Invited Paper EM1-WeM3
Progress and Future Challenges in SiC Material for High-Voltage Power Devices

Wednesday, November 12, 2014, 8:40 am, Room 311

Silicon carbide (SiC) is an emerging wide bandgap semiconductor, by which high-voltage, low-loss power devices can be realized owing to its superior properties. SiC unipolar devices such as Schottky barrier diodes and MOSFETs will replace Si unipolar/bipolar devices in the blocking-voltage range from 600 V to about 6500 V. Regarding SiC power MOSFETs, the authors developed vertical trench MOSFETs in collaboration with ROHM. The trench MOSFETs with cell miniaturization exhibited extremely low on-resistances of 0.79 mΩcm² and 1.4 mΩcm² for 630 V and 1260 V devices, respectively, with normally-off characteristics [1]. After reliability tests, all-SiC power modules (1200 V – 180 A) are now commercial products.

For ultrahigh-voltage applications above 6500 V, SiC bipolar devices such as PiN diodes and IGBTs are promising. Major technological challenges include fast epitaxy of thick (> 100 μm) and high-purity epilayers, stress control, reduction of basal-plane dislocations and stacking faults, enhancement and control of carrier lifetimes. The authors succeeded in fast epitaxial growth of 100-200 μm-thick SiC at a growth rate of 40-85 μm/h with a high purity (1x10¹³ cm^-3) and reduced density of basal-plane dislocations. Through elimination of the Z_1/2 center, a lifetime killer in SiC, via thermal oxidation, the authors obtained about 200 μm-thick, Z_1/2-free n-type SiC epilayers, where the bulk lifetime reaches 30 μs or even longer [2]. The carrier lifetimes can be controlled by low-energy electron irradiation, which can preferentially generate the Z_1/2 center (carbon vacancy) in SiC.

Ultrahigh-voltage PiN diodes were fabricated by using lightly-doped (2-3x10¹⁴ cm^-3) SiC epilayers with different thicknesses (50-260 μm). The breakdown voltage was scaled with increasing the epilayer thickness, as simulated. The maximum breakdown voltage experimentally obtained exceeded 27 kV, which is the highest blocking voltage among any solid state devices. The differential on-resistance was remarkably reduced by enhancement of carrier lifetimes, being 2 mΩcm² and 10 mΩcm² for 13 kV and 27 kV devices, respectively. A SiC bipolar junction transistor with a blocking voltage over 21 kV and current gain over 60 was also demonstrated [3].

This work was supported by the Funding Program for World-Leading Innovative R&D on Science and Technology (FIRST Program) and a Grant-in-Aid for Scientific Research from JSPS. The authors acknowledge Dr. T. Nakamura with ROHM for the MOSFET collaboration.

[1] T. Nakamura et al., Tech. Digest. 2011 IEDM, 26.5.1.

[2] S. Ichikawa et al., Appl. Phys. Exp., 5, 101301 (2012).