High Quality SiO₂/Al₂O₃ Gate Stack for GaN Metal–Oxide–Semiconductor Field-Effect Transistor

High quality SiO 2 /Al 2 O 3 gate stack has been demonstrated for GaN metal–oxide–semiconductor (MOS) transistor. We confirmed that Al 2 O 3 could realize a low interface-state density between Al 2 O 3 and GaN, however, the breakdown field was low. By incorporating the merits of both Al 2 O 3 and SiO 2 , which has a high breakdown field and a large charge-to-breakdown, SiO 2 /Al 2 O 3 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. The SiO 2 /Al 2 O 3 gate stack has also been applied to AlGaN/GaN hybrid MOS heterojunction field-effect transistor (HFET). The MOS-HFET shows excellent properties with the threshold voltage of 4.2 V and the maximum field-effect mobility of 192 cm 2 V -1 s -1 .