Normally-Off Operation GaN Based MOSFETs for Power Electronics

GaN is promising for high-power, high-temperature devices due to some of its large bandgap, high critical electric field, and high saturation velocity compared with Si and SiC. The MOSFET structure can be operated at a positive threshold voltage in the normally-off mode, which is preferable for power transistors in terms of fail-safe operation. In order to realize MOSFET operation, low resistance in the n⁺-contact layer and good interface quality at SiO₂/GaN are strongly required. We could reduce the interface state density at SiO₂/GaN by annealing at 900degC for 30 min. Furthermore, we successfully realized the formation of the n+ contact layer by annealing at 1260degC for 30 s. Finally, we have fabricated GaN MOSFETs and have achieved more than 2.5 A operation in the normally-off mode at more than 250degC. The breakdown voltage was more than 1550 V.