This paper reports on the use of low-damage atomic layer etching (ALE) performed using O₂ and BCl₃ plasma for etching (Al)GaN. The proposed ALE process led to excellent self-limiting etch characteristics with a low direct current (DC) self-bias, which resulted in a high linearity between the etching depth and number of cycles. The etching damage was evaluated using several methods, including atomic force microscopy, photoluminescence (PL), and X-ray photoelectron spectroscopy, and the I–V properties of the recessed Schottky diodes were compared with those of digital etching performed using O₂ plasma and HCl solution. The electrical characteristics of the recessed Schottky diode fabricated using the proposed ALE process were superior to those of the diodes fabricated using the conventional digital etching process. Moreover, the ALE process yielded a higher PL intensity and N/(Al + Ga) ratio of the etched AlGaN surface, along with a smoother etched surface.

This study investigated the effects of a thin aluminum oxynitride (AlO_xN_y) gate insulator on the electrical characteristics of AlGaN/GaN-on-SiC metal-insulator-semiconductor high electron mobility transistors (MIS-HEMTs). The fabricated AlGaN/GaN-on-SiC MIS-HEMTs exhibited a significant reduction in gate leakage and off-state drain currents in comparison with the conventional Schottky-gate HEMTs, thus enhancing the breakdown voltage. The effects of gate recess were also investigated while using recessed MIS-HEMT configuration. The Johnson’s figures of merit (= f_T × BV_gd) for the fabricated MIS-HEMTs were found to be in the range of 5.57 to 10.76 THz·V, which is the state-of-the-art values for GaN-based HEMTs without a field plate. Various characterization methods were used to investigate the quality of the MIS and the recessed MIS interface.

We have developed a novel unidirectional AlGaN/GaN-on-Si heterojunction field-effect transistor (HFET) with reverse blocking drain. A recessed Schottky contact was incorporated into a conventional ohmic drain electrode to prevent the undesired reverse current flow while reducing the turn-on voltage in forward current characteristics. The combination of recessed Schottky and ohmic electrodes significantly reduced the turn-on voltage in comparison with a conventional Schottky drain. A turn-on voltage of 0.4 V, an off-state forward breakdown voltage of 615 V, and a reverse blocking drain voltage of −685 V were achieved for the gate-to-drain distance of 12 µm.

We investigated the hydrogen gas sensors based on AlGaN/GaN high electron mobility transistors (HEMTs) for high temperature sensing operation. The gate area of the sensor was functionalized using a 10 nm Pd catalyst layer for hydrogen gas sensing. A thin WO₃ layer was deposited on top of the Pd layer to enhance the sensor selectivity toward hydrogen gas. At 200 °C, the sensor exhibited high sensitivity of 658% toward 4%-H₂, while exhibiting only a little interaction with NO₂, CH₄, CO₂, NH₃, and H₂S. From 150 °C to 250 °C, the 10 ppm hydrogen response of the sensor was at least eight times larger than other target gases. These results showed that this sensor is suitable for H₂ detection in a complex gas environment at a high temperature.

In this study, we proposed a novel p-GaN/p-AlGaN/AlGaN/GaN heterojunction field-effect transistor (HFET). When a selective etching process is used to precisely etch the p-GaN layer of a conventional p-GaN/AlGaN/GaN HFET structure, over-etching-induced surface damage can degrade the device characteristics. By inserting a p-AlGaN layer between p-GaN and AlGaN, the over-etching-induced plasma damage at the surface of the p-AlGaN layer can subsequently be removed using a low-damage atomic layer etching process. As a result, the surface damage issue can be mitigated. An additional benefit of using the p-AlGaN layer is a higher threshold voltage. The fabricated p-GaN/p-AlGaN/AlGaN/GaN HFET exhibited a remarkably high threshold voltage of 6 V with a specific on-resistance of 7.47 $\text{m}\cdot \Omega $ cm2.

We developed a Schottky barrier diode (SBD) embedded AlGaN/GaN switching transistor to allow negative current flow during off-state condition. An SBD was embedded in a recessed normally-off AlGaN/GaN-on-Si metal-oxide-semiconductor heterostructure field-effect transistor (MOSHFET). The fabricated device exhibited normally-off characteristics with a gate threshold voltage of 2.8 V, a diode turn-on voltage of 1.2 V, and a breakdown voltage of 849 V for the anode-to-drain distance of 8 µm. An on-resistance of 2.66 mΩcm2 was achieved at a gate voltage of 16 V in the forward transistor mode. Eliminating the need for an external diode, the SBD embedded switching transistor has advantages of significant reduction in parasitic inductance and chip area.