• Journal of the Institute of Electronics Engineers of Korea SD
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• v.45 no.11
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• pp.21-28
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• 2008

In this paper, we suggest an analytical model that can derive the I-V characteristics in the saturation region of a short channel GaAs MESFET. Instead of the pinch-off concept that has been used in the conventional models we can derive the two-dimensional potential in the depletion region in order that the velocity saturation region cannot be pinched-off and the current continuity condition can be satisfied. Obtained expression for the velocity saturation length is expressed in terms of the total channel length, channel doping density, gate voltage, and drain voltage. Compared with the conventional channel length shortening models, the present model seems to be considerably accurate and more reasonable in explaining the Early effect.

• JSTS:Journal of Semiconductor Technology and Science
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• v.14 no.5
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• pp.557-565
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• 2014

We report the effect of basal-plane stacking faults (BSFs) on X-ray diffraction (XRD) of non-polar (11$\underline{2}$0) a-plane GaN films with different $SiN_x$ interlayers. Complete $SiN_x$ coverage and increased three-dimensional (3D) to two-dimensional (2D) transition stages substantially reduce BSF density. It was revealed that the Si-doping profile in the Si-doped GaN layer was unaffected by the introduction of a $SiN_x$ interlayer. The smallest in-plane anisotropy of the (11$\underline{2}$0) XRD ${\omega}$-scan widths was found in the sample with multiple $SiN_x$ layers, and this finding can be attributed to the relatively isotropic GaN mosaic resulting from the increase in the 3D-2D growth step. Williamson-Hall (WH) analysis of the (h0$\underline{h}$0) series of diffractions was employed to determine the c-axis lateral coherence length (LCL) and to estimate the mosaic tilt. The c-axis LCLs obtained from WH analyses of the present study's representative a-plane GaN samples were well correlated with the BSF-related results from both the off-axis XRD ${\omega}$-scan and transmission electron microscopy (TEM). Based on WH and TEM analyses, the trends in BSF densities were very similar, even though the BSF densities extracted from LCLs indicated that the values were reduced by a factor of about twenty.

• Proceedings of the Korean Vacuum Society Conference
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• 2016.02a
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• pp.247.1-247.1
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• 2016

Low-cost, high efficiency solar cells are tremendous interests for the realization of a renewable and clean energy source. ZnTe based solar cells have a possibility of high efficiency with formation of an intermediated energy band structure by impurity doping. In this work, ZnO/ZnTe:Cr and ZnO/i-ZnTe structures were fabricated by pulsed laser deposition (PLD) technique. A pulsed (10 Hz) Nd:YAG laser operating at a wavelength of 266 nm was used to produce a plasma plume from an ablated a ZnTe target, whose density of laser energy was 10 J/cm2. The base pressure of the chamber was kept at approximately $4{\times}10-7Torr$. ZnTe:Cr and i-ZnTe thin films with thickness of 210 nm were grown on p-Si substrate, respectively, and then ZnO thin films with thickness of 150 nm were grown on ZnTe:Cr layer under oxygen partial pressure of 3 mTorr. Growth temperature of all the films was set to $250^{\circ}C$. For fabricating ZnO/i-ZnTe and ZnO/ZnTe:Cr solar cells, indium metal and Ti/Au grid patterns were deposited on back and front side of the solar cells by using thermal evaporator, respectively. From the fabricated ZnO/ZnTe:Cr and ZnO/i-ZnTe solar cell, dark currents were measured by using Keithley 2600. Solar cell parameters were obtained under Air Mass 1.5 Global solar simulator with an irradiation intensity of 100 mW/cm2, and then the photoelectric conversion efficiency values of ZnO/ZnTe:Cr and ZnO/i-ZnTe solar cells were measured at 1.5 % and 0.3 %, respectively.

