• Proceedings of the Korean Institute of Electrical and Electronic Material Engineers Conference
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• 2008.06a
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• pp.151-151
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• 2008

갈륨-질화물(GaN) 기반의 고속전자이동도 트랜지스터(high electron mobility transistor, HEMT)는 마이크로파 또는 밀리미터파 등과 같은 고주파 대역의 통신시스템에 널리 사용되는 전자소자로 각광받고 있다. GaN HEMT는 AlGaN/GaN 또는 AlGaN/InGaN/GaN 등과 같은 이종접합구조(heterostructure)로부터 발생하는 이차원 전자가스(two-dimensional electron gas, 2DEG) 채널을 이용하여 캐리어 구속효과(carrier confinement) 및 이동도의 향상이 가능하다. 또한 높은 2DEG 채널의 면밀도(sheet concentration) 와 전자의 포화 속도(saturation velocity)를 바탕으로 고출력 동작이 가능하여 차세대 이동통신용 전력 증폭기로 주목받고 있다. 그러나 이론적으로 우수한 특성과 달리, 실제 소자에서는 epi 성장시의 결함이나 전위, 표면 상태에 따른 2DEG 감소 등의 영향으로 이론보다 높은 누설 전류와 낮은 항복 전압 특성을 가진다. 특히, 기존의 GaN HEMT 구조에서는 Drain-Side Gate Edge에서의 전계 집중이 항복 전압 특성에 미치는 영향이 크다. 본 논문에서는 이러한 문제를 해결하기 위해 Trapezoidal Gate구조를 이용하여 Drain 방향의 Gate Edge가 완만히 변하는 구조를 제안하였다. 이를 위해 $ATLAS^{TM}$ 전산모사 프로그램을 이용하여 Trapezoidal Gate 구조를 구현하여 형태에 따른 전류-전압 특성 및 소자의 스위칭 특성 및 Gate 아래 채널층에 형성되는 Electric Field의 분산을 조사하고, 이를 바탕으로 고속 동작 및 높은 항복 전압을 갖는 AlGaN/GaN HEMT의 최적화된 구조를 제안하였다. 새로운 구조의 Gate를 적용한 AlGaN/GaN HEMT는 Gate edge에서의 전계를 분산시켜 피크 값이 감소되는 것을 확인하였다.

• Korean Journal of Materials Research
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• v.30 no.11
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• pp.615-620
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• 2020

Oxide semiconductor, represented by a-IGZO, has been commercialized in the market as active layer of TFTs of display backplanes due to its various advantages over a-Si. a-IGZO can be deposited at room temperature by RF magnetron sputtering process; however, additional thermal annealing above 300℃ is required to obtain good semiconducting properties and stability. These temperature are too high for common flexible substrates like PET, PEN, and PI. In this work, effects of microwave annealing time on IGZO thin film and associated thin-film transistors are demonstrated. As the microwave annealing time increases, the electrical properties of a-IGZO TFT improve to a degree similar to that during thermal annealing. Optimal microwave annealed IGZO TFT exhibits mobility, SS, V_th, and V_H of 6.45 ㎠/Vs, 0.17 V/dec, 1.53 V, and 0.47 V, respectively. PBS and NBS stability tests confirm that microwave annealing can effectively improve the interface between the dielectric and the active layer.

• Journal of the Microelectronics and Packaging Society
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• v.6 no.2
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• pp.75-81
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• 1999

$CaTiO_3$-$ La(Mg_{2/3}Ta_{1/3})O_3$ solid solutions were prepared in order to improve the microwave dielectric properties of $CaTiO_3$. XRD analysis revealed that the crystal structure of the solid solution changed from orthorhombic to monoclinic as the amount of $ La(Mg_{2/3}Ta_{1/3})O_3$increased. When x=0.3 in (1-x)$CaTiO_3+xLa(Mg_{2/3}Ta_{1/3})O_3$, the dielectric constant was 49, the temperature coefficient of resonance frequency was +$14ppm/^{\circ}C$, and $Q \times f_0$ was 17000.

• Applied Chemistry for Engineering
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• v.19 no.3
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• pp.345-350
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• 2008

Polymer Impregnated Concrete (PIC) prepared from Ordinary Portland Cement Concrete (OPC) has excellent mechanical properties as well as physico-chemical properties. For the manufacturing of PIC, drying process of basis concrete (precast concrete), impregnation process with evacuation system and ultrasonic vibration system, polymerization process of monomers are essential. Modified microwave reactor using magnetron was used for polymerization of styrene/MMA (1 : 1) impregnated in pore volume of basis concrete. From the experimental results, the degree of polymerization increased up to 30% and more homogeneous PIC was prepared as compared to the conventional thermal method. Also the mechanical strengths increased more than 400% ($800{\sim}1200kg_f/cm^2$) and the resistance for corrosion to acids was improved up to 25%. AIBN and BPO as initiators for polymerization were used at the concentration less than 1%. Optimum conditions for polymerization were obtained at the frequency of microwave of 400 W and 2450 MHz, and optimum reaction temperature was $120^{\circ}C$ at an atmospheric pressure.

