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.

A modulated VT HEMT with improved gm flatness is demonstrated for high linearity application. The modulated VT HEMT was achieved by connecting two elements with different VT values in parallel along the gate width, realizing a flat resulting transfer curve, and the two different VT elements were fabricated by recessing part area of the barrier along the gate width under the gate region. The proposed HEMT shows a gate voltage swing as high as 5.4 V, a high drain current of approximately 2 A mm−1, and an fT/fmax of 63/125 GHz with a much flatter profile within the large gate voltage range.

The thermal effect is an important reliability issue for GaN-based devices. The impact of the diamond layer on double-channel AlGaN/GaN HEMTs (DC-HEMTs) is first investigated in this paper by Sentaurus TCAD simulation. By utilizing the diamond layer, the lattice temperature along the channel can be modulated and becomes more even. The results show that the peak lattice temperature can be reduced by 64 K when the power dissipation increases to 46 W mm −1 in the DC-HEMT with a diamond layer. A 1 μm thick diamond layer is regarded as the optimized thickness when considering the temperature reduction and cost. With the help of the diamond layer, the saturated drain current and transconductance of DC-HEMTs are increased by 0.21 A mm −1 and 22 mS mm −1 , respectively. Meanwhile, peak f T and f max can be enhanced by 4.7 GHz and 10.3 GHz, respectively. These results show that diamond layers have great potential in lattice temperature reduction and performance and reliability improvement of DC-HEMTs.

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.

In this work, high-performance millimeter-wave AlGaN/GaN structures for high-electron-mobility transistors (HEMTs) are presented using a Si-rich SiN passivation layer. The analysis of transient and x-ray photoelectron spectroscopy measurements revealed that the presence of the Si-rich SiN layer leads to a decrease in the deep-level surface traps by mitigating the formation of Ga–O bonds. This results in a suppressed current collapse from 11% to 5% as well as a decreased knee voltage (Vknee). The current gain cutoff frequency and the maximum oscillation frequency of the devices with the Si-rich SiN layer exhibit the values of 74 and 140 GHz, respectively. Moreover, load-pull measurements at 30 GHz show that the devices containing the Si-rich SiN deliver excellent output power density of 8.7 W/mm at Vds = 28 V and high power-added efficiency up to 48% at Vds = 10 V. The enhanced power performance of HEMTs using Si-rich SiN interlayer passivation is attributed to the reduced Vknee, the suppressed current collapse, and the improved drain current.