• Journal of IKEEE
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• v.11 no.4
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• pp.272-278
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• 2007

In this paper, we report on the 2D (two-dimensional) simulation result of the DC (direct current) electrical and thermal characteristics of AlGaN/GaN HEMTs (high electron mobility transistors) grown on Si substrate, in comparison with those grown on sapphire and SiC (silicon carbide) substrate, respectively. In general, the electrical properties of HEMT are affected by electron mobility and thermal conductivity, which depend on substrate material. For this reason, the substrates of GaN-based HEMT have been widely studied today. The simulation results are compared and studied by applying general Drift-Diffusion and thermal model altering temperature as 300, 400 and 500 K, respectively. With setting T=300 K and $V_{GS}$=1 V, the $I_{D,max}$ (drain saturation current) were 189 mA/mm for sapphire, 293 mA/mm for SiC, and 258 mA/mm for Si, respectively. In addition, $G_{m,max}$ (maximum transfer conductance) of sapphire, SiC, Si was 38, 50, 31 mS/mm, respectively, at T=500 K.

• Journal of the Institute of Electronics Engineers of Korea SD
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• v.45 no.5
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• pp.12-18
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• 2008

Ag paste was employed for source and drain electrode of OTFTs and for the data metal lines of OTFT-OLED array on PC(polycarbonate) substrate. We tested two kinds of Ag-pastes such as pastes for 325 mesh and 500 mesh screen mask to examine the pattern ability and electrical performance for OTFTs. The minimum feature size was 60 ${\mu}m$ for 325 mesh screen mask and 40 ${\mu}m$ for 500 mesh screen mask. The conductivity was 60 $m{\Omega}/\square$ for 325 mesh and 133.1 $m{\Omega}/\square$ for 500 mesh. For the OTFT performance the mobility was 0.35 $cm^2/V{\cdot}sec$ and 0.12 $cm^2/V{\cdot}sec$, threshold voltage was -4.7 V and 0.9 V, respectively, and on/off current ratio was ${\sim}10^5$, for both screen masks. We applied the 500 mash Ag paste to OTFT-OLED array because of its good patterning property. The pixel was composed of two OTFTs and one capacitor and one OLED in the area of $2mm{\times}2mm$. The panel successfully worked in active mode operation even though there were a few bad pixels.

• Journal of Sensor Science and Technology
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• v.12 no.3
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• pp.103-111
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• 2003

A chromel-alumel thermoelectric flow sensor using $Si_3N_4/SiO_2/Si_3N_4$ thermal isolation membrane was fabricated. Temperature coefficient of resistance of thin film Pt-heater was about $0.00397/^{\circ}C$, and Seebeck coefficient of chromel-alumel thermocouple was about $36\;{\mu}V/K$. The sensor showed that thermoelectric voltage decreased as thermal conductivity of gas increased, and $N_2$-flow sensitivity increased as heater voltage increased or the distance between heater and thermocouple decreased. When heater voltage was about 2.5 V, $N_2$-flow sensitivity and thermal response time of the sensor were about $1.5\;mV/sccm^{1/2}$ and 0.18 sec., respectively. Linear range in flow sensitivity of the flow sensor was wider than that of Bi-Sb flow sensor.

• Journal of IKEEE
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• v.18 no.2
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• pp.272-281
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• 2014

In electrical impedance tomography (EIT), modified Newton Raphson (mNR) method is widely used inverse algorithm for static image reconstruction due to its convergence speed and estimation accuracy. The unknown conductivity distribution is estimated iteratively by minimizing a cost functional such that the residual error namely the difference in measured and calculated voltages is reduced. Although, mNR method has good estimation performance, EIT inverse problem still suffers from ill-conditioned and ill-posedness nature. To mitigate the ill-posedness, generally, regularization methods are adopted. The inverse solution is highly dependent on the choice of regularization parameter. In most cases, the regularization parameter has a constant value and is chosen based on experience or trail and error approach. In situations, when the internal distribution changes or with high measurement noise, the solution does not get converged with the use of constant regularization parameter. Therefore, in this paper, in order to improve the image reconstruction performance, we propose a new scheme to determine the regularization parameter. The regularization parameter is computed based on residual error and updated every iteration. The proposed scheme is tested with numerical simulations and laboratory phantom experiments. The results show an improved reconstruction performance when using the proposed regularization scheme as compared to constant regularization scheme.

