This paper describes device process and characteristics of sub-100-nm-gate AlGaN/GaN heterostructure field-effect transistors (HFETs) for millimeter-wave applications. We developed three techniques to suppress short-channel effects and thereby enhance high-frequency device characteristics: high-AL-composition and thing AlGaN barrier layers, SiN passivation by catalytic chemical vapor deposition, and sub-100-nm Ti-based gates. The Al<sub>0.4</sub>Ga<sub>0.6</sub>N(6 nm)/GaN HFETs with a gate length of 60 nm on a 4H-SiC substrate showed a maximum drain current density of 1.6 A/mm and a maximum transconductance of 424 mS/mm. THe use of the techniques led to record current-gain cutoff frequency (f<sub>T</sub>) and maximum oscillation frequency (f<sub>max</sub>) of 190 and 241 GH, respectively. The f<sub>T</sub> and f<sub>max</sub>kept high values over the wide range of drain voltage and current. These results indicate significantly high potential of GaN HFETs for high-power applications in the millimeter-wave frequency range.