• Korean Journal of Remote Sensing
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• v.14 no.4
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• pp.315-324
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• 1998

The total precipitable water fields derived from HIRS(High Resolution Infrared Radiometer Sounder)and MSU(Microwave Sounding Unit) measurements of TOVS and brightness temperature of SSM/I were used to investigate the evolution of moisture fields for the Typhoon WALT(9407) which after landing in Japan it became tropical depression in Korea-Japan Strait, and FAYE(9503) which was the first tropical storm of 1995 to became a typhoon, respectively. The total precipitable water derived from TOVS observations is delineated according to the evolutions of WALT and FAYE movements because total precipitable water fields of TY WALT(9407) and FAYE9\(9503) were largely controlled by horizontal transport of water vapor over the Northwest Pacific Ocean which dominantly plays an important role in maintaining and accelerating their intensities toward Korea and Japan . These fields demonstrated that two major bands, which imply the rain bands, were locally well-organized and similar to the thick convective cloud features over Japan and the Korean peninsula while WALT and FAYE were approaching away and to. But the values of derived TOVS total precipitable water have shown the underestimate of those of SSM/I total comparatively for two typhoons.

• Applied Chemistry for Engineering
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• v.30 no.1
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• pp.23-28
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• 2019

We report a supercapacitor cable, which consists of three-dimensional (3D) porous graphene coated onto the surface of carbon thread. The 3D porous framework of graphene was constructed by microwave-activated process using a graphene oxide-coated carbon thread. The use of microwave irradiation enabled to convert graphene oxide into reduced graphene oxide without any reducing agents and activate graphene sheets into exfoliated and porous graphene sheets. Combining two wire electrodes with a polymer gel electrolyte successfully completed supercapacitor device in a form of cable construction. The supercapacitor cables were highly flexible, and thus can be transformed into various shapes of devices and be integrated into textile items. A high area-capacitance of 38.1 mF/cm was obtained at a scan rate of 10 mV/s. This capacitance was retained 88% of its original value at 500 mV/s. The cycle life was also demonstrated by repeating a charge/discharge process during 10,000 cycles even under bent states, showing a high capacitance retention of 96.5%.

• Current Research on Agriculture and Life Sciences
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• v.28
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• pp.53-62
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• 2010

The effect of far-infrared (FIR) irradiation on the functional compounds of grape seeds extract were evaluated. Grape seed was dried on different heating power (0, 900, 1800 W) and heating time (0, 20, 40 min) with far-infrared drier. Contents of soluble solids, catechins, total phenolics and total flavonoids along with Hunter's color value, electron donating ability (EDA) and nitrite scavenging activity were analyzed. The soluble solids, Hunter's L value and a value were not significantly different from control samples. Cathechin, procyanidin $B_2$ and epicatechin contents changed with heating time. In the treated sample, total cathechins content was found maximum at 900 W of heating power and 20 min of heating time with a high level of total phenolics, total flavonids and EDA. These results indicated that FIR irradiation of grape seed could enhance antioxidant activities of its extracts by increasing the amounts of functional compounds. Moreover, response surface methodology(RSM) was applied to predict optimum conditions for heating by FIR rays of grape seeds. Based on superimposition of contour map with respect to total phenolics, total catechin and EDA, optimum ranges of heating conditions were heating power of 621.82~818.18 W and heating time of 16.3~19.83 min.

• Journal of the Korean Magnetics Society
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• v.7 no.4
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• pp.191-195
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• 1997

$Co_{84}Hf_{16}$ (1300$\AA$, 2150$\AA$) thin films were prepared by dc magnetron sputtering method. To investigate the uniaxial anisotrpy of the sample, the saturation and effective magnetization of the thin films were measured by VSM and FMR, respectively. The spectroscopic splitting g factor were estimated from the ferromagnetic resonance curves. For 1300$\AA$, 2150$\AA$, the effective magnetization was measured at the temperatures from T=77K to T=300K. The results were analyzed in terms of Bloch's law $M_s(T)=M_s(0)(1BT^{3/2}CT^{5/2}$. The Bloch coefficient B and C were determined by fitting. $M_{eff}(0)$ was obtained by extrapolating $M_{eff}$ to 0 K. From this result, the spinwave stiffness constants D was also determined and the exchange stiffness constants $A_{eff}$ were calculated by Kittel's resonance conditions.

