III-nitride semiconductor materials have potential for high-power device applications by exploiting their high critical electric field. Under high-current operation, however, power performance is limited because of the generation of Joule heat (the so-called self-heating effect). Therefore, thermal management in the GaN-based devices is necessary to further improve high-power performance. One approach to overcome this problem is to grow device structures on highly thermal conductive substrate. However, the lattice mismatch or the difference in thermal expansion coefficients between substrates and GaN often limit the crystal quality and thickness of the layers. We propose a different approach, where the devices are transferred to foreign materials with high thermal conductivity. Recently, we have developed a mechanical transfer technique using release layer (MeTRe mothod), in which h-BN is inserted between a GaN layer and sapphire substrate as a release layer . Using this method, we have demonstrated strong emission from transferred InGaN/GaN multiple quantum well light emitting diodes (LEDs) . In this study, we applied the same approach to electronic devices . We fabricated AlGaN/GaN high-electron-mobility transistors (HEMTs) on h-BN/sapphire substrates and transferred them to copper plates, which led to an improvement of device performance due to enhanced heat dissipation.
Figure 1 shows typical I-V characteristics of an AlGaN/GaN HEMT before release from the substrate and after transfer to the copper plate. We obtained good pinch-off and saturation characteristics before release and after transfer. Before release, a large reduction in drain current Id with increasing Vds, i.e., negative differential resistance, is observed in the saturation region. In contrast, the negative resistance is reduced for the transferred HEMT. The negative resistance is commonly attributed to the self-heating effect. Transfer from sapphire (thermal conductivity κ = 40 W/m K) to the copper plate (κ = 390 W/m K) improves the heat dissipation efficiency. We took temperature images of devices during operation with 1-W power dissipation (Fig. 2). Before release, a hot spot with a temperature of about 50°C is observed. In contrast, the temperature is as low as 30°C for the transferred HEMT. Our results indicate that the transfer technique using the h-BN release layer can enhance the heat dissipation and significantly improve the power performance of GaN-based devises.