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For the first time, an epitaxial beryllium oxide (BeO) film was grown on 4H silicon carbide (4H-SiC) by atomic layer deposition (ALD) at a low temperature of 250 °C. The BeO film had a large lattice mismatch with the substrate (>7–8%), but it was successfully grown to a single crystal by domain-matching epitaxy (DME). The bandgap energy, dielectric constant, and thermal conductivity properties of crystalline BeO are suitable for power transistors that require low leakage currents and fast heat dissipation in high electric fields. Physical characterization confirmed the single-crystalline BeO (0 0 2). Raman analysis showed that the E1 and A1 phonon modes of ALD BeO were intermixed with the E2 and A1 phonon modes of SiC, resulting in a significant increase in phonon intensity. After heat treatment at a high temperature, a small amount of SiO2 interfacial oxide was formed but the stoichiometry of BeO was maintained. From the capacitance-voltage (C-V) curves, we obtained a dielectric constant of 6.9 and calculated a low interface trap density of 6 × 1010 cm−2·eV−1 using the Terman method at Ec-Et = 0.6 eV. The high bandgap, thermal conductivity, and excellent crystallinity reduced the dangling bonds at the interface of BeO-on-SiC.

We optimized a silicon nitride (SiNx) passivation process using a catalytic-chemical vapor deposition (Cat-CVD) system to suppress the current collapse phenomenon of AlGaN/GaN-on-Si high electron mobility transistors (HEMTs). The optimized Cat-CVD SiNx film exhibited a high film density of 2.7 g/cm3 with a low wet etch rate (buffered oxide etchant (BOE) 10:1) of 2 nm/min and a breakdown field of 8.2 MV/cm. The AlGaN/GaN-on-Si HEMT fabricated by the optimized Cat-CVD SiNx passivation process, which had a gate length of 1.5 &mu, m and a source-to-drain distance of 6 &mu, m, exhibited the maximum drain current density of 670 mA/mm and the maximum transconductance of 162 mS/mm with negligible hysteresis. We found that the optimized SiNx film had positive charges, which were responsible for suppressing the current collapse phenomenon.

We demonstrated gallium nitride (GaN) based negative capacitance field-effect transistors (NCFETs) that achieve steep switching operation. Ferroelectric phase characteristics were successfully demonstrated using a 10 nm thick undoped-HfO2 ferroelectric thin film fabricated through atomic layer deposition. The HfO2 metal-ferroelectric-metal (MFM) capacitor was connected in series with the gate electrode of a recessed normally-off AlGaN/GaN MOS-HFET. The forward/reverse subthreshold slopes were dramatically reduced from 104/105 to 22/23 mV/dec.

This paper reports the first-time evaluation of the time-dependent dielectric breakdown of recessed AlGaN/GaN-on-Si metal-oxide-semiconductor heterostructure field-effect transistors (MOS-HFETs) with plasma enhanced chemical vapor deposition (PECVD) SiO2 gate oxide. The interface fixed charge density and oxide bulk charge density extracted from the flat-band voltage characteristics were 2.7 × 1011 ± 6.54 × 1010 cm−2 and -9.71 × 1017 ± 5.18 × 1016 cm−3, respectively. The time dependent dielectric breakdown (TDDB) characteristics exhibited longer lifetime estimation as the SiO2 thickness increased. The excellent reliability of the PECVD SiO2 film was validated for use as the gate oxide of recessed AlGaN/GaN MOS-HFET.

The effects of post-metallization annealing (PMA) have been investigated for thermally-grown SiO2 on 4H-SiC metal-oxide-semiconductor (MOS) with a molybdenum (Mo) gate electrode. Mo is a great metal gate material for Silicon carbide (SiC) due to its high thermal budget and low thermal expansion coefficient, which allows high-temperature processing after the gate electrode formation. In this study, PMA process was carried out after Mo gate formation on thermally-grown SiO2/4H-SiC in the temperature range from 600 to 1000 °C in N2 or a forming gas ambient. The fabricated Mo/SiO2/4H-SiC MOS device annealed at 800 °C exhibited excellent interface characteristics with negligible hysteresis in comparison with as-grown or post-oxidation annealed samples.

