A review of thermoreflectance techniques for characterizing wide bandgap semiconductors’ thermal properties and devices’ temperatures

Thermoreflectance-based techniques, such as pump–probe thermoreflectance (pump–probe TR) and thermoreflectance thermal imaging (TTI), have emerged as the powerful and versatile tools for the characterization of wide bandgap (WBG) and ultrawide bandgap (UWBG) semiconductor thermal transport properties and device temperatures, respectively. This Review begins with the basic principles and standard implementations of pump–probe TR and TTI techniques, illustrating that when analyzing WBG and UWBG materials or devices with pump–probe TR or TTI, a metal thin-film layer is often required. Due to the transparency of the semiconductor layers to light sources with sub-bandgap energies, these measurements directly on semiconductors with bandgaps larger than 3 eV remain challenging. This Review then summarizes the general applications of pump–probe TR and TTI techniques for characterizing WBG and UWBG materials and devices where thin metals are utilized, followed by introducing more advanced approaches to conventional pump–probe TR and TTI methods, which achieve the direct characterizations of thermal properties on GaN-based materials and the channel temperature on GaN-based devices without the use of thin-film metals. Discussions on these techniques show that they provide more accurate results and rapid feedback and would ideally be used as a monitoring tool during manufacturing. Finally, this Review concludes with a summary that discusses the current limitations and proposes some directions for future development.