• Proceedings of the Korean Vacuum Society Conference
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• 2015.08a
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• pp.197.2-197.2
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• 2015

Low-cost, high efficiency solar cells are tremendous interests for the realization of a renewable and clean energy source. ZnTe based solar cells have a possibility of high efficiency with formation of an intermediated energy band structure by impurity doping. In this work, the ZnTe:O/CdS/ZnO structure was fabricated by pulsed laser deposition (PLD) technique. A pulsed (10 Hz) Nd:YAG laser operating at a wavelength of 266 nm was used to produce a plasma plume from an ablated a ZnTe target, whose density of laser energy was 4.5 J/cm2. The base pressure of the chamber was kept at a pressure of approximately $4{\times}10-7Torr$. ZnO thin film with thickness of 100 nm was grown on to ITO/glass, and then CdS and ZnTe:O thin film were grown on ZnO thin film. Thickness of CdS and ZnTe:O were 50 nm and 500 nm, respectively. During deposition of ZnTe:O films, O2 gas was introduced from 1 to 20 mTorr. For fabricating ZnTe:O/CdS/ZnO solar cells, Au metal was deposited on the ITO film and ZnTe:O by thermal evaporation method. From the fabricated ZnTe:O/CdS/ZnO solar cell, current-voltage characteristics was measured by using HP 4156-a semiconductor parameter analyzer. Finally, solar cell performance was measured using an Air Mass 1.5 Global (AM 1.5 G) solar simulator with an irradiation intensity of 100 mW cm-2.

• Journal of the Korean Crystal Growth and Crystal Technology
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• v.10 no.2
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• pp.122-127
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• 2000

Microstructural evolutions in ceria-stabilized zirconia (Ce-TZP) and alumina-dispersed Ce-TZP ceramics were investigated as functions of doping and annealing conditions. All of sintered specimens showed the relative density over 99 %. Sintered specimens had linear grain boundaries and normal grain shapes, but ceria-doped specimens had irregular grain shapes and nonlinear grain boundaries due to the diffusion-induced grain boundary migration during annealing at $1650^{\circ}C$ for 2 h. Mean grain boundary length of Ce-TZP with irregular grain shapes was higher than that of normal grain shapes, and was a value of 23pm at the maximum. Alumina particles dispersed in Ce-TZP inhibited the grain growth of zirconia particles. $Al_2O_3$Ce-TZP doped with ceria and annealed at $1650^{\circ}C$ for 2 h showed irregular grain shapes as well as small grain size. Added alumina particles showed the grain growth during sintering or annealing, and they changed the position from grain boundary to inside of the grains during the annealing. The specimens with normal grain shapes showed an intergranular fracture mode, whereas the specimens with irregular grain shapes showed a transgranular fracture mode during the crack propagation.

• Journal of the Korean Society for Precision Engineering
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• v.28 no.1
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• pp.19-23
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• 2011

Recently, focused ion beam (FIB) applications have been investigated for the modification of VLSI circuit, the MEMS processing, and the localized ion doping, A multi aperture FIB system has been introduced as the demands of FIB applications for high speed and large area processing increase. A liquid metal ion source has problems, a large angular divergence and a metal contamination into a substrate. In this study, a gas ion source was introduced to replace a liquid metal ion source. The gas ion source generated inductively coupled plasma (ICP) in a quartz tube (diameter: 45 mm). Ar gas fed into the quartz was ionized by a 2 turned radio frequency antenna. The Ar ions were extracted by 2 extraction grids. The maximum extraction voltage was 10 kV. A numerical simulation was used to optimize the design of extraction grids and to predict an ion trajectory. As a result, the maximum ion current density was 38 $mA/cm^2$ and the spread of ion energy was 1.6 % for the extraction voltage.

• Progress in Superconductivity
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• v.9 no.1
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• pp.62-67
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• 2007

The effects of the heat-treatment temperature on the carbon (C) substitution amount, full width at half maximum (FWHM) value, critical temperature ($T_c$), critical current density ($J_c$) have been investigated for 10 wt % malic acid ($C_4H_6O_5$)-doped $MgB_2/Fe$ wires. All the samples were fabricated by the in-situ powder-in-tube (PIT) method and heat-treated within a temperature range of $650^{\circ}C$ to $1000^{\circ}C$. As the heat-treatment temperature increased, it seemed that the lattice distortion was increased by a more active C substitution into the boron sites from the malic acid addition. These increased electron scattering defects seemed to enhance the $J_c-H$ properties in spite of an improvement in the crystallinity, such as a decrease of the FWHM value and an increase of the $T_c$. Compared to the un-doped wire heat-treated at $650^{\circ}C$ for 30 min, the $J_c$ was enhanced by the C doping in a high-field regime. The wire heat-treated at $900^{\circ}C$ resulted in a higher magnetic $J_c$ of approximately $10^4\;A/cm^2$ at 5 K and 8 T.

