Benefitting from regrown Ohmic contact with a contact ledge structure, high performance millimeter-wave InAlN/GaN HEMT is fabricated to satisfy low voltage RF applications. Different from the commonly seen fabrication process for regrown Ohmic contact, the scheme proposed in this work features MBE regrowth of n+ GaN on the whole wafer after formation of regrowth well without masks and partial removal of n+ GaN grown on the access region by self-stopping etching. The remaining n+ GaN on the barrier, serving as contact ledges, provides an additional current path to achieve the reduced equivalent source-drain distance and, thus, improved output current, and more current contribution is made by contact ledge as the actual source-drain distance shrinks. With the assistance of contact ledge, the fabricated device demonstrates output current density of 2.8 A/mm, a peak extrinsic transconductance of 823 mS/mm, a knee voltage of 1.6 V, and an on-resistance of 0.47 Ω·mm. Although self-stopping etching is performed on the access region, the device exhibits ignorable current collapse. At 30 GHz and VDS of 6 V, decent power-added-efficiency of 52% together with output power density of 1.2 W/mm is achieved, revealing the great potential of the proposed regrown Ohmic contact with contact ledge structure for low voltage RF applications.

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GaN-High-electron-mobility transistors (GaN-HEMTs) were fabricated and investigated in detail to improve the linearity at high operation voltage. The scheme of dual-threshold coupling was adopted to mitigate the transconductance (Gm) non-linearity, and multi-finger drain field plate was employed to alleviate the high electric field. The proposed devices yielded Gm plateau of 5.5 V, 10 dB improvement in output third order intercept power. The load-pull measurements at 30 GHz delivered peak power-added-efficiency of 52.5 %, and saturation output-power-density of 5.5 W/mm, whose 2.4 dB improvement in gain compression, and 2 dB enhancement in 1-dB compression point, respectively.