Thermal Management of High Power GaN-HEMTs by Surface Activated Bonding of SiC/Diamond

Fujitsu Ltd. has 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 expand the range of radio waves for weather radars and wireless communications and make it possible to increase the output power of GaN-HEMTs.

Recently high-frequency GaN-HEMT power amplifiers have been used for long-range radio applications, such as radar and wireless communications. They are also expected to be used in the forthcoming 5G millimeter-band mobile communications protocols.

Although the SiC substrate has relatively high thermal conductivity, a material with even better thermal conductivity is needed for devices with higher power output to efficiently carry device heat to the cooling structure. Single-crystal diamond has extremely good thermal conductivity - almost five times that of a SiC substrate - and is known as a material that can efficiently spread heat.

A technology was 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, the 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.

Simulations using the GaN-HEMT/SiC on a single-crystal diamond by SAB showed that this technology would significantly reduce the total thermal resistance by more than 30 percent 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, Fujitsu 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.

Use of this technology promises to yield GaN-HEMT power amplifiers for transmitters with even higher power output. When used in systems such as weather radars, GaN-HEMT power amplifiers for transmitters can be expected to increase the radar’s observable range by a factor of 1.5. This would allow quicker detection of the cumulonimbus clouds that can produce sudden rainstorms, contributing to greater disaster readiness, says Fujitsu.

Fujitsu plans to assess the thermal resistance and output performance of GaN-HEMTs using this technology, aiming to implement it in high-output, high-frequency power amplifiers in fiscal 2020, for application in weather radar and 5G wireless communications systems.