• Bulletin of the Korean Chemical Society
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• v.34 no.3
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• pp.856-862
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• 2013

The photoanode electrode of $TiO_2$ nanotubes (NTs) anchored with ZnS/CdSSe/CdS quantum dots (QDs) was prepared by anodization of Ti metal and successive ionic layer adsorption and reaction (SILAR) procedure. The tuning of the band gap of CdSSe was done with controlled composition of Cd, S, or Se during the SILAR. A ladder-like energy structure suitable for carrier transfer was attained with the photoanode electrode. The power conversion efficiency (PCE) of our solar cell fabricated with the regular array of $TiO_2$ NTs anchored with CdSSe/CdS or CdSe/CdS QDs [i.e., (CdSSe/CdS/$TiO_2NTs$) or (CdSe/CdS/$TiO_2NTs$)] was PCE = 3.49% and 2.81% under the illumination at 100 mW/$cm^2$, respectively. To protect the photocorrosion of our solar cell from the electrolyte and to suppress carrier recombination, ZnS was introduced onto CdSSe/CdS. The PCE of our solar cell with the structure of a photoanode electrode, (ZnS/CdSSe/CdS/$TiO_2$ NTs/Ti) was 4.67% under illumination at 100 mW/$cm^2$.

• Journal of the Korean Applied Science and Technology
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• v.32 no.1
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• pp.1-6
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• 2015

We investigated green phosphorescent organic light-emitting diodes with stepwise doping to improve efficiency roll-off and operational lifetime by efficient distribution of triplet excitons. The host material which was 4,4,N,N'-dicarbazolebiphenyl (CBP) of bipolar characteristic that can control the carrier in emitting layer (EML). When the EML devided into four parts with different doping concentration, each devices shows various efficiency roll-off and lifetime enhancement. The distribution of the carrier and excitons in the EML can be confirmed by using stepwise doping structure. The properties of device C exhibited luminous efficiency of 51.10 cd/A, external quantum efficiency of 14.88%, respectively. Lifetime has increased 73.70% compared to the reference device.

• Proceedings of the Korean Vacuum Society Conference
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• 2014.02a
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• pp.320-320
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• 2014

Amorphous silicon alloy (a-Si) solar cells and modules have been receiving a great deal of attention as a low-cost alternate energy source for large-scale terrestrial applications. Key to the achievement of high-efficiency solar cells using the multi-junction approach is the development of high quality, low band-gap materials which can capture the low-energy photons of the solar spectrum. Several cell designs have been reported in the past where grading or buffer layers have been incorporated at the junction interface to reduce carrier recombination near the junction. We have investigated profiling the composition of the a-SiGe alloy throughout the bulk of the intrinsic material so as to have a built-in electrical field in a substantial portion of the intrinsic material. As a result, the band gap mismatch between a-Si:H and $a-Si_{1-x}Ge_x:H$ creates a barrier for carrier transport. Previous reports have proposed a graded band gap structure in the absorber layer not only effectively increases the short wavelength absorption near the p/i interface, but also enhances the hole transport near the i-n interface. Here, we modulated the GeH4 flow rate to control the band gap to be graded from 1.75 eV (a-Si:H) to 1.55 eV ($a-Si_{1-x}Ge_x:H$). The band structure in the absorber layer thus became like a U-shape in which the lowest band gap was located in the middle of the i-layer. Incorporation of this structure in the middle and top cell of the triple-cell configuration is expected to increase the conversion efficiency further.

• JSTS:Journal of Semiconductor Technology and Science
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• v.14 no.4
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• pp.495-502
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• 2014

Fast recovery diodes (FRDs) were developed using the $p^{{+}{+}}/n^-/n^{{+}{+}}$ epitaxial layers grown by low temperature epitaxy technology. We investigated the effect of electrostatic discharge (ESD) stresses on their electrical and switching properties using current-voltage (I-V) and reverse recovery time analyses. The FRDs presented a high breakdown voltage, >450 V, and a low reverse leakage current, < $10^{-9}$ A. From the temperature dependence of thermal activation energy, the reverse leakage current was dominated by thermal generation-recombination and diffusion, respectively, at low and high temperature regions. By virtue of the abrupt junction and the Pt drive-in for the controlling of carrier lifetime, the soft reverse recovery behavior could be obtained along with a well-controlled reverse recovery time of 21.12 ns. The FRDs exhibited excellent ESD robustness with negligible degradations in the I-V and the reverse recovery characteristics up to ${\pm}5.5$ kV of HBM and ${\pm}3.5$ kV of IEC61000-4-2 shocks. Likewise, transmission line pulse (TLP) analysis reveals that the FRDs can handle the maximum peak pulse current, $I_{pp,max}$, up to 30 A in the forward mode and down to - 24 A in the reverse mode. The robust ESD property can improve the long term reliability of various power applications such as automobile and switching mode power supply.

• Proceedings of the Korean Institute of Electrical and Electronic Material Engineers Conference
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• 2004.07a
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• pp.292-295
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• 2004

The SIT was introduced by Nishizawa. in 1972. When compared with high-voltage, power bipolar junction transistors, SITs have several advantages as power switching devices. They have a higher input impedance than do bipolar transistors and a negative temperature coefficient for the drain current that prevents thermal runaway, thus allowing the coupling of many devices in parallel to increase the current handling capability. Furthermore, the SIT is majority carrier device with a higher inherent switching speed because of the absence of minority carrier recombination, which limits the speed of bipolar transistors. This also eliminates the stringent lifetime control requirements that are essential during the fabrication of high-speed bipolar transistors. This results in a much larger safe operating area(SOA) in comparison to bipolar transistors. In this paper, vertical SIT structures are proposed to improve their electrical characteristics including the blocking voltage. Besides, the two dimensional numerical simulations were carried out using ISE-TCAD to verify the validity of the device and examine the electrical characteristics. A trench gate region oxide power SIT device is proposed to improve forward blocking characteristics. The proposed devices have superior electrical characteristics when compared to conventional device. Consequently, the fabrication of trench oxide power SIT with superior stability and electrical characteristics is simplified.

