Kohei Sasaki
Senior Manager, Novel Crystal Technology, Inc.

个人简介

Kohei Sasaki was born in Sendai, Japan, in 1981. After receiving the M.Eng. degree from Nagaoka University of Technology in 2006, he joined Tamura Corporation where he developed gallium oxide homoepitaxial growth technique by using molecular beam epitaxy. In 2015, he joined Novel Crystal Technology, Inc. to develop epitaxial growth technique and power devices of gallium oxide. He received the Ph.D. degree in Engineering from Kyoto University in 2016.
 
Development of gallium oxide power devices
 
Gallium oxide (β-Ga2O3) is a suitable material for next generation high power devices because of its excellent material properties and mass producibility. Table 1 shows the material properties of β-Ga2O3 and other major semiconductors. β-Ga2O3 has the large bandgap of about 4.5-4.9 eV. Electric field strength of β-Ga2O3 is estimated to be 8.0 MV/cm from relationship between bandgap and electric field strength. Baliga’s figure of merit of β-Ga2O3 calculated from material properties is ten times higher than that of silicon carbide (SiC). This mean the conduction loss of β-Ga2O3 will be ten times smaller than that of SiC. Another big advantage of β-Ga2O3 is that bulk crystal can be fabricated by using conventional melt growth method such as Czochralski, edge-defined film-fed growth (EFG), floating zone, or vertical Bridgaman method at atmospheric pressure. Figure 1 shows the 6-inch β-Ga2O3 single crystal substrate grown by EFG method. We will start to sell that near future.

β-Ga2O3 has the suitable material properties for power device applications. However, most current device structures are not enough to take advantage of the full potential of β-Ga2O3 because these structures are optimized for material properties of silicon. To bring out the potential of β-Ga2O3, we propose a trench structure. We will share our recent progress with trench-type β-Ga2O3 devices.

Figure 2(a) is a schematic illustration of β-Ga2O3 metal-oxide-semiconductor Schottky barrier diodes (MOSSBDs) [1, 2]. Si-doped β-Ga2O3 film was grown on a Sn-doped (001) β-Ga2O3 substrate by using halide vapor phase epitaxy (HVPE). Donor concentrations and thickness are shown in fig. 2(a). The HfO2 film was deposited on the trench bottom and sidewall. This device had a simple field plate structure made with SiO2 films. Figure 2(b) shows the reverse characteristics of β-Ga2O3 MOSSBD and commercially available 600 V class SiC SBDs. β-Ga2O3 MOSSBD had a similar small leakage current level for SiC SBDs. Figure 2(c) shows the forward characteristics. The β-Ga2O3 MOSSBD had about a 40% lower forward voltage than that of the SiC SBDs. We thus successfully demonstrated that the performance of β-Ga2O3 devices can exceed that of SiC devices.

Figure 3(a) is a schematic illustration of the β-Ga2O3 junction barrier Schottky (JBS) diode [3]. The p-type region was made by using p-NiO. Normal SBDs were also fabricated on the same wafer. Moreover, a p-NiO/n-Ga2O3 pn diode (PND) was fabricated for comparison. Figure 3(b) shows the reverse characteristics. The JBS had several orders of magnitude less leakage current than that of the normal SBD. This result indicates that the electric field at the Schottky metal/β-Ga2O3 interface decreased as a result of using the junction barrier structure. Figure 3(c) shows the forward characteristics. The SBD and JBS show similar characteristics because the forward current of JBS flowed by using the Schottky junction as well as the SBDs. These results indicate that incorporating a JBS structure is highly effective for decreasing leakage current and that a p-NiO/n-Ga2O3 hetero-junction can be a useful pn junction.

Figure 4(a) is the schematic illustration of the β-Ga2O3 trench MOS field effect transistors [4]. We used a static induction transistor-type structure that can be made only with n-type semiconductors. Si-doped β-Ga2O3 n+ contact and n- drift layers were grown on Sn-doped (001) β-Ga2O3 substrate with HVPE. The gate dielectric was HfO2. SiO2 film was used for isolation between a source and gate electrodes. Figure 4(b) shows drain current density-voltage (Jds-Vds) characteristics. The device showed clear current modulation characteristics and a maximum current density of 1.36 kA/cm2. The Jds-Vds curves did not pinch-off due to a short channel effect. Figure 4(c) shows transfer characteristics at a Vds of 10 V. The device had a high on-off ratio of over 107. In mid. 2018, Novel Crystal Technology and Cornell University developed β-Ga2O3 FETs with a similar structure to what is shown in fig. 4(a) [5]. The device had normally off characteristics and a high breakdown voltage over 1 kV despite its provisional structure. These results clearly indicate that β- Ga2O3 has excellent potential for power-device applications. In the future, we hope to devise ultra-low-loss and high-voltage β-Ga2O3 FETs by further improving the device structure.
 
ACKNOWLEDGEMENTS
 
This study was supported by the NIMS Nanofabrication Platform of the Nanotechnology Platform Project sponsored by the Ministry of Education, Culture, Sports, Science and Technology (MEXT), Japan and by Grant 12025014(F-17-IT-0002).
 
REFERENCES
[1] K. Sasaki et al., IEEE Electron Device Lett., vol. 38, no. 6, pp. 783-785, (2017).
[2] K. Sasaki et al., IWGO2017, O12.
[3] K. Sasaki et al., ICSCRM2017, WE.E1.7.
[4] K. Sasaki et al., Appl. Phys. Express, vol. 10, pp. 124201, (2017).
[5] Z. Hu et al., IEEE Electron Device Lett. Vol. 39, No. 6, 869 (2018).