400V Vertical GaN Transistor Switches 15A in 40ns
During the recent IEEE International Electron Devices Meeting (IEDM) in San Francisco, researchers from Panasonic Corp. described a normally-off vertical GaN-based transistor on a bulk GaN substrate with low specific on-state resistance of 1.0mÎ©âˆ™cmÂ² and high off-state breakdown voltage of 1.7kV. P-GaN/AlGaN/GaN triple layers were epitaxially regrown over V-shaped grooves formed over the drift layer. The channel utilizes so-called semi-polar face with reduced sheet carrier concentration at the AlGaN/GaN interface, which enables a threshold voltage of 2.5V and stable switching operations. The result was a 400V vertical GaN transistor that switches 15A in 40ns.
â€œThese results indicate that the demonstrated vertical GaN transistor is very promising for future high power switching applications,â€ stated the authors which included Daisuke Shibata, Ryo Kajitani, Masahiro Ogawa, Kenichiro Tanaka, Satoshi Tamura, Tsuguyasu Hatsuda, Masahiro Ishida, and Tetsuzo Ueda from Panasonicâ€™s Automotive and Industrial Systems Company.
The paper continues: GaN power transistors are emerging alternatives for future power switching systems with high efficiency and compact size. So far, lateral transistors using AlGaN/GaN heterojunction have been intensively investigated for commercialization. At present, various commercial samples of the lateral AlGaN/GaN transistors on Si substrates with the blocking voltage of 600 V or lower are available demonstrating superior performances of switching systems to those by conventional Si devices.
Although the performances including the reliability are sufficiently good for practical use, increasing the total output power of the switching systems using the lateral GaN transistors is limited up to several kW. Increasing both the output current and the breakdown voltage is necessary to enable higher output power, which has been difficult by the lateral device configuration.
Large area of the electrode on the surface and the spacing between the source and the drain are significant on the surface of the lateral device in case the device is designed for high power. In addition, the breakdown voltage is limited up to 1kV for the lateral GaN transistor on Si owing to the difficulty of increasing the thickness of GaN on Si.
Vertical GaN transistors on bulk GaN substrates are very promising to overcome the above limitation by the lateral device. Various prototypes have been reported, however, these are not sufficiently good for practical applications. One critical issue is smaller threshold voltage with poor pinch-off characteristics even though the devices claim that these are normally-off transistors.
Another issue is the stability of gate structure. Most of the normally-off vertical GaN transistors utilize oxide gate that people cannot believe as a practical choice at present owing to its low channel mobility and the inherent instability at the interface between the oxide and GaN. The authors then detail the structure and fabrication process employed to produce their vertical device.
They conclude: â€œA vertical GaN-based transistor on a bulk GaN substrate with low specific on-state resistance of 1.0mÎ©Â·cm2 and high off-state breakdown voltage of 1.7kV is demonstrated. Here, semipolar p-GaN/AlGaN/GaN triple layers are epitaxially regrown over V-shaped grooves formed over the drift layer resulting in high threshold voltage of 2.5 V. Stable gate characteristics and successful 400V/15A fast switching by the high current device are confirmed indicating that the presented device is satisfactory good for future high power application that cannot be operated by conventional lateral GaN transistors.â€