Effects of barrier thinning on small-signal and 30-GHz power characteristics of AlGaN/GaN heterostructure field-effect transistors

Short-gate AlGaN/GaN heterostructure field-effect transistors (HFETs) with extremely thin AlGaN barrier layers were fabricated and characterized from the viewpoint of millimeter-wave applications. The devices showed good direct-current and small-signal characteristics; however, the 30-GHz power characteristics were degraded due to frequency dispersion caused by the SiN _x/AlGaN interface states. The dispersive behavior of the thin barrier devices measured in pulse I-V curves was different from the commonly observed one for the devices with normal AlGaN barrier thicknesses of 20-30 nm. The first characteristic point was that it happened only for structures with extremely thin barrier layers. As another unique point, the drain current collapse in the pulsed modes was observed only at a positive gate bias. We consider that the unique dispersion of the thin barrier HFETs was caused by the different charging paths related to hot electrons accelerated in a high electric field region of a 2-D electron gas (2DEG) channel. It seems reasonable to suppose that, for the extremely thin barrier structures, the AlGaN surface states can be charged not only by injected electrons from the gate metal through the SiN_x /AlGaN interface but also by the hot electrons overcoming the barrier from the 2DEG formed at the AlGaN/GaN interface.