Small-signal and 30-GHz power performance of AlGaN/GaN HFETs without back barriers

Recently, back barrier structures using AlGaN or InGaN have commonly been employed for GaN heterostructure field-effect transistors (HFETs) with a goal of developing high-power amplifiers in the mm-wave frequency range. However, the back barrier possibly causes some disadvantages such as decreases in electron density (n_s) 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 n_s caused by the thin AlGaN barrier can be compensated by SiN_x deposition. Similar device structures with a gate length (L_g) of 60 nm have demonstrated record RF small-signal characteristics in previous work; therefore, we studied the applicability of this concept to large-signal applications.