AlGaN/GaN heterostructure field-effect transistors (HFETs) with a gate length (L _G) of 60–250 nm were fabricated on a sapphire substrate. The HFET structure was grown by plasma-assisted molecular-beam epitaxy, and a 2-nm-thick SiN film was formed on the device surface by catalytic chemical vapor deposition. All of the HFETs showed outstanding DC device performance. They exhibited maximum drain current densities of 1.50–1.55 A/mm and extrinsic transconductances of 340–400 mS/mm. The 60-nm-gate HFET had a current gain cut-off frequency ( f _T) of 152 GHz and a maximum oscillation frequency ( f _max) of 173 GHz. To our knowledge, the f _T and f _max are the highest ever reported for GaN-based transistors. These superior high-frequency characteristics were achieved with a process using a thin and high-Al-content barrier layer, high-quality catalytic chemical vapor deposition (Cat-CVD) SiN passivation, and sub-0.1-µm gates defined by electron-beam lithography.