Diamond wafer as the heat spreader for the thermal dissipation of GaN-based electronic devices

The current GaN wafers are typically grown on sapphie, silicon (Si), silicon carbide (SiC), or free-standing GaN substrates, whose thermal conductivities are 35, 150, 400, and 280W/mK, respectively, which are far from the requirements. The ideal heat spreader would be a substrate that is both highly thermally conductive and electrically insulating. Diamond, with the thermal conductivity up to 2400W/mK at room temperature for the single crystals, and approaching 2000W/mK for the polycrystals, is the best candidate as a heat spreader for GaN power transistors . 

Over the past twenty years, a variety of methods have been developed to utilize diamond as the heat spreader  for AlGaN/GaN power transistors. The developed wafer is therefore called “GaN-on-diamond wafer”. The approaches are summarized in Figure 1, which include: (1) bonding of diamond to GaN wafers or directly to the HEMT devices with/without an adhesion layer, (2) GaN epitaxial growth on single-crystal or poly-crystal diamond substrate, then fabrication of HEMT devices, and (3) nanocrystalline or poly-crystalline diamond growth on the frontside or backside of GaN or the HEMTs devices. For the three approaches, the thermal resistance at the GaN/diamond, which is referred to the “effective thermal boundary resistance (TBReff)”, is one of the factors that significantly increases the overall temperatures during device operation. 

The bonding of GaN or the well-fabricated AlGaN/GaN HEMTs device with diamond can maintain the quality of both the GaN devices and diamond substrate. However, the dielectric interlayers are necessary for either the high-temperature or room-temperature bonding along with an amorphous layer induced by the surface activation. This interlayer impedes the heat flow from the device channel, leading to a large TBReff. The growth technique of the GaN or AlGaN/GaN HEMTs structures on the SCD substrates were developed in recent years. A relatively lower TBReff<10 m2K/GW was reported at the interface between epitaxial GaN and diamond substrate. However, although the HEMTs devices are demonstrated, the crystalline quality of the material and the mobility of the HEMT devices are still far from those grown on the conventional substrates of sapphire, Si, SiC and free-standing GaN substrates due to the large lattice mismatch and thermal mismatch. The CVD 

growth of the polycrystalline or nanocrystalline diamond on the GaN or HEMTs devices has the uniquenessofthe direct growth as close as possible to the Joule hot spots, which may be highly effective for the thermal dissipation. However, the damage to the GaN by hydrogen during CVD growth and the less nucleation of the diamond restrict the film quality, leading to a poor thermal conductivity of diamond and large TBReff. In addition, the large thermal expansion coefficient between diamond and GaN results in the stress problems in the GaN-on-diamond wafers, leading to the layer cracking and wafer bow and impact the electrical performance of the devices. Up to now, a variety of efforts have been performed to challenge the above problems with different methods. Without doubt, the heat dissipation using diamond wafer as heat spreader for the AlGaN/GaN HEMTs devices is a highly effective way to reduce the operation junction temperature compared to the devices on sapphire, Si and SiC substrates. Although the large TBReff exists between GaN and diamond, Fujitsu laboratory has reduced the device temperature during HEMT operation by more than 40%, and the temperature can be lowered by 100 °C or more due to the high thermal conductivity of diamond . Nevertheless, to take the full advantage of the GaN technology for the high-power applications, the reduction of the TBReff between GaN and diamond is still a challenge. Novel strategies and concepts are still required for the effective thermal management of GaN-based power devices using the diamond heat spreader.