• The Journal of the Korea institute of electronic communication sciences
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• v.15 no.4
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• pp.665-670
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• 2020

Buffer layer in CIGS thin-film solar cells improves energy conversion efficiency through band alignment between the absorption layer and the window layer. ZnS is a non-toxic II-VI compound semiconductor with direct-transition band gaps and n-conductivity as well as with excellent lattice matching for CIGS absorbent layers. In this study, the structural and optical properties of ZnS thin films, deposited by RF magnetron sputtering method and subsequently performed by the rapid thermal annealing treatment, were investigated for the buffer layer. The zincblende cubic structures along (111), (220), and (311) were shown in all specimens. The rapid thermal annealed specimens at the relatively low temperatures were polycrystalline structure with the wurtzite hexagonal structures along (002). Rapid thermal annealing at high temperatures changed the polycrystalline structure to the single crystal of the zincblende cubic structures. Through the chemical analysis, the zincblende cubic structure was obtained in the specimen with the ratio of Zn/S near stoichiometry. ZnS thin film showed the shifted absorption edge towards the lower wavelength as annealing temperature increased, and the mean optical transmittance in the visible light range increased to 80.40% under 500℃ conditions.

• Proceedings of the Korean Institute of Electrical and Electronic Material Engineers Conference
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• 2006.06a
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• pp.154-154
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• 2006

Samsung Electro Mechanics ink jet has developed ultra high resolution alignment system. The alignment system has been developed for repeatable printing of conductive ink. The resolution of alignment system is 0.5um and the velocity of printing working plate is 1.5m/s. So far repeated printing results included sintering process have over 30um of drop mislocation data. In order to improve line thickness and conductivity of metal line, we need to develop the higher mechanical accurate align system. On the demand, this developed align system has under $1{\sim}2{\mu}m$ mispositioning performance and can measure of mechanical accuracy of inkjet printer, as well as the straightness of jetted drop from inkjet head. There is no kinds limit of substrate and ink to use SEM alignment system. By using this alignment system, we progress two experiment of reiterate printing drop and making conductive line on the glass and photo paper. Optical microscope and 3D profiler has been used for measurement of printed ink.

• Proceedings of the Korean Vacuum Society Conference
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• 2010.02a
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• pp.74-74
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• 2010

One-dimensional nanosturctures such as nanowires and nanotube have been mainly proposed as important components of nano-electronic devices and are expected to play an integral part in design and construction of these devices. Silicon carbide(SiC) is one of a promising wide bandgap semiconductor that exhibits extraordinary properties, such as higher thermal conductivity, mechanical and chemical stability than silicon. Therefore, the synthesis of SiC-based nanowires(NWs) open a possibility for developing a potential application in nano-electronic devices which have to work under harsh environment. In this study, one-dimensional nanowires(NWs) of cubic phase silicon carbide($\beta$-SiC) were efficiently produced by thermal chemical vapor deposition(T-CVD) synthesis of mixtures containing Si powders and hydrocarbon in a alumina boat about $T\;=\;1400^{\circ}C$ SEM images are shown that the temperature below $1300^{\circ}C$ is not enough to synthesis the SiC NWs due to insufficient thermal energy for melting of Si Powder and decomposition of methane gas. However, the SiC NWs are produced over $1300^{\circ}C$ and the most efficient temperature for growth of SiC NWs is about $1400^{\circ}C$ with an average diameter range between 50 ~ 150 nm. Raman spectra revealed the crystal form of the synthesized SiC NWs is a cubic phase. Two distinct peaks at 795 and $970\;cm^{-1}$ over $1400^{\circ}C$ represent the TO and LO mode of the bulk $\beta$-SiC, respectively. In XRD spectra, this result was also verified with the strongest (111) peaks at $2{\theta}=35.7^{\circ}$, which is very close to (111) plane peak position of 3C-SiC over $1400 ^{\circ}C$ TEM images are represented to two typical $\beta$-SiC NWs structures. One is shown the defect-free $\beta$-SiC nanowire with a (111) interplane distance with 0.25 nm, and the other is the stacking-faulted $\beta$-SiC nanowire. Two SiC nanowires are covered with $SiO_2$ layer with a thickness of less 2 nm. Moreover, by changing the flow rate of methane gas, the 300 sccm is the optimal condition for synthesis of a large amount of $\beta$-SiC NWs.

