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.

In this work, ultrathin barrier (∼6 nm) AlGaN/GaN high-electron-mobility transistors (HEMTs) with in situ SiN gate dielectric and slant-field plate (SFP) T-gates were fabricated and analyzed. Since the proposed scheme of gate dielectric and SFP effectively suppresses the gate leakage and alleviates the peak electric field (E-field) around gate region, the maximum breakdown voltage ( VBK) was improved to 92 V, which is 54 V higher than that of the conventional device. The fabricated ultrathin AlGaN/GaN HEMT with 60-nm SFP-T-gate exhibited the peak fT of 177 GHz and peak fmax of 393 GHz, yielding high figure-of-merits of fT · VBK = 16 THz V and fmax·VBK = 36 THz V. Moreover, load-pull measurements at 30 GHz reveal that these devices deliver output power density ( Pout) of 4.6 W/mm at Vds  =  20 V and high power-added efficiency up to 52.5% at Vds  =  10 V. Essentially, the experimental results indicate that the employment of SFP and in situ SiN gate dielectric is an attractive approach to balance the breakdown and speed for millimeter wave devices.

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In this study, AlGaN/GaN nanochannel high-electron-mobility transistors (HEMTs) with tri-gate (TGN-devices) and dual-gate (DGN-devices) structures were fabricated and investigated. It was found that the peak value of the transconductance (Gm), current gain cut-off frequency (fT) and power gain cut-off frequency (fmax) of the TGN-devices were larger than that of the DGN-devices because of the enhanced gate control from the top gate. Although the TGN-devices and DGN-devices demonstrated flattened transconductance, fT and fmax profiles, the first and second transconductance derivatives of the DGN-devices were lower than those of the TGN-devices, implying an improvement in linearity. With the nanochannel width decreased, the peak value of the transconductance and the first and second transconductance derivatives increased, implying the predominant influence of sidewall gate capacitance on the transconductance and linearity. The comparison of gate capacitance for the TGN-devices and DGN-devices revealed that the gate capacitance of the tri-gate structure was not simply a linear superposition of the top planar gate capacitance and sidewall gate capacitance of the dual-gate structure, which could be attributed to the difference in the depletion region shape for tri-gate and dual-gate structures.

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