We have developed a silicon oxynitride (SiON) deposition process using a plasma-enhanced chemical vapor deposition system for the gate dielectric of GaN-on-Si metal–insulator–semiconductor field-effect transistors (MISFETs). The optimized SiON film had a relative dielectric constant of 5.3 and a breakdown field of 12MV/cm. A normally-off GaN-on-Si MISFET fabricated with a 33-nm SiON gate dielectric exhibited a threshold voltage of ~2 V, an ON-resistance of $7.85~\mathsf {m}\Omega \cdot \mathsf {cm}^{{\mathsf {2}}}$ , and a breakdown voltage of ~640 V at the OFF-state current density of $1~\mu \text{A}$ /mm. The extracted interface trap density was $\mathsf {1} \times \mathsf {10}^{{\mathsf {12}}}$ cm $^{{\mathsf {-2}}}\cdot \mathsf {eV}^{\mathsf {-1}}$ at ${E}_{c}- {E}_{t}= \mathsf {0.442}$ eV, which resulted in negligible hysteresis and excellent dynamic characteristics.

We have investigated the channel mobility of AlGaN/GaN-on-Si recessed-metal-oxide-semiconductor-heterojunction field-effect transistors (recessed-MOS-HFET) with SiO 2 gate oxide. Both field-effect mobility and effective mobility for the recessed-MOS channel region were extracted as a function of the effective transverse electric field. The maximum field effect mobility was 380 cm 2 /V·s near the threshold voltage. The effective channel mobility at the on-state bias condition was 115 cm 2 /V·s at which the effective transverse electric field was 340 kV/cm. The influence of the recessed-MOS region on the overall channel mobility of AlGaN/GaN recessed-MOS-HFETs was also investigated.

A high quality SiO 2 deposition process using a plasma enhanced chemical vapor deposition system has been developed for the gate insulator process of normally-off recessed-gate AlGaN/GaN metal–oxide–semiconductor-heterostructure field-effect transistors (MOS-HFETs). SiO 2 films were deposited by using SiH 4 and N 2 O mixtures as reactant gases. The breakdown field increased with increasing the N 2 O flow rate. The optimum SiH 4 /N 2 O ratio was 0.05, which resulted in a maximum breakdown field of 11 MV/cm for the SiO 2 film deposited on recessed GaN surface. The deposition conditions were optimized as follows; a gas flow rate of SiH 4 /N 2 O (=27/540 sccm), a source RF power of 100 W, a pressure of 2 Torr, and a deposition temperature of 350 °C. A fabricated normally-off MOS-HFET exhibited a threshold voltage of 3.2 V, a specific on-resistance of 4.46 mΩ cm 2 , and a breakdown voltage of 810 V.

This paper reports a new method to enable the normally-off operation of AlGaN/GaN heterojunction field-effect transistors (HFETs). A capacitor was connected to the gate input node of a normally-on AlGaN/GaN HFET with a Schottky gate where the Schottky gate acted as a clamping diode. The combination of the capacitor and Schottky gate functioned as a clamp circuit to downshift the input signal to enable the normally-off operation. The normally-off operation with a virtual threshold voltage of 5.3 V was successfully demonstrated with excellent dynamic switching characteristics.

