AlGaN/GaN high-electron-mobility transistors (HEMTs) are widely used in high-frequency and high-power applications owing to the high two-dimensional electron gas (2DEG) concentration. However, the microscopic origin of the 2DEG remains unclear. This hinders the development of device fabrication technologies, such as threshold voltage modulation, current collapse suppression, and 2DEG concentration enhancement technologies, as well as AlGaN/GaN sensors with very high sensitivity to polar liquids. To clarify the 2DEG microscopic origin, we studied the effects of gas molecules on AlGaN/GaN surfaces through various experiments and first-principles calculations. The results indicated that the adsorption of gas molecules on the AlGaN/GaN surface is an important phenomenon, clarifying the microscopic origin of the 2DEG. This study elucidates the properties of AlGaN/GaN heterojunctions and promotes the development of new fabrication technologies for AlGaN/GaN devices.

This letter reports the phenomenon of current drops in an AlGaN/GaN heterojunction with CF₄ plasma treated in a polar gas ambient. Ungated AlGaN/GaN HEMT with CF₄ plasma treatment was tested in ethanol, acetonitrile, and an acetic acid gas ambient, and the current dropped by 52%, 51% and 61%, respectively, which are much higher than the drops (6%, 7%, and 8%) in normal ungated HEMT. On the one hand, based on “ambient doping”, a possible model was proposed to explain this phenomenon. On the other hand, the lower initial current was very important to increase the sensitivity. This means that the initial current should be elaborately designed to achieve maximum sensitivity.