In this article, we report on the effective transconductance ( g _m ) and gain linearity improvement of submicrometer gate AlN-barrier-based transistors using GaN/InGaN coupling-channel structures. The fabricated AlN/GaN/InGaN coupling-channel high electron mobility transistor (CC-HEMT) showed flat g _m profile, greatly reduced g _m derivatives, and constant dynamic source resistance compared with an AlN/GaN HEMT using the same fabrication process. The highest extrinsic current gain cutoff frequency ( f _T ) of 55 GHz and the maximum oscillation frequency ( f _max ) of 80 GHz were obtained for 0.15- μm gate-length transistors, both of which remain constant values across wide input voltage ( V _GS ) of 4 V. Moreover, a significant theoretical OIP3 value boost by 7.1 dB has been observed using the CC-HEMT as compared to the AlN/GaN HEMT. The superior linearity performance of the CC-HEMT can be attributed to the strong channel-to-channel coupling effect. The drain bias-dependence of g _m , f _T , and f _max versus V _GS profiles illustrate that the linearity characteristics of the CC-HEMT greatly improve with an increase in V _DS . These results demonstrate the AlN/GaN/InGaN material system as a viable platform for high frequency requiring high-linearity applications.

In this article, the impact of the graded AlGaN back barrier and Fe\C co-doping buffer structure on the AlGaN $/$ GaN high electron mobility transistors (HEMTs) is proposed and systematically investigated. Due to effective suppression of Fe tail in unintentionally doped GaN (uid-GaN) layer by the insertion of the thick graded AlGaN back barrier layer, a large maximum drain current density and a transconductance peak are achieved. Meanwhile, the breakdown voltage is significantly improved by the use of Fe\C co-doping GaN buffer design. More importantly, it is revealed that the graded-AlGaN design can reduce the range and intensity of the electrical potential distribution in uid-GaN layer and then effectively suppress the acceptor-induced trapping\detrapping effect under high drain voltage. The RF small-signal performance of Fe-doped GaN (GaN:Fe)\C co-doping buffer HEMT exhibits significant improvement. In addition, load-pull measurement at 8 GHz revealed that a saturation power increases from 38.04 to 41.07 dB, a power gain increases from 9.06 to 10.58 dB, and an associate power-added-efficiency (PAE) increased from 40.27% to 50.18%. Our proposed GaN-based epitaxial structure can not only suppress the gate lag by reduce AlGaN surface electric field, but it can also suppress the drain lag by reduce the amplitude and range of potential distribution. It indicates that our proposed device has great potential for future high-voltage RF power amplifier application.

In this work, low voltage RF power capability on AlGaN/GaN and InAlN/GaN HEMTs is analyzed from the perspective of DC and pulse characteristics, for terminal applications whose operating voltage is usually in the range of 3 to 15 V. Device fabrication is performed on mature AlGaN/GaN heterojunction as well as strongly polarized InAlN/GaN heterojunction, to make a comparison of low voltage RF power capability between two devices. Although it suffers from relatively severe RF dispersion, InAlN/GaN HEMT delivers higher output power density (P _out ) than its opponent, benefiting from the lower parasitic resistance and knee voltage as well as higher output current density. At 8 GHz, P _out of 1.62 W/mm and 1.10 W/mm are achieved for InAlN/GaN and AlGaN/GaN HEMT, respectively, both of which are biased at class AB operation and V _ds of 9 V. However, the tremendous degradation of power added efficiency (PAE) occurs as the higher drain voltage is applied on InAlN/GaN HEMT, as the result of the severe gate leakage. What is more, a higher PAE is more necessary than P _out for terminal applications. Either InAlN/GaN HEMT or AlGaN/GaN HEMT has its own specific voltage range to deliver higher PAE. Concretely, InAlN/GaN HEMT is more suitable to applications with operating voltage not exceeding 6 V, and AlGaN/GaN HEMT is preferred for ones with relatively higher voltage, accompanied by the decent PAE as high as 62% to 66% and moderate P _out to meet the demand of various low voltage terminal applications.

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 V_DS 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.

The high power and linearity performance of GaN-based HEMT for X-band application was achieved using the barrier layer of sandwich structure and Al_0.05GaN back barrier. The AlGaN-sandwich-barrier can modulate polarization-graded field for more flat transconductance profile under the high drain bias. Only about 7.5% current collapse (CC) occurs for drain quiescent bias of 40 V. Due to the Al_0.05GaN back barrier, the three-terminal off-state breakdown voltage (BV_DS) of 260 V and a very small drain-induced barrier lowering (DIBL) of 2.7 mV V⁻¹ is achieved. The AlGaN sandwich barrier combined with Al_0.05GaN back barrier device exhibits a high current-gain cutoff frequency f _T of 42 GHz@V _DS = 10 V, and a high power-gain cutoff frequency f _MAX of 130 GHz@V _DS = 60 V. Load-pull measurement at 10 GHz revealed a saturated power density of 7.3 W mm⁻¹ was achieved with an associated PAE of 29.2% and Gain of 10.6 dB. Two-tone measurement at 10 GHz showed an OIP3 of 38 dBm and a corresponding linearity figure-of-merit OIP3/P _DC of 4.5 dB. These results demonstrate the great potential of AlGaN-sandwich -barrier/GaN/Al_0.05GaN HEMTs as a very promising alternative to high power and high linearity X-band power amplifier.

In this work, high-efficiency InAlN/GaN high electron mobility transistor (HEMT) is fabricated by recess and oxidation process under the gate (RAO) for low-voltage RF applications. Compared with conventional HEMT fabricated on the same heterojunction, RAO-HEMT exhibits a significant decrease in gate leakage, attributed to the thin oxide layer formed around and under the gate as well as adjustment of the location of the peak electric field and confinement of the strong electric field in the oxide. Due to the alleviated electric field as well as adoption of N $_{\text{2}}$ O for plasma treatment, suppressed RF dispersion including the extremely low current collapse and negligible knee voltage walkout is obtained for RAO-HEMT. Although RAO-HEMT delivers lower saturation current as the result of the partial oxidation of the barrier under the gate, it shows a similar ON-resistance to conventional HEMT, which contributes to the reduction in knee voltage. Besides, higher $\textit{f}_{\textit{T}}$ / $\textit{f}_{\text{MAX}}$ is achieved for RAO-HEMT, as the result of the higher dc transconductance and suppressed RF transconductance collapse as well as larger output resistance. Eventually, benefiting from the improvement in gate leakage, RF dispersion, knee voltage, and $\textit{f}_{\text{MAX}}$ by RAO, RAO-HEMT delivers both higher power-added-efficiency (PAE) and output power ( $\textit{P}_{\text{OUT}}$ ) than conventional HEMT. At 3.6 GHz, RAO-HEMT provides a high PAE of 71% (74%) at $\textit{V}_{\text{\text{DS}}}$ of 6 V (9 V), which performs better than most of the existing Si, GaAs, or GaN RF devices in PAE, revealing the great potential of InAlN/GaN RAO-HEMT in low-voltage low-power consumption RF applications.