High integrity SiO2/Al2O3 gate stack has been demonstrated for GaN metal-oxide-semiconductor (MOS) transistors. The SiO2 film formed on GaN by the microwave-excited plasma enhanced chemical vapor deposition (MW-PECVD) exhibits good properties compared that by the LP (Low Pressure)-CVD. Then, by incorporating the advantages of both of SiO2 with a high insulating and Al2O3 with good interface characteristics, the SiO2/Al2O3 gate stack structure has been employed in GaN MOS devices. The structure shows a low interface state density between gate insulator and GaN, a high breakdown field, and a large charge-to-breakdown by applying 3-nm Al2O3. The SiO2/Al2O3 gate stack has also been applied to AlGaN/GaN hybrid MOS heterojunction field-effect transistor (HFET) and the HFET shows excellent properties with the threshold voltage of 4.2 V and the maximum field-effect mobility of 192 cm2/Vs.

We have demonstrated enhancement-mode n-channel gallium nitride (GaN) MOSFETs on Si (111) substrates with high-temperature operation up to 300 °C. The GaN MOSFETs have good normally-off operation with the threshold voltages of +2.7 V. The MOSFET exhibits good output characteristics from room temperature to 300 °C. The leakage current at 300°C is less than 100 pA/mm at the drain-to-source voltage of 0.1 V. The on-state resistance of MOSFET at 300°C is about 1.5 times as high as that at room temperature. These results indicate that GaN MOSFET is suitable for high-temperature operation compared with AlGaN/GaN HFET.

In this report, we have demonstrated enhancement-mode n-channel GaN MOSFETs on silicon (111) substrates. We observe a high field-effect mobility of 115 cm2/Vs, the best report for GaN MOSFET fabricated on a silicon substrate to our knowledge. The threshold voltage was estimated to be +2.7 V, and the maximum operation current was over 3.5 A. This value is the largest which have ever been reports.