The AlInN/GaN high-electron-mobility-transistor (HEMT) indicates better performances compared with the traditional AlGaN/GaN HEMTs. The present work investigated the pH sensor functionality of an analogous HEMT AlInN/GaN device with an open gate. It was shown that the Al_0.83In_0.17N/GaN device demonstrates excellent pH sense functionality in aqueous solutions, exhibiting higher sensitivity (−30.83 μA/pH for AlInN/GaN and −4.6 μA/pH for AlGaN/GaN) and a faster response time, lower degradation and good stability with respect to the AlGaN/GaN device, which is attributed to higher two-dimensional electron gas (2DEG) density and a thinner barrier layer in Al_0.83In_0.17N/GaN owning to lattice matching. On the other hand, the open gate geometry was found to affect the pH sensitivity obviously. Properly increasing the width and shortening the length of the open gate area could enhance the sensitivity. However, when the open gate width is too larger or too small, the pH sensitivity would be suppressed conversely. Designing an optimal ratio of the width to the length is important for achieving high sensitivity. This work suggests that the AlInN/GaN-based 2DEG carrier modulated devices would be good candidates for high-performance pH sensors and other related applications.

The effects of interface charges on the performances of gate-all-around (GAA) GaN vertical nanowire MOSFETs with different geometries have been studied. Geometrical effect on the gate current of vertical GAA GaN nanowire MOSFET has also been analysed for the first time. In the ideal condition, the circular geometry nanowire (CGN) MOSFET exhibits the best performance with subthreshold swing (SS) of 62 mV/dec, drain-induced barrier lowering (DIBL) of 14 mV/V, and ON/OFF current ratio ( I _ON /I _OFF ) of ~10 ⁸ . The triangular or hexagonal geometry nanowire (TGN or HGN) MOSFET suffer from large gate leakage current due to the field enhancement at sidewall corners. It is also known that interface traps at the sidewall surface of vertical nanowires deteriorate the overall device performance. The HGN MOSFET with m-plane sidewall demonstrates the best performance with SS of 69 mV/dec and DIBL of 13 mV/V, while the TGN MOSFET with a-plane sidewall exhibits the worst performance with SS of 112 mV/dec and DIBL of 101 mV/V.

To improve the stability and sensitivity of pH sensors based on AlGaN/GaN high electron mobility transistors (HEMTs), we proposed a modified sensing structure by integrating a reference HEMT device. This structural pH sensor exhibits a typical Nernstian behavior with a sensitivity of 54.38 mV/pH, which is higher than the value of 49.43mV/pH derived from the AlGaN/GaN HEMT-based sensor without integrating the reference HEMTs device. Furthermore, the stability of the new structural sensor is enhanced by approximately 19.2% in comparison with that of its traditional counterpart. The improved performances are analyzed using an electrical double-layer model together with an equivalent circuit model, and we find that the new sensor structure has a larger capacitor and exhibits a better rectification effect, hence decreasing the electrical noise and enhancing the stability of the testing signal. Meanwhile, the new sensor structure displays a smaller equivalent resistance and results in a larger available output current, hence exhibiting a higher sensitivity.

A major obstacle in the commercialization of GaN-on-Si RF HEMTs lies in the elevated thermal resistance introduced by the AlN/AlGaN multi-layer buffer, employed for stress management. This issue adversely impacts device performance and reliability. In this study, the structure function method was utilized to precisely determine the intrinsic thermal resistance of GaN-on-Si materials. Results reveal that a single-layer AlN buffer demonstrates a significantly enhanced heat dissipation capability compared to conventional AlN/AlGaN multi-layer or superlattice buffer. Additionally, the thermal performance of device under operation was quantitatively assessed using static-pulsed I–V measurements. Through theoretical simulations, the influence of GaN buffer structures on heat distribution within GaN-on-Si RF devices was explored, indicating that optimizing GaN buffer thickness can further enhance thermal performance. This research offers a thorough understanding of the relationship between material structures and RF device thermal behavior, providing crucial insights for the thermal management design of GaN-on-Si RF HEMTs.

This work presents an Al_0.25Ga_0.75N/GaN RF MIS-HEMT nominally free of access regions on Si substrate with excellent output performance for low-voltage RF applications at Sub-6 GHz bands. The series resistances introduced by the access regions are effectively suppressed in this device by extending the source/drain ohmic contact regions with heavily $n$ -doped ( $n^{\mathrm {++}}$ ) GaN to the gate edge. The ohmic contact resistance is reduced to as low as $0.09~\Omega \cdot $ mm due to the excellent regrowth of $n^{\mathrm {++}}$ GaN. As a result, with a gate length of 550 nm and width of $2\times 50~\mu $ m, the device exhibits a superior drain current density of 2.11 A/mm, a peak transconductance up to 300 mS/mm with a gate-to-channel distance of approximately 40 nm, and a low on-resistance of $0.61~\Omega \cdot $ mm. Pulsed load–pull characterization at 3.5 GHz reveals an output power density of 0.81 W/mm, with a linear gain of 10.5 dB and a peak power added efficiency of 19.4%, biased at 5 V drain voltage. By linearly extrapolating the RF output power ( $P_{\mathrm {OUT}}$ ) versus drain voltage ( $V_{\mathrm {DD}}$ ), a knee voltage of 0.75 V is obtained, which is the lowest value among the previous reports related to GaN low-voltage RF devices. These results above demonstrate that the MIS-HEMT device nominally free of access regions is promising for low-voltage Sub-6 GHz power amplifier (PA) applications, notably distinct from the conventional fabrication route featured by deeply scaling the device architectures.