Materials such as GaN, AlN, and InN that are nitride semiconductors and their mixed crystals have a wide band gap and are used for devices such as high output electronic devices or short wavelength light-emitting devices. For high output devices, techniques relating to Field-Effect Transistors (FET) and High Electron Mobility Transistors (HEMT) are developed (for example, Patent Document 1). HEMTs using such nitride semiconductors are used for devices such as high output/high efficiency amplifiers or high power switching devices.
As for a FET using nitride semiconductors, a HEMT, which uses GaN in an electron transport layer and uses AlGaN in an electron supply layer, is known. Two-Dimensional Electron Gas (2DEG) is generated in the electron supply layer through piezoelectric polarization or spontaneous polarization in GaN. Further, so as to make output and efficiency of a HEMT higher, a HEMT, which uses GaN in an electron transport layer and uses InAlN in an electron supply layer, is known. Spontaneous polarization of InAlN is high. Therefore, by using InAlN in the electron supply layer, it is possible to generate high concentration 2DEG and to cause a drain current to flow more than that of the HEMT using AlGaN in the electron supply layer.
When InAlN is used in an electron supply layer, the surface of InAlN is easily oxidized, current collapse is caused by indium oxide (InOx) included in oxide of InAlN, and a drain current decreases. Because InOx formed by oxidation of InAlN is chemically unstable, an oxygen defect is likely to occur. When an electron is trapped in the oxygen defect in InOx, a concentration of the 2DEG decreases, current collapse occurs, and a drain current decreases.