Road deposited sediments (RDS) swept from highways in South Korea were characterized to quantitatively evaluate the reduction in non-point source pollutants by sweeping. The swept RDS consisted primarily of sand (63 μm to 2 mm) particles (80.34 ± 8.33% of total weight) highly contaminated by organics, nutrients and heavy metals. The average concentrations of total organic carbon (TOC), biochemical oxygen demand (BOD), volatile solids (VS), total nitrogen (T-N), and total phosphorus (T-P) were 20.17 ± 9.13, 1.04 ± 0.62, 39.92 ± 16.55, 1.99 ± 0.96, and 0.54 ± 0.19 g kg−1 (±one standard deviation), respectively, for 63 μm to 2 mm RDS. The concentrations of the pollutants were high for RDS smaller than 63 μm, but most of the mass was associated with the 63 μm to 2 mm RDS. The results suggest that the pollutants associated with RDS swept from highways originated mainly from engine wear, exhaust emissions, and tire wear. These results were different from the RDS on roads in residential and commercial areas, where natural particles and brake wear contribute significantly to RDS. In addition, the reductions in TOC, BOD, VS, T-N, T-P, Cu, Pb, Zn, Fe, and As based on the swept RDS measurements were calculated to be 3,355.3, 175.1, 6,621.4, 323.0, 88.3, 30.3, 13.7, 1.0, 303.4, 11,198.7, and 0.4 g km−1, respectively.

We have investigated the effects of a silicon nitride (SiNx) passivation process using plasma-enhanced chemical vapor deposition (PECVD) on a ultra-thin-barrier AlGaN/GaN heterostructure field-effect transistors (HFETs). The bulk charge characteristics of the PECVD SiNx films were dependent on the film deposition conditions, which strongly influenced the sheet resistance (Rsh) and flat-band voltage characteristics of AlGaN/GaN HFETs. The reduction in Rsh is a strong function of the amount of positive bulk charges in the SiNx passivation film. An optimized PECVD SiNx process was used to drastically decrease the Rsh from 45,450 Ω/sq to 732 Ω/sq. A Mo/Au Schottky-gate device fabricated with PECVD SiNx passivation exhibited a maximum drain current density of 172 mA/mm, quasi-normally-off operation, and breakdown voltage of > 1100 V

This study investigated the effects of a thin aluminum oxynitride (AlOxNy) 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 (= fT × BVgd) 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.

In this study, we have investigated the effects of SF6 plasma treatment on the isolation leakage current characteristics of AlGaN/GaN heterojunction field-effect transistors (HFETs). The device isolation was performed by Cl2/BCl3-based plasma etching process followed by SF6 plasma treatment for 1 min. It was found that the SF6 plasma post-etching treatment was an effective method to improve the electrical isolation characteristics. X-ray photoelectron spectroscopy analysis was carried out to investigate the chemical bonding states of the etched GaN surfaces.

We investigated the growth behavior of GaN grown on AlN along with Ⅴ/Ⅲ ratio and pressure variation, and found out lateral growth regime for fully coalesced channel layer of the AlN-based double-hetero structure high electron mobility transistor (HEMT). When the Ⅴ/Ⅲ ratio increases and pressure decreases, compressive stress in the GaN channel increases, and pit formation occurs to release the stress. The AlN-based HEMT structure was grown and the device was fabricated with an optimized channel layer. The two-dimensional electron gas mobility, sheet density, and sheet resistance were 1480 cm2/Vs, 1.32 × 1013 cm-2, and 319 Ω/, respectively, at room temperature. The device was characterized; direct current output result showed that the maximum current was ~620 mA/mm, on-resistance was 6.4 Ωmm, transconductance was ~140 mS/mm, and current on/off ratio was ~104, respectively.