• Proceedings of the Korean Vacuum Society Conference
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• 2013.08a
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• pp.167-167
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• 2013

최근에 산화물 형광체는 황화물 형광체에 비해 높은 화학적 안정성을 나타내기 때문에 백색 발광 다이오드, 전계방출 디스플레이와 플라즈마 디스플레이 패널에 그 응용성을 넓히고 있다. 마그네슘 니오베이트(magnesium niobate, MgNb2O6)는 우수한 유전 특성(상대 유전상수=18.4)을 나타내기 때문에 마이크로파 유전체로 응용 가능하며, 단일상 릴랙서 페라브스카이트(relaxor perovskite) Pb(Mg1/3Nb2/3)O3을 합성하기 위한 전구체 (precursor)로 널리 사용되고 있으며, 나이오븀산염 이온에서 다양한 색상을 방출하는 활성제 이온으로 효율적인 에너지 전달이 일어남으로써 Sm3+, Dy3+, Eu3+와 같은 희토류 이온의 좋은 모체 격자로 개발할 수 있다. 본 연구에서는 마그네슘 니오베이트 MgNb2O6 모체 결정에 다양한 활성제 이온, 즉 Eu3+, Sm3+, Dy3+, Tb3+를 선택적으로 주입하여 발광 효율이 높은 천연색 형광체를 합성하고자 한다. 특히, 모체 결정에 주입되는 활성제 이온 주위의 국소적인 환경이 반전 대칭에서 변형되는 척도를 조사하여 활성제의 주 발광 파장의 세기가 최대가 되는 최적의 조건을 결정하고자 한다. Mg1-1.5xNb2O6:REx3+ 형광체 분말 시료는 초기 물질 MgO, Nb2O5와 희토류 이온을 화학 반응식에 맞게 정밀 저울로 측량하여 플라스틱 용기에 ZrO2 볼과 함께 넣고, 소정의 에탄올을 채운 뒤 밀봉하고서, 300 rpm의 속도로 20시간 볼밀 (ball-mill) 작업을 수행하였다. 그 후, 체(sieve)로 ZrO2 볼을 걸러낸 다음에 혼합된 용액을 각 비커에 담아서 $40^{\circ}C$의 건조기에서 24시간 건조하였고, 건조된 시료를 막자 사발에 넣고 잘게 갈고 80 ${\mu}m$의 체로 걸러낸 후에, 알루미나 도가니에 활성제 이온별로 각각 담아, 전기로에 장입하여 매분당 $5^{\circ}C$의 비율로 온도를 상승시켜 $350^{\circ}C$에서 5시간 동안 하소 공정을 실시한 후에, 온도를 계속 일정한 율로 증가시켜 $1,200^{\circ}C$에서 5시간 동안 소성하여 합성하였다. 합성된 형광체 분말의 결정 구조는 $Cu-K{\alpha}$ 복사선(파장: 1.5406)을 사용하여 X-선회절장치로 측정하였으며, 형광체의 표면 형상은 전계형 주사전자현미경으로 관측하였다. 흡광와 발광스펙트럼은 제논 램프를 광원으로 갖는 형광 광도계를 사용하여 측정하였다. 모체 결정에 활성제 이온 Eu3+, Sm3+, Dy3+, Tb3+가 도핑된 형광체 분말은 각각 적색, 주황색, 황색, 녹색 발광이 관측되었다. 각 발광 스펙트럼과 결정 입자의 크기와 형상 사이의 상호 관계를 조사하였다. 실험 결과로부터, 각 형광체의 발광 파장은 활성제 이온의 종류 와 서로 밀접하게 관련되어 있으며, 형광체 시료 합성시 활성제 이온의 농도를 선택적으로 조절함으로써 발광의 세기를 제어할 수 있음을 확인하였다.

• Korean Journal of Remote Sensing
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• v.38 no.6_1
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• pp.1141-1180
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• 2022

The application of satellite imageries has increased in the field of hydrology and water resources in recent years. However, challenges have been encountered on obtaining accurate evapotranspiration and soil moisture. Therefore, present researches have emphasized the necessity to obtain estimations of satellite-based evapotranspiration and soil moisture with related development researches. In this study, we presented the research status in Korea by investigating the current trends and methodologies for evapotranspiration and soil moisture. As a result of examining the detailed methodologies, we have ascertained that, in general, evapotranspiration is estimated using Energy balance models, such as Surface Energy Balance Algorithm for Land (SEBAL) and Mapping Evapotranspiration with Internalized Calibration (METRIC). In addition, Penman-Monteith and Priestley-Taylor equations are also used to estimate evapotranspiration. In the case of soil moisture, in general, active (AMSR-E, AMSR2, MIRAS, and SMAP) and passive (ASCAT and SAR)sensors are used for estimation. In terms of statistics, deep learning, as well as linear regression equations and artificial neural networks, are used for estimating these parameters. There were a number of research cases in which various indices were calculated using satellite-based data and applied to the characterization of drought. In some cases, hydrological cycle factors of evapotranspiration and soil moisture were calculated based on the Land Surface Model (LSM). Through this process, by comparing, reviewing, and presenting major detailed methodologies, we intend to use these references in related research, and lay the foundation for the advancement of researches on the calculation of satellite-based hydrological cycle data in the future.