Development of the First Symmetric Blocking SiC P-type Gate Turn-off Thyristor
In February 2015, researchers at the Future Renewable Electric Energy Delivery and Management Systems (FREEDM) Center, a National Science Foundation (NSF) Engineering Research Center (ERC) based at North Carolina State University in partnership with Arizona State University, Florida State University, Florida A&M University, and the Missouri University of Science and Technology, completed development of the world’s first symmetric blocking, silicon carbide (SiC) positive charge (p-type) gate turn-off (GTO) thyristor. Testing of the thyristor confirms symmetric blocking capability of 3.3 kiloVolts (kV) forward and 3.8 kV reverse blocking voltages, respectively.
For alternating current (AC) power grid applications such as the FREEDM Fault Isolation Device, power semiconductor devices that can block voltages in both directions are needed. Unfortunately, most devices being developed can only block voltage in one direction; therefore, another diode in series is needed. This causes higher losses, hence lower energy efficiency. Therefore, development of a symmetric power blocking device can improve energy efficiency. Improvements to this initial thyristor design would improve the voltage blocking capabilities to well over 10 kV to assure even greater energy efficiency.
In previous years, an orthogonal positive bevel, fabricated by dicing, was proposed and demonstrated to achieve the thyristor’s novel reverse blocking capability. This approach has been successfully applied to the developed GTO. In reverse blocking mode, the bevel allows the depletion region to extend wider along the bevel surface to reduce the electric field to less than one half of that in the active area.
Chip size of the fabricated device measures 6.37 millimeters (mm) by 6.37 mm. The active device area is about 17mm2 (4.12mm by 4.12 mm). Forward I-V characteristics at elevated temperatures confirm the conductivity modulation in the drift layer. The symmetric blocking capability showed 3.3kV and 3.8kV forward and reverse blocking voltages, respectively.