We developed a high-quality plasma enhanced atomic layer deposited (PEALD) aluminum oxynitride (AlO x N y ) process for the metal-oxide-semiconductor (MOS) gate insulator of fully-recessed-gate AlGaN/GaN-on-Si MOS-heterojunction field-effect transistors (MOS-HFETs). It was found that the cyclic nitrogen incorporation into aluminum oxide (Al2O3) during PEALD process improved the conduction band offset at GaN interface resulting in higher forward breakdown field strength, which also contributed to suppressed trapping effects under forward gate bias stress. Improved interface characteristics, which resulted from suppressed surface oxidation, led to significant improvement of threshold voltage stability. A low threshold voltage hysteresis of 180 mV at maximum gate sweep voltage of 10 V was obtained with AlO x N y gate insulator. The MOS channel mobility was also improved to 235 cm2 V−1 s−1. The fabricated fully-recessed-gate AlGaN/GaN-on-Si MOS-HFET with PEALD AlO x N y gate insulator exhibited excellent overall performances such as a threshold voltage of 3.2 V, a maximum drain current density of 481 mA mm−1, an on/off current ratio of ~1010, an ON-resistance of 12 mΩ mm, and a breakdown voltage of 1050 V.

Biodiversity in forests is strongly affected by forest management practices, such as clearcutting and aggregated retention. Therefore, the assessment of the effects of forest management on biodiversity is a major concern in forest ecology. In the present study, we aimed to characterize the effects of forest management practices, after one year, on the abundance, species richness, community composition, and functional groups of moths in forests. The moths were sampled in four different forest stands: three stands (clearcutting, aggregated retention, and no cutting) in a planted Japanese larch forest and one stand in a natural Mongolian oak forest. The results revealed that the moth communities changed in response to the changes in vegetation after the implementation of forest management practices, and clearcutting increased the abundance and species richness of herbivorous and warm-adapted species. The structure and function of moth communities were affected by the forest management practices such as clearcutting and aggregated retention, which were reflected by a decrease in community indices and change in moth community composition with changes in vegetation.

A normally-off AlGaN/GaN-on-Si metal–oxide–semiconductor-heterojunction field-effect transistor (MOS-HFET) with an integrated single-stage inverter is developed. The integrated single-stage GaN inverter consisted of an AlGaN/GaN driver MOS-HFET and a resistive load. With the monolithically integrated gate driver, the gate charging current was boosted from 10 to 170 mA, which reduced the charging and discharging times from 626 to 107 ns and 553 to 196 ns, respectively.

Radio frequency (RF)-sputtered ZnO gate dielectrics for AlGaN/GaN metal-oxide-semiconductor high-electron-mobility transistors (MOS-HEMTs) were investigated with varying O2/Ar ratios. The ZnO deposited with a low oxygen content of 4.5% showed a high dielectric constant and low interface trap density due to the compensation of oxygen vacancies during the sputtering process. The good capacitance–voltage characteristics of ZnO-on-AlGaN/GaN capacitors resulted from the high crystallinity of oxide at the interface, as investigated by x-ray diffraction and high-resolution transmission electron microscopy. The MOS-HEMTs demonstrated comparable output electrical characteristics with conventional Ni/Au HEMTs but a lower gate leakage current. At a gate voltage of −20 V, the typical gate leakage current for a MOS-HEMT with a gate length of 6 μm and width of 100 μm was found to be as low as 8.2 × 10−7 mA mm−1, which was three orders lower than that of the Ni/Au Schottky gate HEMT. The reduction of the gate leakage current improved the on/off current ratio by three orders of magnitude. These results indicate that RF-sputtered ZnO with a low O2/Ar ratio is a good gate dielectric for high-performance AlGaN/GaN MOS-HEMTs.

This study was aimed to investigate the response of bark beetle community structure to green-tree retention in a Korean pine tree forest between 2013 and 2015. Five types of retention methods were evaluated to investigate the retention effect on insect community structure at various sites in Keunjeogol, Samcheok, and Gangwon-do. Lindgren funnel traps were installed to collect insects from July to August, over a 3-year period. Overall, 690 individuals and 29 species of Scolytinae were collected, with populations of insects appearing to gradually increase by each year of the study. Results of the insect community analysis showed that most survey sites presented a higher diversity than the control site annually except in 2015. This study can be used as a baseline dataset for the long-term study of early changes in insect community in response to green-tree retention in forests.