The AlInGaN/GaN heterojunction epitaxy material with high cutoff frequency and saturated power density has become a very popular candidate for millimeter wave applications in next-generation mobile communication. In this study, an advanced voltage-controllable capacitor based on the AlInGaN/GaN-Si (111) epitaxy was proposed by employing a bi-directional series MIS capacitor structure. The capacitor was fabricated by using a pad area of 40 μm × 40 μm, with a 1 μm distance between the positive and negative electrodes. The test results show that the capacitance is turned on with a saturation capacitance density and a maximum leakage current density of 0.30 fF/μm² of 0.37 pA/μm², respectively, for the control voltage from −6.5 V to 6 V. In particular, in the proposed design method, the saturation capacitance required for the practical application can be obtained by simply adjusting the capacitance area. The capacitor showcases characteristics of rapid turn-on and turn-off responses coupled with low loss, underscoring its promising prospects for deployment in RF switching applications.

The etching process is pivotal in the fabrication of GaN-based devices, directly influencing their performance. This study presents an enhanced low-damage hybrid cyclic etching technology for AlGaN/GaN heteroepitaxy structures, combining oxygen plasma oxidation with HCl etching to achieve atomic-scale precision. By carefully controlling the oxidation layer thickness, this method preserves a low surface roughness of 0.228 nm and achieves an etching rate of 0.45 nm/cycle with excellent consistency. Compared to conventional ICP dry etching and KOH wet etching evaluated under identical experimental conditions, the proposed technique significantly reduces etching-induced-damage. When applied to the ohmic contact formation of AlGaN/GaN HEMTs using the same metallization scheme and annealing process, devices fabricated with the new hybrid etching method demonstrated a significantly lower contact resistance of 0.76 Ω7mm compared to those using conventional ICP etching (1.21 Ω7mm) or no additional etching after SiN removal (1.37 Ω7mm). Furthermore, these devices exhibited a threshold voltage of -6.5 V, a saturation drain current density of 880.55 mA/mm, and a knee voltage of approximately 4 V. This method also demonstrates potential for recessed gate etching and post-dry-etching damage repair, offering a promising solution for low-roughness surfaces processing with broad application prospects.