Diamond-based GaN Heat Dissipation

Diamond-based GaN heat dissipation

The third generation semiconductor industry is the cornerstone of the development of modern society, and is a strategic, fundamental and leading industry that supports economic and social development and guarantees national security. It has broad prospects in optoelectronics, high-power devices and high-frequency microwave devices, among which, GaN is characterized by large band gap (3.39eV at room temperature), high breakdown electric field intensity (3.3MV/cm), high saturation electron velocity (2.5×107cm/s), high thermal conductivity (1.5W·cm-1·K-1), strong radiation resistance and easy to form heterogeneous structures. It is very suitable for the development of high frequency, high power microwave, millimeter wave devices and circuits, and has extremely high application value in 5G communications, aerospace, national defense and other fields. But the thermal effect seriously restricts its performance.

Diamond, as the material with the highest thermal conductivity in nature, can reach 2000W/mK. Using diamond with high thermal conductivity as the cooling substrate or heat sink of Gan-based power devices is expected to improve its "self-heating effect" and realize the best choice for high frequency and high power applications, which can achieve very close to the effective thermal conductivity surface of chips. However, it is difficult to combine these two things. The technical difficulties of diamond-based GaN devices lie in the interface thermal resistance between diamond and GaN, lattice mismatch, thermal stress, and low thermal conductivity of CVD grown nanocrystalline diamond. CVD growth is generally at 800 or 800 degrees Celsius. Because of the difference between the thermal expansion coefficient of GaN and diamond, when the device is cooled to normal temperature, there will be great thermal stress at the interface, which will cause the device to crack. The growth of diamond on GaN requires a protective layer. This extra protective layer increases the interfacial thermal resistance and greatly reduces the heat dissipation efficiency of diamond. Moreover, the diamond grown by CVD is nanocrystal near the interface, and the thermal conductivity is very low, only tens of W/m·K. After the CVD grows several microns thick, the thermal conductivity of polycrystalline diamond increases to the near-bulk material. This layer of low quality diamond also impedes the heat dissipation of GaN devices. Therefore, how to use diamond as heat sink or substrate for GAN-based power devices has become a research hotspot. At present, a variety of technologies have been reported, including GaN heat dissipation technology on polycrystalline diamond substrate, single crystal diamond substrate heat dissipation technology, high thermal conductivity diamond passivation layer heat dissipation technology, etc.

Combination of GaN & Diamond:

01 Diamond on GaN

Diamond growth on GaN HEMT structure.

The epitaxial growth of polycrystalline diamond with < 10um thickness on a 50.8 mm (2 in) silicon-based gallium nitride HEMT has been achieved by a microwave plasma chemical vapor deposition facility. Scanning electron microscope and X-ray diffractometer were used to characterize the surface morphology, crystal quality and grain orientation of the diamond film. The results show that the surface morphology of the sample is relatively uniform, and the diamond grains basically show the growth of (111) plane with high crystal orientation. During the growth process, gallium nitride (GaN) is effectively avoided by hydrogen plasma etching, so that gallium nitride properties do not change significantly before and after diamond coating.

02 GaN on Diamond

GaN structures are grown by direct epitaxy on diamond substrate.

In the epitaxial growth of GaN on Diamond, aluminum nitride AIN is grown as GaN epitaxial layer by Compound Semiconductor (Xiamen) Technology Co., Ltd. with a special process. Currently, the products of Compound Semiconductor (Xiamen) Technology Co., Ltd. are available: Epi-ready-GaN on Diamond (AlN on Diamond).

03 GaN/ diamond bonding

GaN HEMTs are transferred and bonded to diamond substrates after preparation.

The technical indexes of the diamond and wafer grade diamond products of Compound Semiconductor (Xiamen) Technology Co., Ltd. have reached the world leading level. The surface roughness of the wafer grade diamond growth surface Ra < 1nm, and the thermal conductivity of the diamond hot sink sheet has reached 1000-2000W/m.K. By bonding with GaN, the temperature of the device can be reduced effectively, and the stability and life of the device can be improved.