• Proceedings of the Korean Vacuum Society Conference
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• 2015.08a
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• pp.87.2-87.2
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• 2015

Transparent conductors are commonly used in photoelectric devices, where the electric energy converts to light energy or vice versa. Energy consumption devices, such as LEDs, Displays, Lighting devices use the electrical energy to generate light by carrier recombination. Meanwhile, solar cell is the only device to generate electric energy from the incident photon. Most photoelectric devices require a transparent electrode to pass the light in or out from a device. Beyond the passive role, transparent conductors can be employed to form Schottky junction or heterojunction to establish a rectifying current flow. Transparent conductor-embedded heterojunction device provides significant advantages of transparent electrode formation, no need for intentional doping process, and enhanced light-reactive surface area. Herein, we present versatile applications of transparent conductors, such as NiO, ZnO, ITO in photoelectric devices of solar cells and photodetectors for high-performing UV or IR detection. Moreover, we also introduce the growth of transparent ITO nanowires by sputtering methods for large scale application.

• Korean Journal of Materials Research
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• v.18 no.11
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• pp.571-576
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• 2008

The simulation program for solar cells, PC1D, was briefly reviewed and the device modeling of a multicrystalline Si solar cell using the program was carried out to understand the internal operating principles. The effects of design parameters on the light absorption and the quantum efficiency were investigated and strategies to reduce carrier recombination, such as back surface field and surface passivation, were also characterized with the numerical simulation. In every step of the process, efficiency improvements for the key performance characteristics of the model device were determined and compared with the properties of the solar cell, whose efficiency (20.3%) has been confirmed as the highest in multicrystalline Si devices. In this simulation work, it was found that the conversion efficiency of the prototype model (13.6%) can be increased up to 20.7% after the optimization of design parameters.

• Journal of the Korean Institute of Electrical and Electronic Material Engineers
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• v.23 no.6
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• pp.436-439
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• 2010

SiC power device possesses attractive features, such as high breakdown voltage, high-speed switching capability, and high temperature operation. In general, device design has a significant effect on the switching characteristics. In this work, we report the effect of the interface states ($Q_f$) on the transient characteristics of SiC DMOSFETs. The key design parameters for SiC DMOSFETs have been optimized by using a physics-based two-dimensional (2-D) mixed device and circuit simulator by Silvaco Inc. When the $SiO_2$/SiC interface charge decreases, power losses and switching time also decrease, primarily due to the lowered channel mobilities. High density interface states can result in increased carrier trapping, or more recombination centers or scattering sites. Therefore, the quality of $SiO_2$/SiC interfaces has a important effect on both the static and transient properties of SiC MOSFET devices.

• Proceedings of the KIEE Conference
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• 1988.11a
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• pp.168-171
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• 1988

The p-n homo junction diode of the III-V ternary alloy semiconductor $In_{1-x}Ga_xP$ : S grown by the temperature gradient solution (TGS) was fabricated by Zn-diffusion, and it's characteristics was investigated. The carrier concentration of $In_{1-x}Ga_xP$ doped with sulfur, 0.5 mol %, was $1{\times}10^{17}cm^{-3}$ and the mobility was varied with the composition. In the case that the diffusion time was constant as 30 minutes. The temperature dependence of diffusion coefficient was decreased from D= $4.2{\times}10^{-5}$ exp (-1.74/$k_{B}T$) to D= $2.5{\times}10^{-5}$ exp (-3.272/$k_{B}T$) with increasing of composition $\times$ from 0.43 to 0.98. The major peak of E.L spectrum was due to D-A pair recombination and the peak intensity was increased with the increasing of input current. And the E.L intensity was decreased with the increasing temperature, and shift to the long wavelength. The luminescence efficiencies measured at $5^{\circ}C$, atmosphere temperature, was decreased from $2.6{\times}10^{-4}$% to $9.49{\times}10^{-6}$ % with increasing of composition it from 0.39, direct transition region, to 0.98, indirect transition region.

• Korean Journal of Materials Research
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• v.12 no.6
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• pp.431-435
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• 2002

The dark current and photocurrent(PC) spectrum of Mg-doped GaN thin film were investigated with various bias voltages and temperatures. At high temperature and small bias, the dark current is dominated by holes thermally activated from an acceptor level Al located at about 0.16 eV above the valence band maximum $(E_v)$, The PC peak originates from the electron transition from deep level A2 located at about 0.34 eV above the $E_v$ to the conduction band minimum $(E_ C)$. However, at a large bias voltage, holes thermally activated from A2 to Al experience the field-in-duces tunneling to form one-dimensional defect band at Al, which determines the dark current. The PC peak associated with the transition from Al to $E_ C$ is also observed at large bias voltages owing to the extended recombination lifetime of holes by the tunneling. In the near infrared region, a strong PC peak at 1.20 eV appears due to the hole transition from deep donor/acceptor level to the valence band.