It is influenced by several factors. For instance, in a multilayer IGBT module that handles a wide range of input supplies, any random input  
Figure 3: Schematic diagrams for; (**a**) Si IGBT and (**b**) SiC MOSFET. Figure 2: Comparison of the operating power and frequency for Si and SiC transistors (IGBT and MOSFET). voltage fluctuation causes the module to repeatedly hold up the thermal cycle's shock for an extended period. Thus, junction temperature also fluctuates during this thermal cycle, giving rise to alternating thermal stress. Similarly, for SiC MOSFET, thermal stress influences the junction temperature variation due to high switching frequency [30]. In general, when there is a degradation in electron mobility, a further increase in power generation will also increase the junction temperature of these devices due to power dissipation. On the other hand, the aging of the solder layer can also contribute to an increase in thermal resistance, which in turn raises the junction temperature of the power switching semiconductor chips [31; 32]. The internal structure of these semiconductor devices and the comparison are discussed in the subsequent paragraph. Considering Figure 3a, the trench gate structure of Si IGBT that runs through the \(n^{+}\)-emitter and \(p\)-base regions facilitates an increase in the channel density and eliminates the usual channel voltage drop inherent to junction MOSFETs. Moreover, the IGBT chip thickness is reduced by introducing an \"\(n\)-fieldstop\" layer that lowers the static and dynamic losses. Conversely, the conventional planar structure of SiC MOSFET, illustrated in Figure 3b, has the \(n^{+}\) substrate region in contact with the drain electrode at the bottom of the device instead of the collector.