We previously reported that a vertical InGaAs MISFET with an electron launcher, undoped channel to prevent electron scattering, and 15-nm-wide mesa achieved a high current density of 7 MA/cm 2. However, the reported structure was designed only for DC operation, as it had a large parasitic capacitance between the gate electrode and source. Here we report on the impact of this parasitic capacitance on high-speed operation and the effectiveness of a BCB insulating layer in mitigating the capacitance. In measurements on a test element group, insertion of a BCB layer reduced the parasitic capacitance from 27.6 pF/cm to 1.68 pF/cm, and transistor operation with an inserted BCB layer was confirmed. © VDE VERLAG GMBH.