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We developed an AlGaN/GaN heterojunction phototransistor with a recessed detection area to enhance the photoresponsivity. The recessed-AlGaN/GaN phototransistor exhibited a maximum photoresponsivity of 1.6 × 107 A/W at 375 ㎚, which was ~30% higher than that obtained with a conventional AlGaN/GaN phototransistor. A comparable photoresponsivity was also achieved at 260 ㎚ in UV-C range due to the dual absorption process in conjunction with the polarization induced built-in electric field characteristics of AlGaN/GaN heterojunction.

Natural regeneration in forest management, which relies on artificial planting, is considered a desirable alternative to reforestation. However, there are large uncertainties regarding the natural regeneration processes, such as seed production, seed dispersal, and seedling establishment. Among these processes, seed dispersal by wind must be modeled accurately to minimize the risks of natural regeneration. This study aimed to (1) review the main mechanisms of seed dispersal models, their characteristics, and their applications and (2) suggest prospects for seed dispersal models to increase the predictability of natural regeneration. With improving computing and observation systems, the modeling technique for seed dispersal by wind has continued to progress steadily from a simple empirical model to the Eulerian-Lagrangian model. Mechanistic modeling approaches with a dispersal kernel have been widely used and have attempted to be directly incorporated into spatial models. Despite the rapid development of various wind-dispersal models, only a few studies have considered their application in natural regeneration. We identified the potential attributes of seed dispersal modeling that cause high uncertainties and poor simulation results in natural regeneration scenarios: topography, pre-processing of wind data, and various inherent complexities in seed dispersal processes. We suggest that seed dispersal models can be further improved by incorporating (1) seed abscission mechanisms by wind, (2) spatiotemporally complex wind environments, (3) collisions with the canopy or ground during seed flight, and (4) secondary dispersal, long-distance dispersal, and seed predation. Interdisciplinary research linking climatology, biophysics, and forestry would help improve the prediction of seed dispersal and its impact on natural regeneration.

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.

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 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.

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 studied the genetic impact of natural regeneration practices, such as Single seed tree, Group seed tree, Patch clear cutting and Alternate strip clear cutting systems, by comparing the nuclear microsatellite(nSSR) variation of post-practice natural regeneration one-year old seedlings of Pinus densiflora to that of pre-practice mature trees. The levels of genetic diversity of seedlings (A=13.6, =4.3, =0.571, =0.597) were similar to those of mature trees (A=13.4, =4.3, =0.596, =0.598) and the differences in the level of genetic diversity between seedlings and mature trees for each of the practices were not statistically significant. The degree of genetic differentiation between seedlings and mature trees was very low (=0.002) and the pairwise values between seedlings and mature trees for all practices were less than 0.01. Overall, the natural regeneration practices appeared to have only minor impacts on the genetic diversity and the genetic composition in the studied P. densiflora stands. For a better understanding of the genetic effects of natural regeneration practices, subsequent studies such as temporal genetic variation of seedlings formed by crossing among post-practice mature trees should be considered.

This study was conducted to analyze the relationship among climatic factors and radial growth of Pinus koraiensis in South Korea. To determine climate-growth relationships, cluster analysis was applied to group climatically similar surveyed regions, and dendroclimatological model was developed to predict radial growth for each climate group under the RCP 4.5 and RCP 8.5 scenarios for greenhouse gases. The dendroclimatological models were developed through climatic variables and standardized residual chronology for each climatic cluster of P. koraiensis. 2 to 4 climatic variables were used in the models (R 2 values between 0.35~0.49). For each of the climatic clusters for Pinus koraiensis, the growth simulations obtained from two RCP climate-change scenarios were used for growth prediction. The radial growth of the Clusters 2 and 3, which grow at high elevation, tend to increase. In contrast, Cluster 1, which grows at low elevation, tends to decrease with a large difference. Thus, the growth of Pinus koraiensis, which is a boreal species, could increase along with increasing temperature up to a certain point.

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.

This study examined the effects of thinning intensities on carbon (C) storage of soil, forest floor and coarse woody debris (CWD) in Larix kaempferi stands, Korea. Two study stands were located in Gwangneung Experiment Forest (Stand 1: 31–40 years old) and Muju (Stand 2: 51–60 years old). These stands were thinned in 2011. Each stand was divided into three plots by different thinning intensities based on removed volume: no thinning (control, 0%), moderate thinning (M, 20%), and heavy thinning (H, 30%). The C storage of soil at 0–50 cm depth, forest floor, and CWD was measured in 2011. Total C storage of H plot was significantly higher than that of control plot and M plot in Stand 1 (control: 64.6 t C ha−1, M: 62.2 t C ha−1, H: 83.7 t C ha−1). A similar tendency was also found in Stand 2; total C storage of H plot was significantly higher than that of control plot and M plot (control: 137.8 t C ha−1; M: 138.0 t C ha−1; H: 169.6 t C ha−1). Initial effects of thinning intensities on C storage were analyzed a...

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.

This study was conducted to investigate effects of thinning on carbon (C) storage of soil, forest floor and coarse woody debris (CWD) in Pinus densiflora stands, central Korea. Two study stands (25- and 55-year-old stands) were located in P. densiflora forests of Gangwon-do and thinned in 2008. Each stand was divided into three plots by different thinning intensities based on stand density: no thinning (control, 0%), moderate thinning (M, 30%), and heavy thinning (H, 50%) in the 25-year-old stand and no thinning (control, 0%), light thinning (L, 20%), and moderate thinning (M, 30%) in the 55-year-old stand. We measured C storage of 0–30 cm depth soil, forest floor and CWD in 2009. Total C storage (t C ha−1) of soil, forest floor and CWD in two thinned plots was significantly higher than that in the control plot only in the 55-year-old stand: 66.4 for control

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.

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.