Recently, back barrier structures using Al GaN orlnGaN have commonly been employed for GaN hetero structure field-effect transistors (HFETs) with a goal of developing high-power amplifiers in the mm-wave frequency range [1-4]. However, the back barrier possibly causes some disadvantages suchas decreases in electron density (ns) and thermal conductivity in exchange for an improvement on output conductance due to its strong carrier confinement. In this work, we characterized capabilities of AlGaN/GaN HFETs without a back barrier for the mm-wave high-power applications. To enhance charge control, the device relied solely on a high gate aspect ratio design by using an extremely thin AlGaN top barrier instead of the back barrier. The decrease in ns caused by the thin AlGaN barrier can be compensated by SiNx deposition . Similar device structures with a gate length (Lg) of 60 nm have dem onstrated record RF small-signal characteristics in previous work ; therefore, we studied the applicability of this concept to large-signal applications.