• 한국신재생에너지학회:학술대회논문집
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• 2011.05a
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• pp.112.1-112.1
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• 2011

The back contact solar cell (BCSC) has several advantages compared to the conventional solar cell since it can reduce grid shadowing loss and contact resistance between the electrode and the silicon substrate. This paper presents the effect of the surface texturing of the silicon BCSC by varying the texturing depth or the texturing gap in the commercially available simulation software, ATHENA and ATLAS of the company SILVACO. The texturing depth was varied from $5{\mu}m$ to $150{\mu}m$ and the texturing gap was varied from $1{\mu}m$ to $100{\mu}m$ in the simulation. The resulting efficiency of the silicon BCSC was evaluated depending on the texturing condition. The quantum efficiency and the I-V curve of the designed silicon BCSC was also obtained for the analysis since they are closely related with the solar cell efficiency. Other parameters of the simulated silicon BCSC are as follows. The substrate was an n-type silicon, which was doped with phosphorous at $6{\times}10^{15}cm^{-3}$, and its thickness was $180{\mu}m$, a typical thickness of commercial solar cell substrate thickness. The back surface field (BSF) was $1{\times}10^{20}\;cm^{-3}$ and the doping concentration of a boron doped emitter was $8.5{\times}10^{19}\;cm^{-3}$. The pitch of the silicon BCSC was $1250{\mu}m$ and the anti-reflection coating (ARC) SiN thickness was $0.079{\mu}m$. It was assumed that the texturing was anisotropic etching of crystalline silicon, resulting in texturing angle of 54.7 degrees. The best efficiency was 25.6264% when texturing depth was $50{\mu}m$ with zero texturing gap in case of low texturing depth (< $100{\mu}m$).

• Current Photovoltaic Research
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• v.8 no.1
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• pp.6-11
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• 2020

The power conversion efficiency (PCE) of a two-terminal tandem solar cell depends upon the tunnel-recombination junction (TRJ) between the top and bottom sub-cells. An optimized TRJ in a tandem cell helps improve its open-circuit voltage (V_oc), short-circuit current density (J_sc), fill factor (FF), and efficiency (PCE). One of the parameters that affect the TRJ is the buffer layer thickness. Therefore, we investigated various TRJs by varying the thickness of the buffer or intermediate layer (TRJ-buffer) in between the highly doped p-type and n-type layers of the TRJ. The TRJ-buffer layer was p-type nc-Si:H, with a doping of 0.06%, an activation energy (E_a) of 43 meV, an optical gap (E_g) of 2.04 eV, and its thickness was varied from 0 nm to 125 nm. The tandem solar cells we investigated were a combination of a heterojunction with intrinsic thin layer (HIT) bottom sub-cell and an a-Si:H (amorphous silicon) top sub-cell. The initial cell efficiency without the TRJ buffer was 7.65% while with an optimized buffer layer, its efficiency improved to 11.74%, i.e., an improvement in efficiency by a factor of 1.53.

• Journal of the Korean Institute of Electrical and Electronic Material Engineers
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• v.17 no.3
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• pp.289-293
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• 2004

PNW-PMN-PZT ceramics were fabricated with the variations of Nb$_2$O$_{5}$ addition and their microstructural and piezoelectric characteristics were investigated. When the amount of Nb$_2$O$_{5}$ increased, grain size decreased gradually. At 0.3wt％ Nb$_2$O$_{5}$ which is the same weight percent with Fe$_2$O$_3$, maximum tetragonality(c/a) and density were shown due to the complexed doping effects. Also, this composition that showed Qm of 2,041, kp of 0.55, grain size of 2.5${\mu}{\textrm}{m}$ and $\varepsilon$r of 1704 were proper for high power application. Using this composition, Rosen-type piezoelectric transformer was fabricated as the size of 1 ${\times}$ 16 ${\times}$ 5㎣ and its electrical characteristics were investigated with the variations of load resistance and driving frequency. At the resistance of 200㏀, maximum step-up ratio of 13.68 was shown. After driving PDA CCFL for 25 min using the inverter circuit, at driving frequency of 214.4KHz, input voltage of 31.78 V and input current of 21.1mA were measured at the input part of piezoelectric transformer. And then, output voltage of 293.2 V and output current of 2.2mA were shown at the output part of piezoelectric transformer. At the same time, efficiency of 96.2％ and temperature rise of 3.5$^{\circ}C$ were appeared at the piezoelectric transformer.ormer.