# Junction Temperature Optical Sensing Techniques for Power Switching Semiconductors: A Review
## 1 Introduction  
Power switching semiconductors are indispensable elements in inverters and converters used in power grids/systems, automobiles, data centers, and renewable energy for reliability towards a more intelligent control system. While conventional switching devices applications are limited due to low switching speed and massive size, power switching semiconductors exhibit fast switching that can meet the load requirements and operating frequency of today's technology [1; 2]. Nowadays, over 1000 gigawats of renewable energy incorporated into the power grids is controlled by power-switching semiconductors [3]. Additionally, power electronic converters and switches, which contain semiconductor devices, are utilized to regulate almost 60% of the supplied electrical energy consumed in industrialized countries [4]. Nevertheless, during power and thermal cycling, one of the most common failures is the wear-out caused by thermal stress on these power-switching semiconductor devices due to variations in their junction temperature [5; 6; 7; 8]. Hence, real-time temperature sensing of circuits, including these devices, is of paramount importance. Recently, composite power switching devices such as IGBT- and Silicon Carbide (SiC)-based MOSFETs have gathered attention due to their improved performance characteristics(higher frequency operation, current handling capacity, and switching speed) than the conventional MOSFETs and bipolar junction transistors (BJT) [9]. Under normal operating conditions, the temperature of the semiconductor power switching devices rise above the ambient around the junction and other parts of the circuit. Furthermore, these devices are subjected to repetitive heat pulses due to high-frequency switching, which leads to the junction temperature variations as a result of the power losses [10]. This high junction temperature variations could be excessive and hamper the performance of the device due to overloading and abrupt changes in the input and output power of the circuit [3]. Although there is a relationship between the performance of an electronic component and its range of operating temperature, it has been well established in the literature that the susceptibility of an electronic assembly to fail increases exponentially with the junction temperature [11]. As such, this may lead to devastating effects of shutting down the power grid/system. Hence, several techniques have been proposed in the literature to measure the junction temperature of these critical power switching semiconductor devices. These techniques can be broadly classified as electrical-, physical-, and optical-based as illustrated in Figure 1.  
The electrical-based technique uses electrical devices or electrical parameters for temperature measurement. Typical electrical devices include thermal-sensitive electrical devices (TSED) that employ additional electronic components such as resistors, diodes, and externally designed electrical circuits for measurement. Although this method provides excellent spatial resolution, it requires high costs and adds to the system's complexity [10]. The temperature-sensitive electrical parameters (TSEP), such as the gate threshold voltage, saturation current, and short circuit current, are also suitable for online junction temperature sensing, but incur power loss to the system and thus are not suitable for measurement when the device is in operation. Another disadvantage of TSEP is that the device's temperature distribution cannot be obtained since this measurement provides a point temperature value of the chip [12; 13]. On the other hand, physical techniques include a thermistor and thermocouple (TC) that measure temperature differences, which are external to the system. Their techniques are simple to implement with excellent spatial resolution; however, the slow response in the measurement, especially in high-frequency circuits, remains the constraint for its deployment.