Electron‐beam/ultraviolet (UV) exposure technology to produce undercut T‐shaped resist cavities with bottom openings as small as 0.15 μm is demonstrated with a novel bilayer resist system for AlInAs/InGaAs high electron mobility transistors operated at the millimeter‐wave band. We employed an image reversal resist (AZ5206E) for the top layer and a polydimethyl glutarimide (PMGI) for the bottom layer. The top layer is delineated by UV exposure and the bottom layer is delineated by electron‐beam direct writing. These resist layers are developed layer by layer in different content aqueous tetramethyl ammonium hydroxide solution. Resist profiles are extremely well controlled because exposure and development of both layers are completely independent. A reliable overhang structure for metal liftoff, with a 0.15 μm footprint, was obtained. Gate length variation of less than ±10% on a 3‐in.‐diam InP substrate was successfully accomplished. In addition, an interesting phenomenon was noted. The combination of these two resists leads to an electron‐beam sensitivity decrease of PMGI. This phenomenon is closely related to the change of molecular weight distribution in PMGI. The novel bilayer resist system also allows the further improvement of resolution. This would provide a practical means for electron‐beam lithography in the nanometer region.