In this letter, we demonstrated a normally-off AlGaN/GaN HEMT using p-NiO as a gate stack combined with a recess structure. The fabricated HEMT exhibits a positive threshold voltage of 1.73 V, a saturation output current of 524 mA/mm, a small subthreshold swing of 79.7 mV/dec and a maximum transconductance as high as 143 mS/mm. This is the first time to demonstrate the breakdown characteristics of a p-NiO gate HEMT with a high breakdown voltage of 1205 V and a low specific ON-resistance of 2.22 $\text{m}\Omega \cdot $ cm ² , yielding a competitive Baliga’s figure-of-merit of 0.65 GW/cm ² . The instability evaluation of ${V}_{\text {TH}}$ by step stress and pulse transfer curves shows that the p-NiO gate HEMT has a negligible ${V}_{\text {TH}}$ shift in the entire measured gate bias range, which can be attributed to the counteraction between the electron trapping-induced positive $\text{V}_{\text {TH}}$ shift and hole accumulation induced-negative ${V}_{\text {TH}}$ shift. It is well understood in terms of the carrier transport model based on the large band discontinuity at the interface of the p-NiO/AlGaN type-II heterojunction, which is further verified by transient gate current spectra.

Normally off AlGaN/GaN high electron mobility transistors with a p-type gate are promising for power switching applications, with advantages of low energy consumption and safe operation. In this work, p-NiO is employed as a gate stack, and the interfacial reconstruction and band structure modification at the p-NiO/AlGaN interface have been demonstrated to manipulate channel transport of AlGaN/GaN high electron mobility transistors by post-annealing. In addition to achieving a positive threshold voltage of 0.6 V and a large saturation output current of 520 mA/mm, we found that the gate leakage and On/Off drain current ratio can be improved significantly by more than 10⁴ due to the p-NiO/AlGaN interfacial reconstruction. However, high annealing temperature also results in an increasing ON-resistance and a dramatically increased knee voltage (V_K), which can be attributed to the formation of an ultra-thin γ-Al₂O₃ layer and the substitution of O on N site as a shallow donor at the p-NiO/AlGaN interface confirmed by experimental analyses. Theoretical calculations indicate that such interface reconstruction facilitates an additional potential well at the p-NiO/AlGaN interface to which electrons are spilled out from a two-dimensional electron gas channel under high forward gate voltage, resulting in the increased V_K. Finally, an optimized annealing condition was confirmed that can eliminate this increased V_K phenomenon and simultaneously remain these significantly improved device performances. These findings provide deep understanding of the performance manipulation of AlGaN high electron mobility transistors, which is very important for engineering the p-NiO/AlGaN interface toward high-performance and stable devices.

AlGaN/GaN high electron mobility transistors (HEMTs) have demonstrated their extraordinary potential in developing solid-state microsensors for detecting gases, metal ions, anions, biomolecules, and other substances due to their excellent chemical stability, high surface charge sensitivity, high temperature-tolerance performance, and low power consumption characteristics. In this paper, only three types of AlGaN/GaN HEMT-based sensors used for detecting the p H value, heavy metal ions, and harmful anions, which are suitable for water quality monitoring, will be discussed. First, we introduce the structural design, detection principle, and fabrication processes of AlGaN/GaN HEMT-based sensors. Then, surface functionalization methods for the gate region, sensing mechanisms, and the sensitivity and selectivity performances based on different gate region treatments are reviewed and analyzed. Finally, some challenging problems that hinder the practical application of the sensors are proposed.

A two-dimensional hole gas (2DHG) induced by polarization charges at the GaN/AlGaN hetero-interface is attracting much attention because of its potential to develop p-channel transistors required for GaN complementary logic integrated circuits. This platform is compatible with commercial AlGaN/GaN n-channel electronics, but the performance of GaN p-channel transistors has been far behind. In this work, 2DHGs in GaN/AlGaN/GaN heterostructures grown by plasma-assisted molecular beam epitaxy have been investigated. The Al composition of the AlGaN barrier has been pushed as high as possible without obvious strain relaxation, and the record high 2DHG sheet density and conductivity on the GaN/AlGaN/GaN platform have been obtained. By adopting a parallel conduction model, a dependent relationship of the 2DHG density on temperature has been extracted. The temperature dependent Hall-effect results have demonstrated that the 2DHG density boosts by 75 times and 46 times at room temperature and 77 K, respectively, when the Al composition is pushed from 0.18 to 0.45 for the AlGaN barriers. The 2DHG sheet density reaches 3.6 × 10¹³ and 2.1 × 10¹³ cm⁻² at room temperature and 77 K, respectively, and the lowest sheet resistance is 8.9 kΩ/□ at 77 K. Such a 2DHG is beneficial for fabrication of p-channel GaN transistors with lower on-resistance on the already-industrialized platform.

In this work, fine carrier transport and recombination processes in p-NiO gate AlGaN/GaN high electron mobility transistors were investigated by analyzing their electroluminescence under forward gate bias, with photoluminescence spectrum as a reference. Red luminescence with a peak of 1.9 eV was captured when the gate bias voltage exceeded 4 V, which was verified to originate from the tunneling enhanced interface recombination of injected holes from the gate metal and spilled electrons from the 2DEG channel at the type-II band aligned p-NiO/AlGaN heterostructure interface. Under higher gate bias voltage, holes were further injected into the GaN buffer layer, producing ultraviolet luminescence and yellow luminescence, corresponding respectively to the band edge emission and defect-assisted radiative recombination of GaN. Threshold voltage shift measurements under forward gate bias were conducted to further investigate the carrier transport and recombination processes.

In this Letter, we investigated the electrostatic control capability and gate reliability of the BaTiO3 integrated AlGaN/GaN HEMTs. The fabricated HEMT exhibits a high peak maximum transconductance of 141 mS/mm, a large ON/OFF current ratio of 4.1 × 109, and a small subthreshold swing of 70.6 mV/dec, attributed to the gate leakage suppression and prominent gate coupling. These parameters demonstrate that the HEMT device with extreme permittivity BaTiO3 dielectric has the excellent electrostatic control capability. A high breakdown voltage of 1348 V was also achieved thanks to the screening of the negative gate image charges and the resulting improvement of electric field distribution by using double-layer BaTiO3 with the wrapped gate structure. Furthermore, gate bias step-stress and pulse I–V measurements show that the device exhibits a small VTH shift of 0.06 V at 2 V bias stress and a slight current collapse of ∼2.7% accompanied with a 6.0% RON increment at a quiescent drain bias VDSQ = 30 V, which benefits from the high quality of the dielectrics/AlGaN interface with Dit of 8.87 × 1012 eV−1/cm2. These findings elucidate the immense potential of extreme permittivity dielectrics engineering in achieving high-performance AlGaN/GaN power electronic devices.