Normally off operation GaN-based MOSFETs for power electronics applications

Gallium nitride (GaN) is a promising electronic semiconductor material for high-power, high-temperature devices due to its remarkable material properties like wide bandgap, large critical electric field and high saturation velocity compared with Si. The metal-oxide-semiconductor (MOS) field-effect transistor (MOSFET) structure can be operated at a positive threshold voltage, namely the normally off mode, which is preferable for power transistors in terms of fail-safe operation. However in order to minimize the power losses in MOSFET operation, good interface quality at SiO₂/GaN and low resistance in the n⁺-contact layer are strongly required. The MOS capacitors were used to characterize the interface states at SiO₂/GaN, and the interface state density at E_c - 0.4 eV was less than 1 × 10¹¹ cm^-2 eV^-1 after annealing at 900 °C for 30 min by the furnace. In addition, the activation annealing of Si-implanted GaN was performed at 1260 °C for 30 s in rapid thermal annealing (RTA) and its sheet resistance was 23 kΩ sq^-1. Finally, we have fabricated GaN MOSFETs and have achieved more than 1 A operation in the normally off mode at more than 250 °C. The breakdown voltage was more than 1500 V. We also confirmed more than 100 h of consecutive operation at 250 °C at the moment.