Diamond Breakthrough: Waseda University and PDS Unveil Normally-Off MOSFET for Advanced Power Electronics

Waseda University and Power Diamonds Systems (PDS) have developed a structure in which the surface of a diamond is covered with silicon dioxide terminals (C-Si-O terminals). In this structure, when the gate voltage is 0V, the transistor is turned off. As a result, they have announced the development of a “normally-off” diamond MOSFET.

The achievement was contributed by Professor Hiroshi Kawarada, FU Yu, Norito Narita, Xiahua Zhu from Waseda University, part-time professor Atsushi Hiraiwa from Waseda University, Kosuke Ota from PDS, and Tatsuya Fujishima, co-founder and CEO of PDS, among others. Detailed information was presented on December 13th at the International Electron Devices Meeting (IEDM 2023), a semiconductor device/process international conference organized by the IEEE.

MOSFET, standing for Metal-Oxide-Semiconductor Field-Effect Transistor, is a type of transistor with characteristics such as high speed, low on-resistance, and high breakdown voltage, making it particularly suitable as a switch element for motor drive and high-speed switching of large currents, which has already been achieved.

Concerning diamond semiconductors, often referred to as the ultimate power semiconductor material, research and development on diamond MOSFETs with hydrogen termination (CH structure) are being conducted worldwide. However, due to 2DHG, even with a gate voltage of 0V, the transistor conducts in a “normally-on” operation, making it impossible to achieve a normally-off state when the gate voltage is 0V.

Therefore, if normally-on devices were applied to power electronic devices, it would be impossible to safely stop the device when it ceases normal operation. Hence, achieving normally-off operation is crucial. In this context, PDS and the research group from Waseda University discovered that due to high-temperature oxidation, the C-H bonds of hydrogen atoms covering the diamond surface transform into CO bonds, turning the surface into an electron defect, leading to performance degradation. The company has been committed to improving this to achieve stable FET operation.

In this research, a device structure was adopted where the diamond surface has silicon dioxide (C-Si-O) bonds instead of the traditional CO-Si bonds. As a result, the hole mobility of p-channel MOSFET reaches 150 cm^2/V·s, surpassing the electron channel mobility of SiC n-channel MOSFET. The signal threshold voltage for normally-off operation is 3-5V, which is claimed to be a value that prevents accidental conduction (short circuit) and cannot be achieved with traditional diamond semiconductors.

Furthermore, PDS reported that the maximum drain current for the lateral silicon dioxide terminated diamond MOSFET exceeds 300 mA/mm, and for the vertical silicon dioxide terminated diamond MOSFET, it exceeds 200 mA/mm. This is said to be the highest value among normally-off diamond MOSFETs in this series.

Both companies assert that by covering the surface with C-Si-O bonds, they have created a more temperature-resistant and oxidation-resistant stable device compared to the traditional CH surface. The companies believe that its usability makes it suitable for large-scale production. Professor Kawarada believes that a diamond power semiconductor that is easy to implement in society has already been achieved, and PDS will continue to enhance the development of diamond MOSFETs, aiming for the popularization and practical use of diamond semiconductors. Their goal is to develop a device process suitable for mass production and achieve higher voltage resistance with a simpler structure.