Improvement of fT in InAl(Ga)N barrier HEMTs by plasma treatments

GaN-based high electron mobility transistors (HEMTs) have been developed for high-temperature, high-frequency and high-power applications. To improve the transistor speed, various techniques have been explored in addition to scaling down the gate length and top barrier thickness: ultrathin SiN passivation to reduce access resistance and parasitic capacitances; re-grown ohmic contacts and self-alignment to minimize access resistances; O₂ plasma treatment in the gate region prior to the metal deposition to suppress rf transconductance collapse; and dielectric-free passivation (DFP) by a O₂-containing plasma treatment in the access region to shorten the gate extension in InAlN HEMTs. Here we report a comparative study on the impact of various plasma treatments in the access region (DFP) as well as under the gate for InAl(Ga)N barrier HEMTs, and propose a model for the observed f_T improvement.