Summary form only given. We fabricated nanoscale-gate Al<sub>0.25</sub>In<sub>0.75</sub>Sb/InSb high electron mobility transistors (HEMTs) grown on a (100) GaAs structure by molecular beam epitaxy (MBE) and measured their DC and RF performances at room temperature. The epitaxial structure had an electron mobility (μ) of 17,700 cm<sup>2</sup>/Vs with a sheet carrier concentration (N<sub>s</sub>) of 1.06×10<sup>12</sup> cm<sup>-2</sup>. We also introduced an evaporated SiO<sub>x</sub> passivation film instead of SiO<sub>x</sub> and SiN deposited by CVD method to prevent plasma damage in the passivation deposition process. As the result, we obtained a maximum transconductance (g<sub>m_max</sub>) of 0.88 S/mm and a cutoff frequency (f<sub>T</sub>) of 302 GHz for the 50-nm-gate HEMT when biased at a drain-source voltage (V<sub>ds</sub>) of 0.5 V, which was because the evaporated SiO<sub>x</sub> passivation film could prevent electrical degradation of the HEMT structure caused by not only plasma damage but also thermal damage in whole HEMT fabrication process.