This study investigates the operating characteristics of AlGaN/GaN high-electron-mobility transistors (HEMTs) by applying HfO₂ as the passivation layer. Before analyzing HEMTs with various passivation structures, modeling parameters were derived from the measured data of fabricated HEMT with Si₃N₄ passivation to ensure the reliability of the simulation. Subsequently, we proposed new structures by dividing the single Si₃N₄ passivation into a bilayer (first and second) and applying HfO₂ to the bilayer and first passivation layer only. Ultimately, we analyzed and compared the operational characteristics of the HEMTs considering the basic Si₃N₄, only HfO₂, and HfO₂/Si₃N₄ (hybrid) as passivation layers. The breakdown voltage of the AlGaN/GaN HEMT having only HfO₂ passivation was improved by up to 19%, compared to the basic Si₃N₄ passivation structure, but the frequency characteristics deteriorated. In order to compensate for the degraded RF characteristics, we modified the second Si₃N₄ passivation thickness of the hybrid passivation structure from 150 nm to 450 nm. We confirmed that the hybrid passivation structure with 350-nm-thick second Si₃N₄ passivation not only improves the breakdown voltage by 15% but also secures RF performance. Consequently, Johnson’s figure-of-merit, which is commonly used to judge RF performance, was improved by up to 5% compared to the basic Si₃N₄ passivation structure.

This study investigates the operational characteristics of AlGaN/GaN high-electron-mobility transistors (HEMTs) by applying a slant-gate structure and drain-side extended field-plate (FP) for improved breakdown voltage. Prior to the analysis of slant-gate-based HEMT, simulation parameters were extracted from the measured data of fabricated basic T-gate HEMTs to secure the reliability of the results. We suggest three different types of slant-gate structures that connect the basic T-gate electrode boundary to the 1st and 2nd SiN passivation layers obliquely. To consider both the breakdown voltage and frequency characteristics, the DC and RF characteristics of various slant-gate structures including the self-heating effect were analyzed by TCAD simulation. We then applied a drain-side extended FP to further increase the breakdown voltage. The maximum breakdown voltage was achieved at the FP length of 0.4 μm. Finally, we conclude that the slant-gate structures can improve breakdown voltage by up to 66% without compromising the frequency characteristics of the HEMT. When the drain-side FP is applied to a slant-gate structure, the breakdown voltage is further improved by up to 108%, but the frequency characteristics deteriorate. Therefore, AlGaN/GaN HEMTs with an optimized slant-gate-based structure can ultimately be a promising candidate for high-power and high-frequency applications.

Stretchable strain sensors are capable of acquiring data when in contact with human skin or equipment and are widely used in wearable applications. Most strain sensors have tensile properties of less than 20% and have limitations regarding body motion linkage, complex sensor structure, and motion nonreliability. To address these problems, we developed a high tension and high sensitivity sensor with a gauge factor over 40 and tensile stress about 50%. Polydimethylsiloxane (PDMS) was selected as the flexible substrate to ensure tensile strength, and polyaniline (PANI) was used to measure the resistance changes in the sensor. In particular, problems regarding poor uniformity of PANI on PDMS were resolved by surface treatment of the PDMS, wherein PANI polymerization was performed sequentially after forming a self-assembled monolayer (SAM) on the PDMS substrate. O₂ plasma and (3-aminopropyl)triethoxysilane were used to form the SAM. It is expected that this sensor can obtain stable characteristics even under high tensile stress through the evenly formed PANI films on the surface-treated PDMS substrate and may be used in various flexible sensor applications.

In this study, we propose an optimized AlGaN/GaN high-electron-mobility transistor (HEMT) with a considerably improved breakdown voltage. First, we matched the simulated data obtained from a basic T-gate HEMT with the measured data obtained from the fabricated device to ensure the reliability of the simulation. Thereafter, to improve the breakdown voltage, we suggested applying a gate-head extended structure. The gate-head-top and gate-head-bottom lengths of the basic T-gate HEMT were symmetrically extended by 0.2 μm steps up to 1.0 μm. The breakdown voltage of the 1.0 μm extended structure was 52% higher than that of the basic T-gate HEMT. However, the cutoff frequency (${\mathrm{f}}_{\mathrm{T}}$) and maximum frequency (${\mathrm{f}}_{\mathrm{m}\mathrm{a}\mathrm{x}}$) degraded. To minimize the degradation of ${\mathrm{f}}_{\mathrm{T}}$ and ${\mathrm{f}}_{\mathrm{m}\mathrm{a}\mathrm{x}}$, we additionally introduced a gate-recessed structure to the 1.0 μm gate-head extended HEMT. The thickness of the 25 nm AlGaN barrier layer was thinned down to 13 nm in 3 nm steps, and the highest ${\mathrm{f}}_{\mathrm{T}}$ and ${\mathrm{f}}_{\mathrm{m}\mathrm{a}\mathrm{x}}$ were obtained at a 6 nm recessed structure. The ${\mathrm{f}}_{\mathrm{T}}$ and ${\mathrm{f}}_{\mathrm{m}\mathrm{a}\mathrm{x}}$ of the gate-recessed structure improved by 9% and 28%, respectively, with respect to those of the non-gate-recessed structure, and further improvement of the breakdown voltage by 35% was observed. Consequently, considering the trade-off relationship between the DC and RF characteristics, the 1.0 μm gate-head extended HEMT with the 6 nm gate-recessed structure was found to be the optimized AlGaN/GaN HEMT for high-power operations.