While high-power, high-frequency SiC and GaN-based third generation components and systems are becoming widely used, fourth generation ultra-wide bandgap gallium oxide, diamond and other new generation materials are gaining popularity.
Technical Principles and Advantages
Although the current semiconductor industry is dominated by IC and electronic components based on Si substrates maturely, such products still face many limits, in both high-power and high-frequency components and systems. Structural designs are continuously being improved to deal with these limitations. In addition, new, innovative materials are being developed. The high-power, high-frequency SiC and GaN-based third generation components and systems are becoming more and more widely used in various related industries.
Despite this, fourth generation ultra-wide bandgap gallium oxide (Ga2O3), diamond and other new generation materials are gaining popularity. In particular, Ga2O3 substrate’s production is easier than that of SiC and GaN, and the material with ultra-wide bandgap also enables it to sustain a high breakdown voltage and critical electric field at higher voltages. As such, its potential applications in the field of ultra-high power components should not be underestimated. Figure (1) (a) shows the applicable frequency and operating power range of commonly used semiconductor materials today, and (b) shows the corresponding bandgap and breakdown voltage of commonly used semiconductor materials today . As you can see, the power range of Ga2O3 applications can be as high as 1kW-10kW.
Ga2O3 has five crystalline phases: the polymorphs monoclinic (β-Ga2O3), rhombohedral (α), defective spinel (γ), cubic (δ), and orthorhombic (ε)). It also has an ultra-wide energy gap of 4.5-4.9eV and an Electrical breakdown field (Ebr) of 8MV/cm. In contrast, GaN has a bandgap of only 3.4 eV, and SiC has a bandgap of only 3.3 eV. According to the Baliga’s Figure-Of-Merits (BFOM), the wide bandgap semiconductor coefficient for Ga2O3 can be as high as 3444, which is ten times that of SiC and four times that of GaN. This coefficient is related to the highest voltage a component can withstand. From this BFOM coefficient, we can see Ga2O3 the material’s great potential for application in the field of high-power components. See Table (1) for a comparison of the related characteristics of these materials.
Figure 1 (a) The applicable frequency and operating power range of commonly used semiconductor materials and (b) The bandgap and breakdown field strength of commonly used semiconductor materials
Table 1 Comparison of Relevant Material Properties
Though a sufficiently high breakdown electric field is important for application in high-power components, the ON-resistance is also an important parameter. As you can see in Figure (2), the ON-resistance of Ga2O3 is lower than that of GaN and SiC. Therefore, Ga2O3 has potential applied as a rectifier in both industry and military applications . Nowadays, the industrial applications of SiC and GaN have matured a great deal. In contrast, Ga2O3 applications are still in development. We can see that it has a promising future, but there are still many problems that must be overcome.
Figure 2 The relationship between the breakdown voltage and the ON-resistance of wide bandgap materials 
At present, the main problem with Ga2O3 is that it has difficulty with heat dissipation and P-type doping. In terms of heat dissipation, it has been found that its thermal conductivity (0.25W/cm.K) is poorer than that of other high-power materials. SiC has a thermal conductivity of 4.9W/cm.K, while GaN has a thermal conductivity of 2.3W/cm.K. Poor thermal conductivity can cause the component functions to thermally-induced breakdown during operation. Currently, the issue is mainly improved through structural designs such as using a substrate with high thermal conductivity to help shunt the heat from high temperatures during operation.
Power management applications in the booming electric vehicle (EV) sector and consumer electronics devices, such as chargers or adapters, are fueling demand for silicon carbide (SiC) and gallium nitride (GaN) components. This month, we look into the latest developments in the wide-bandgap (WBG) materials and the way forward for the industry.
The issue with P-type doping, however, is trickier to deal with. At present, there is not enough relevant literature on the subject of hole mobility for reference. There are three main reasons behind this issue. One, since the covalent bond of oxygen in Ga2O3 is a 2p orbital, the electrons are difficult to take away, resulting in the deep acceptor state. Second, Ga2O3‘s effective mass is too high, which causes the edge of the flat valence band to lean towards oxygen. Finally, because free electron holes are easily self-trapped in lattice distortions, it is impossible to achieve diffusion and low electric field drift. These are some of the problems currently faced by Ga2O3, and they need to be improved if the material is to be used in more diverse applications.
In terms of crystal growth, the main techniques include the floating zone (FZ), edge defined film (EFG), and Czochralski (CZ) methods. These techniques have been used for years for the production of sapphire substrates. Compared to GaN and SiC compound semiconductor materials, Ga2O3 is better able to achieve mass production and reduced production costs. Today’s commercial production mainly uses the EFG crystal growth method. As shown in Figure (3) , this method can produce large numbers of high purity Ga2O3 wafers. Melt high-purity (5N) Ga2O3 powder in a crucible under a N2/O2 environment. Then extract the ingots from the crystal at a rate of 15mm per hour. Finally, it’s time to clean and cut. For n-type substrate, use elements such as Sn or Si doped further.
Figure 3 Schematic Diagram of Ga2O3 Ingots Grown Using the EFG Method 
Ga2O3 can be used in many ways because of its many excellent characteristics [4, 5]. In particular, as shown in Figure (4), its wide bandgap characteristics mean it has great potential for application in power devices. According to Figure (4) (a), it can make significant contributions to everything from the electric car and power systems to wind turbines and more. In terms of optoelectronic components, because Ga2O3 film is transparent, it can be used as the components of transparent panels. It can also be applied in the fields of light and gas sensors, as you can see in Figure (5) (b).
In terms of industry prospects, though Ga2O3 is already widely used, its great potential means that there are still many electronic components just waiting to be developed and commercialized. Therefore, it can be said to be truly a material of the next generation!
Figure 4 Ga2O3 Current and Future Sensor Applications
Figure 4b Ga2O3 Current and Future Sensor Applications (Prof. Klaus-Dieter Kohl et al. Journal of Materiomics Volume 5, Issue 4, December 2019, Pages 542-557)
In conclusion, from homojunction epitaxy to heterojunction epitaxy, Ga2O3 is still a very new material. However, whether it is crystalline phase identification, physical measurements, surface topography or even composition identification and doping concentration measurement, MA-tek can provide related analysis and testing services.
Higashiwaki, K. Sasaki, A. Kuramata, T. Masui, and S. Yamakoshi, “Development of gallium oxide power devices,” Phys. Status Solidi A 211, 21–26 (2014).
Higashiwaki, K. Sasaki, A. Kuramata, T. Masui, and S. Yamakosh. “Gallium oxide (Ga2O3) metal-semiconductor field-effect transistors on single-crystal b-Ga2O3 (010) substrates”, Appl. Phys. Lett, 100, 013504 (2012)
Kuramata, K. Koshia, S. Watanabe, Y. Yamaoka, T. Masui, and S. Yamakoshia, “Bulk Crystal Growth of Ga2O3”, Proc. SPIE 10533, Oxide-based Materials and Devices IX, 105330E (2018).
Pearton, F. Ren, M. Tadjer, and J. Kim. “Perspective: Ga2O3 for ultra-high power rectifiers and MOSFETS”, J. Appl. Phys. 124, 220901 (2018).
Afzal, “b-Ga2O3 nanowires and thin films for metal oxide semiconductor gas sensors: Sensing mechanisms and performance enhancement strategies”, J. Materiomics, 5, 542 (2019).
About the Author
Ray-Hua Horng is a distinguished professor of the Institute of Electronics, National Yang Ming Chiao Tong University, in Taiwan.
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