In this work, we investigate the thermal response of GaN PIN diodes grown on a sapphire substrate and compare the results to GaN PIN diodes grown on a free standing GaN substrate (FS-GaN). Until now, thermal characterization techniques have been developed to assess the temperature distribution across lateral devices. Raman thermometry has shown to accurately measure the temperature rise across the depth of the GaN layer. Implementing this technique to assess the temperature distribution across the depth of a vertical GaN device is more challenging since a volumetric depth average is measured. The use of TiO2 nanoparticles is shown to overcome this issue and reduce the uncertainty in the peak temperature by probing a surface temperature on top of the device. For the sapphire substrate, an additional temperature rise of about 15 K was seen on the surface of the PIN diode as compared to the average in the bulk. While the steady state thermal measurements show an accurate estimation of the device’s peak temperature, the PIN diodes are normally operated under pulsed conditions and the thermal response of these devices under periodic joule heating must be assessed. A recently developed transient thermoreflectance imaging technique (TTI) is used in this study to monitor transient temperature rise and decay of top metal contact. Under the same biasing conditions, the FS-GaN PIN diode is found to result in less than half the temperature rise obtained by the sapphire substrate diode. Extracting time constants, a longer rise and decay is also observed in the sapphire substrate diode.

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