A systematic study of the selective etching of p-GaN over AlGaN was carried out using a BCl₃/SF₆ inductively coupled plasma (ICP) process. Compared to similar chemistry, a record high etch selectivity of 41:1 with a p-GaN etch rate of 3.4 nm/min was realized by optimizing the SF₆ concentration, chamber pressure, ICP and bias power. The surface morphology after p-GaN etching was characterized by AFM for both selective and nonselective processes, showing the exposed AlGaN surface RMS values of 0.43 nm and 0.99 nm, respectively. MIS-capacitor devices fabricated on the AlGaN surface with ALD-Al₂O₃ as the gate dielectric after p-GaN etch showed the significant benefit of BCl₃/SF₆ selective etch process.

The V_th stability and gate reliability of AlGaN/GaN metal–insulator–semiconductor high-electron-mobility transistors (MIS-HEMTs) with alternating O₂ plasma treatment were systematically investigated in this article. It was found that the conduction band offset at the Al₂O₃/AlGaN interface was elevated to 2.4 eV, which contributed to the suppressed gate leakage current. The time-dependent dielectric breakdown (TDDB) test results showed that the ALD-Al₂O₃ with the alternating O₂ plasma treatment had better quality and reliability. The AlGaN/GaN MIS-HEMT with the alternating O₂ plasma treatment demonstrated remarkable advantages in higher V_th stability under high-temperature and long-term gate bias stress.

A novel monocrystalline AlN interfacial layer formation method is proposed to improve the device performance of the fully recessed-gate Al₂O₃/AlN/GaN Metal-Insulator-Semiconductor High Electron Mobility Transistors (MIS-HEMTs), which is achieved by plasma-enhanced atomic layer deposition (PEALD) and in situ N₂ plasma annealing (NPA). Compared with the traditional RTA method, the NPA process not only avoids the device damage caused by high temperatures but also obtains a high-quality AlN monocrystalline film that avoids natural oxidation by in situ growth. As a contrast with the conventional PELAD amorphous AlN, C-V results indicated a significantly lower interface density of states (D_it) in a MIS C-V characterization, which could be attributed to the polarization effect induced by the AlN crystal from the X-ray Diffraction (XRD) and Transmission Electron Microscope (TEM) characterizations. The proposed method could reduce the subthreshold swing, and the Al₂O₃/AlN/GaN MIS-HEMTs were significantly enhanced with ~38% lower on-resistance at V_g = 10 V. What is more, in situ NPA provides a more stable threshold voltage (V_th) after a long gate stress time, and ΔV_th is inhibited by about 40 mV under V_g,stress = 10 V for 1000 s, showing great potential for improving Al₂O₃/AlN/GaN MIS-HEMT gate reliability.

The high-order mechanical resonances of the sensing element in a high-vacuum environment can significantly degrade the noise and distortion performance of seismic-grade sigma–delta MEMS capacitive accelerometers. However, the current modeling approach is unable to evaluate the effects of high-order mechanical resonances. This study proposes a novel multiple-degree-of-freedom (MDOF) model to evaluate the noise and distortion induced by high-order mechanical resonances. Firstly, the MDOF dynamic equations of the sensing element are derived using the principle of modal superposition and Lagrange’s equations. Secondly, a fifth-order electromechanical sigma–delta system of the MEMS accelerometer is established in Simulink based on the dynamic equations of the sensing element. Then, the mechanism through which the high-order mechanical resonances degrade the noise and distortion performances is discovered by analyzing the simulated result. Finally, a noise and distortion suppression method is proposed based on the appropriate improvement in high-order natural frequency. The results show that the low-frequency noise drastically decreases from about −120.5 dB to −175.3 dB after the high-order natural frequency increases from about 130 kHz to 455 kHz. The harmonic distortion also reduces significantly.

A novel atomic-level post-etch-surface-reinforcement (PESR) process is developed to recover the p-GaN etching induced damage region for high performance p-GaN gate HEMTs fabrication. This process is composed of a self-limited surface modification step with O₂ plasma, following by an oxide removal step with BCl₃ plasma. With PESR process, the AlGaN surface morphology after p-GaN etching was comparable to the as-epitaxial level by AFM characterization, and the AlGaN lattice crystallization was also recovered which was measured in a confocal Raman system. The electrical measurement further confirmed the significant improvement of AlGaN surface quality, with one-order of magnitude lower surface leakage in a metal-semiconductor (MS) Schottky-diode and 6 times lower interface density of states (D_it) in a MIS C-V characterization. The XPS analysis of Al₂O₃/AlGaN showed that the p-GaN etching induced F-byproduct and Ga-oxide was well removed and suppressed by PESR process. Finally, the developed PESR process was successfully integrated in p-GaN gate HEMTs fabrication, and the device performance was significantly enhanced with ~20% lower of on-resistance and ~25% less of current collapse at V_ds,Q bias of 40 V, showing great potential of leverage p-GaN gate HEMTs reliability.

In this work, we demonstrated a low current collapse normally on Al₂O₃/AlGaN/GaN MIS-HEMT with in situ H-radical surface treatment on AlGaN. The in situ atomic pretreatment was performed in a specially designed chamber prior to the thermal ALD-Al₂O₃ deposition, which improved the Al₂O₃/AlGaN interface with ${\mathrm{D}}_{\mathrm{i}\mathrm{t}}$ of ~2 × 10¹² cm⁻² eV⁻¹, and thus effectively reduced the current collapse and the dynamic ${\mathrm{R}}_{\mathrm{o}\mathrm{n}}$ degradation. The devices showed good electrical performance with low ${\mathrm{V}}_{\mathrm{t}\mathrm{h}}$ hysteresis and peak trans-conductance of 107 mS/mm. Additionally, when the devices operated under 25 °C pulse-mode stress measurement with ${\mathrm{V}}_{\mathrm{D}\mathrm{S},\mathrm{Q}}$ = 40 V (period of 1 ms, pulse width of 1 $\mathsf{\mu }$s), the dynamic ${\mathrm{R}}_{\mathrm{o}\mathrm{n}}$ increase of ~14.1% was achieved.