We have developed a unique technology for the surface activated bonding (SAB) of a single-crystal diamond to a SiC substrate at room-temperature (RT) for high-efficiency cooling of high-power GaN high electron mobility transistors (GaN-HEMTs). This technology is expected to significantly expand the range of radio waves for weather radars and wireless communications and make it possible to increase the output power of GaN-HEMTs.
First, we will introduce the technology we developed that bonds a single-crystal diamond to a SiC substrate using SAB. With existing SAB, the argon (Ar) beam exposure used for surface activating creates a low-density damaged layer on the surface of the diamond, which weakens bonding strength. However, our technology can prevent the formation of the low-density damaged layer by protecting the surface of the diamond with an extremely thin metallic film. Consequently, the bonding strength is improved by eliminating the damaged layer and the SiC/diamond interface was found to have an extremely low thermal resistance of 67 m2K/GW.
Next, we will report on thermal analysis of GaN-HEMT/SiC on a single-crystal diamond by SAB. Simulations using the above parameter showed that this technology would significantly reduce the total thermal resistance by more than 30% compared to the conventional GaN-HEMT/SiC without diamond. This indicates the possibility of further increasing the output power of GaN-HEMT power amplifiers (PAs). In addition, we simulated a cost-reduced diamond-bonded structure that assumes the use of low thermal conductive poly-crystalline diamond and heterogeneous integration for GaN-HEMT PAs. Consequently, to balance the cost with the cooling efficiency for GaN-HEMT PAs, it is necessary to optimize the size and thermal conductivity of the diamond heat spreader.
Finally, we will describe the device performance of our GaN-HEMT/SiC bonded on single-crystal diamond by SAB.