Multiwavelength Laser-Based Transient Thermoreflectance for Channel-Temperature Monitoring of GaN HEMTs

Simultaneous high-spatial and high-temporal resolution channel-temperature characterization methods are in great demand but still lacking for GaN devices. In this work, we developed a multiwavelength laser-based transient thermoreflectance technique (MWL-TTR) by using a 320-nm continuous wave (CW) laser to monitor the channel temperature and a 532-nm CW laser to monitor the metal contacts, achieving submicron spatial and nanosecond temporal resolution. The photocurrents effects induced by the above-bandgap 320 nm laser on temperature monitoring, which are used to be ignored, are quantitatively investigated and eliminated. A reliable calibration of the thermoreflectance coefficient (<inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex-math notation="LaTeX">${C}_\text{th}$</tex-math></inline-formula>) is realized by MWL-TTR, laying the foundation of accurate channel-temperature monitoring. Based on this technique, several GaN HEMTs are measured to assess the validity of this technique.