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AlN amorphous films were prepared by RF magnetron sputtering, and applied to the AlGaN/GaN metal-insulator-semiconductor heterostructure field-effect transistors (MIS-HFETs) as a gate insulator. X-ray photoelectron spectroscopy (XPS) was used to characterize the AlN films, showing their chemical bonds and the bandgap by Nls electron energy loss spectroscopy. The AlGaN/GaN MIS-HFET with a gate length of 150 nm was successfully fabricated, exhibiting low gate leakage currents for both reverse and forward biases, which are at least four orders of magnitude lower than those of reference Schottky-HFETs. These results support a possibility of sputtered amorphous AlN as a gate insulator. Moreover, AlN/AlGaN interface states were investigated by the frequency dispersion in the C-V characteristics.