• Bulletin of the Korean Chemical Society
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• v.10 no.6
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• pp.504-509
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• 1989

The effect of oxygen-deficiency on the charge distributions and orbital energies for small copper oxide clusters representing the superconducting materials $YBa_2Cu_3O_x (6{\leq}x{\leq}7)$ were investigated by the extended Huckel molecular orbital (EHMO) method with the tight-binding model. Our calculations show +3 oxidation state of Cu(1) in the $CuO_3$ chain and +2 or +1 of Cu(2) in the $CuO_2$ layers for $YBa_2Cu_3O_7$ with the nominal charge of $Cu_3$ = +7 (or +5), while for $YBa_2Cu_3O_6$ +1 oxidation state of Cu(1) and +3 (or +2) of Cu(2) in the $CuO_2$ layers with the nominal charge of $Cu_3$ = +7 (or +5). For $Cu_3O_{12}$ cluster representing $YBa_2Cu_3O_7$ with the nominal charge of $Cu_3$ = +7 the Cu(2) $d_{{x^2}-{y^2}}$ orbitals in the $CuO_2$ layers is a typical Jahn-Teller $d^9$ system with the partial hole and the Cu(1) $d_{{_z2}-{_y2}}$ orbital in the $CuO_3$ chain contains hole occupancy. For $Cu_3O_{10}$ cluster representing $YBa_2Cu_3O_6$ with the nominal charge of Cu = +5 the orbital character of the highest partially occupied MO (HPOMO) and the lowest completely unoccupied MO (LCUMO) of $Cu_3O_{12}$ representing $YBa_2Cu_3O_7$ with the nominal charge of $Cu_3$ = +7 is reversed, and the character of Cu(1) $d{{x^2}-{y^2}}$ orbital of LCUMO of the $Cu_3O_{12} $cluster is vanished. It is suggested that the local crystal field environment of Cu(1) by the oxygens in the Cu(1) chain may play a vital role in conductivity and superconductivity, either alone or through cooperative electronic coupling with the Cu(2) layers in $YBa_2Cu_3O_7.$.

• Journal of the Korean Institute of Electrical and Electronic Material Engineers
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• v.34 no.4
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• pp.251-255
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• 2021

a-Si is commonly considered as a primary candidate for the formation of passivation layer in heterojunction (HIT) solar cells. However, there are some problems when using this material such as significant losses due to recombination and parasitic absorption. To reduce these problems, a wide bandgap material is needed. A wide bandgap has a positive influence on effective transmittance, reduction of the parasitic absorption, and prevention of unnecessary epitaxial growth. In this paper, the adoption of a-SiO_x:H as the intrinsic layer was discussed. To increase lifetime and conductivity, oxygen concentration control is crucial because it is correlated with the thickness, bonding defect, interface density (D_it), and band offset. A thick oxygen-rich layer causes the lifetime and the implied open-circuit voltage to drop. Furthermore the thicker the layer gets, the more free hydrogen atoms are etched in thin films, which worsens the passivation quality and the efficiency of solar cells. Previous studies revealed that the lifetime and the implied voltage decreased when the a-SiOx thickness went beyond around 9 nm. In addition to this, oxygen acted as a defect in the intrinsic layer. The D_it increased up to an oxygen rate on the order of 8%. Beyond 8%, the D_it was constant. By controlling the oxygen concentration properly and achieving a thin layer, high-efficiency HIT solar cells can be fabricated.

• Composites Research
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• v.34 no.6
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• pp.357-372
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• 2021

The energy harvesting device is known to be promising as an alternative to solve the resource shortage caused by the depletion of petroleum resources. In order to overcome the limitations (environmental pollution and low mechanical properties) of piezoelectric elements capable of converting mechanical motion into electrical energy, many studies have been conducted on a polymer matrix-based composite piezoelectric energy harvesting device. In this paper, the output performance and related applications of the reported piezoelectric composites are reviewed based on the applied materials and processes. As for the piezoelectric fillers, zinc oxide, which is advantageous in terms of eco-friendliness, biocompatibility, and flexibility, as well as ceramic fillers based on lead zirconate titanate and barium titanate, were reviewed. The polymer matrix was classified into piezoelectric polymers composed of polyvinylidene fluoride and copolymers, and flexible polymers based on epoxy and polydimethylsiloxane, to discuss piezoelectric synergy of composite materials and improvement of piezoelectric output by high external force application, respectively. In addition, the effect of improving the conductivity or the mechanical properties of composite material by the application of a metal or carbon-based secondary filler on the output performance of the piezoelectric harvesting device was explained in terms of the structure of the composite material. Composite material-based piezoelectric harvesting devices, which can be applied to small electronic devices, smart sensors, and medicine with improved performance, can provide potential insights as a power source for wireless electronic devices expected to be encountered in future daily life.