Enhancement of Heat Dissipation in AlGaN/GaN High-electron-mobility Transistors Using Substrate-transfer Technique

Masanobu Hiroki, Kazuhide Kumakura, and Hideki Yamamoto
Materials Science Laboratory

　III-nitride semiconductor materials can be applied for high-power devices by exploiting their high critical electric field. Under high current operation, however, power performance is limited due to the generation of Joule heat. We transferred AlGaN/GaN high electron mobility transistors (HEMTs) from a sapphire substrate to a copper plate using the hexagonal boron nitride epitaxial lift-off technique [1,2]. In this study, we transferred the HEMTs to a copper plate using Au-Au thermocompression bonding and found an increase in drain current (I_d) and significant improvement of the heat dissipation [3].
　Figure 1 shows an I-V characteristics of an AlGaN/GaN HEMT before release and after transfer. Before release, a large reduction in drain current I_d vs drain bias (V_ds) was observed in the saturation region. In contrast, for the HEMT after transfer, the negative slope of I_d vs V_dsdecreased. In addition, we found a significant increase in maximum I_d from 0.72 to 0.91 A/mm after transfer. The maximum transconductance also increased from 110 to 140 mS/mm. These indicate an increase in two-dimensional electron gas (2DEG) density. The increase in 2DEG density is likely caused by the reduction in compressive stress in the GaN layer, which is revealed from the E₂ peak shift of –1.3 cm^–1 in Raman spectroscopy measurement.
　Figure 2 shows temperature maps of a HEMT transferred to a copper plate and another HEMT on a sapphire substrate. For the transferred HEMT, the temperature increased from 50 to 150°C with increasing power dissipation (P) from 0.3 to 1.5 W. For the HEMT on sapphire (before release), the temperature reached 240°C at P of 0.7 W. Our results indicate that the transfer technique using the h-BN release layer can enhance the heat dissipation and boost the power performance of GaN-based devises.


Fig. 1. I-V characteristics of AlGaN/GaN HEMT before release and after transfer.	Fig. 2. Temperature maps of a HEMT transferred to copper and another HEMT on sapphire.