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Featuring broadband operation and high efficiency, gallium nitride (GaN)-based radio frequency (RF) power amplifiers are key components to realize the next generation mobile network. However, to fully implement GaN high electron mobility transistors (HEMT) for such applications, it is necessary to overcome thermal reliability concerns stemming from localized extreme temperature gradients that form under high voltage and power operation. In this work, we developed a deep-ultraviolet thermoreflectance thermal imaging capability, which can potentially offer the highest spatial resolution among diffraction-limited far-field optical thermography techniques. Experiments were performed to compare device channel temperatures obtained from near-ultraviolet and deep-ultraviolet wavelength illumination sources for the proof of concept of the new characterization method. Deep-ultraviolet thermoreflectance imaging will facilitate the study of device self-heating within transistors based on GaN and emerging ultra-wide bandgap semiconductors (e.g., β-Ga2O3, AlxGa1-xN, and diamond) subjected to simultaneous extreme electric field and heat flux conditions. 

Abstract Researchers have been extensively studying wide-bandgap (WBG) semiconductor materials such as gallium nitride (GaN) with an aim to accomplish an improvement in size, weight, and power of power electronics beyond current devices based on silicon (Si). However, the increased operating power densities and reduced areal footprints of WBG device technologies result in significant levels of self-heating that can ultimately restrict device operation through performance degradation, reliability issues, and failure. Typically, self-heating in WBG devices is studied using a single measurement technique while operating the device under steady-state direct current measurement conditions. However, for switching applications, this steady-state thermal characterization may lose significance since the high power dissipation occurs during fast transient switching events. Therefore, it can be useful to probe the WBG devices under transient measurement conditions in order to better understand the thermal dynamics of these systems in practical applications. In this work, the transient thermal dynamics of an AlGaN/GaN high electron mobility transistor (HEMT) were studied using thermoreflectance thermal imaging and Raman thermometry. Also, the proper use of iterative pulsed measurement schemes such as thermoreflectance thermal imaging to determine the steady-state operating temperature of devices is discussed. These studies are followed with subsequent transient thermal characterization to accurately probe the self-heating from steady-state down to submicrosecond pulse conditions using both thermoreflectance thermal imaging and Raman thermometry with temporal resolutions down to 15 ns.