# Junction Temperature Optical Sensing Techniques for Power Switching Semiconductors: A Review
## 3 Junction Temperature Optical Sensing Techniques
### TSOP Sensing Technique
For SiC MOSFET, the body diode only acts as a parasitic light-emitting diode during passive third quadrant (forward bias) operation [89], in other words, when the MOSFET is in the OFF state (i.e., the gate-source voltage \(V_{gs}\) is nearly zero, and the current flow is in the opposite direction to the device). In the case of Si IGBT, the \(p\)-\(n\) junction near the collector is forward-biased, and a collector current \(I_{c}\) flows into the device when it is in ON state. As a result, its \(p\)-\(n\) junction acts as a parasitic light-emitting diode in the first quadrant operation. To establish third quadrant operation, antiparallel freewheeling diodes are usually included with IGETs chips. Since the light emission from power semiconductors is obtained during the passive third quadrant operation, the measurement period is limited to the dead time in power electronic applications [90; 91]. The spectral sensitivities of SiC MOSFET's EL are influenced by the gate bias voltage and bias temperature instability (BTI). Both alter the effective electrical field across the oxide and cause changes in the current that flows through the body diode, which, in turn, impacts EL extraction [92; 93; 94]. To mitigate this, [PERSON] et al. [95] proposed a post-processing method of minimizing SiC's sensitivity to gate bias voltage using the estimated intensity ratio of both spectral peaks (UV and blue-green peaks). Experimental work in [96; 97; 98] has established that two notable spectrum peaks are significant in a typical SiC EL spectrum. As shown in Figure 7a, the UV peak centered at \(\sim\)390 nm due to band-to-band recombination, and the blue-green peak centered at \(\sim\)500 nm owing to the recombination of deep Boron states with the conduction band and acceptor states caused by doping elements and lattice impurities [99; 100; 101]. Since the remaining energy in the recombination process is released in  
Figure 6: (a) EL of a SiC MOSFET body diode. Reprinted with permission from Ref. [83]. 2023, IEEE. (b) Generic schematic of TSOP sensing technique with SiC MOSFET module. the form of a photon, the relationship between the emitted light peak wavelength \(\lambda\) (nm) and photon energy \(E\) (eV) is given as:  
\[\lambda\ (\text{nm})=1240/E\ (\text{eV}) \tag{6}\]  
Since the energy bandgap of semiconductors is temperature-dependent, the spectral power distribution properties of Si and SiC, and other features such as the peak wavelength and spectral bandwidth, can be used to characterize junction temperature variations [102; 103]. The experimental work in [104] investigated the EL of SiC MOSFET, and developed an electrothermal-optic model that showed the relationship between the EL intensity, junction temperature, and forward current, which is given in the form of total light intensity \(I_{EL}\) as:  
\[I_{EL}=a_{o}i[1+\ k_{1}\ \Delta T]+b_{o}i^{2}\ [1+\ k_{2}\ \Delta T] \tag{7}\]  
where \(a_{o}\) and \(b_{o}\) are the coefficients due to the effect of junction temperature, \(i\) is the forward current, \(k_{1}\) and \(k_{2}\) are the constant coefficients, and \(\Delta T\) is the change in the junction temperature of the device. Thus, at a given forward current condition, light intensity varies with respect to junction temperature. Moreover, the device output voltage \(V_{0}\) is not only a function of \(V_{T0}\) (initial junction voltage) but also the temperature change, and is expressed as:  
\[V_{0}=V_{T0}\big{[}1+\ k_{1}\ \big{(}T_{0}\ -T_{j}\big{]} \tag{8}\]  
where \(T_{0}\) and \(T_{j}\) are the given temperature at \(V_{T0}\) and the junction temperature, respectively. Thus, the expressions for \(I_{EL}\) and \(V_{0}\) in Equations (7) and (8), show that junction temperature varies linearly with an increase in light intensity extracted from the device, corresponding to the spectral waveform as shown in Figure 7a [105].