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Using this technology, heat can efficiently cool high-power gallium nitride (GaN) high electron mobility transistors (HEMTs), allowing the power amplifier to operate stably at high power levels. Traditional GaN HEMT - SiC Substrate Heat Dissipation In recent years, high frequency GaN-HEMT power amplifiers have been widely used in remote radio fields such as radar and wireless communications, and are expected to be used for weather radar to observe local rainstorms, or upcoming 5G mm. Band mobile communication protocol. For these microwave-to-millimeter-wave radar or wireless communication systems, by increasing the output power of the GaN-HEMT power amplifier for transmission, the distance that radio waves can travel will increase, and the radar observation range can be extended to achieve farther and more High-capacity communication. 
Figure 1. Traditional GaN HEMT Power Amplifier In Figure 1, some of the input power of a GaN HEMT power amplifier is converted into heat and then dispersed into the SiC sinker. Increasing the range and power of radar and wireless communications also increases the heat generated by the device, which adversely affects its performance and reliability, so the device heat needs to be efficiently transferred to the cooling structure (heat sink). Diamond- SiC heat dissipation Although SiC substrates have a relatively high thermal conductivity, for devices with increasingly higher power output, materials with better thermal conductivity are needed to efficiently transport device heat to the cooling structure. . Single crystal diamond has very good thermal conductivity - almost five times that of SiC substrates - and is called a material that can effectively dissipate heat. Diamond-SiC bonding method In order to bond single crystal diamond to a device as a cooling material, a normal production process uses an argon (Ar) beam to remove impurities, but this forms a low-density damaged layer on the surface, which weakens A bond that may be formed by single crystal diamond. Further, an insulating film such as silicon nitride (SiN) is used for bonding, and thermal conductivity is impaired due to the thermal resistance of SiN. To prevent the Ar beam from forming a damaged layer on the diamond surface, Fujitsu has developed a technique to protect the surface with a very thin metal film before exposure to the Ar beam (see Figure 2). In order to ensure that the surface is planar (for good bonding at room temperature), the thickness of the metal film needs to be limited to 10 nm or less. 
Figure 2. GaN HEMT power amplifier structure bonded to diamond. This technique has been proven to prevent damage to the diamond surface after the Ar beam is exposed (Figure 3), thereby increasing the bond strength and allowing the single crystal diamond to be at room temperature. SiC substrate bonding. 
Fig. 3 Thermal resistance test results of diamond cross section after exposure of Ar beam The thermal resistance of the viscous sample was measured at room temperature, and the thermal resistance of the SiC/diamond interface was found to be extremely low at 6.7×10-8 m2K/W. Simulations using this measurement parameter show that this technique will significantly reduce the thermal resistance of a 200W GaN-HEMT device down to 61% of existing devices (equivalent to a surface temperature drop of 80°C), as shown in Figure 4. 
Figure 4. Simulation of a 200W class GaN HEMT power amplifier. This technique can therefore be used to produce GaN-HEMT power amplifiers with higher output power emitters. When used in systems such as weather radars, GaN-HEMT power amplifiers for transmitters are expected to increase the radar's observable range by a factor of 1.5, which allows for faster detection of cumulonimbus clouds that can produce sudden heavy rains. be prepared. Fujitsu plans to use this technology to evaluate the thermal resistance and output performance of GaN-HEMT, and plans to apply it to high-output, high-frequency power amplifiers for weather radar and 5G wireless communication systems by 2020. The Ministry of Defence of Japan provided research funding to obtain the Innovative Science and Technology Initiative for Security set up by the Japanese Ministry of Defense's Purchasing, Technology and Logistics Bureau (ALTA). This research was partially supported by the Innovation and Technology Security Initiative established by the Japanese Ministry of Defence, the Technology and Logistics Bureau (ALTA).
Japan's Defense Ministry sponsors Fujitsu to study the heat dissipation technology of diamond and silicon carbide substrates, which will significantly improve the performance of GaN HEMT
At the IEEE Semiconductor Interface Expert Meeting (SISC2017) in California from December 6th to 9th, Fujitsu Laboratories Ltd. and its subsidiary Fujitsu Laboratories Ltd introduced the allegedly the first single crystal at room temperature. The key to welding diamond and silicon carbide (SiC) substrates is that both are hard materials but have different coefficients of thermal expansion.