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Use MU-MIMO at your own risk — Why we don’t get Gb/s Wi-Fi
Choi, Hyunwoo,Gong, Taesik,Kim, Jaehun,Shin, Jaemin,Lee, Sung-Ju Elsevier 2019 Ad hoc networks Vol.83 No.-
<P><B>Abstract</B></P> <P>With the ever growing popularity of mobile devices, the demand for wireless bandwidth has also increased, with the mobile users now expecting wireless network quality similar to what they experience with wired networks. Wireless LANs have evolved over the last twenty years, with major breakthrough technologies such as OFDM (Orthogonal Frequency Division Multiplexing), MIMO (Multiple Input Multiple Output), and MU (Multi-User)-MIMO. The latest IEEE 802.11ac standard supports up to 6.9 Gb/s theoretical capacity, but it could only be achieved with 8-streams in a “perfect” environment. Commercial 802.11ac <I>wave 2</I> APs that include MU-MIMO capability, have only recently been made available in the market. We deployed a few APs from different vendors (that uses chipsets from different vendors) in various office environments and measured user throughput on smartphone mobile devices. We observe an enormous gap between theory and practice, with MU-MIMO often providing less throughput than SU (Single User)-MIMO in various network environments. We analyze the root cause of performance issues and suggest future research directions to achieve Gb/s Wi-Fi in practical deployments.</P>
SH003 selectively induces p73-dependent apoptosis in triple-negative breast cancer cells
Choi, Eun Kyoung,Kim, Seung-Mi,Hong, Seung-Woo,Moon, Jai-Hee,Shin, Jae-Sik,Kim, Jeong Hee,Hwang, Ih-Yeon,Jung, Soo-A,Lee, Dae-Hee,Lee, Eun Young,Lee, Seul,Kim, Hyunwoo,Kim, Daejin,Kim, Yeong Seok,Choi Spandidos Publications 2016 MOLECULAR MEDICINE REPORTS Vol. No.
Choi, Hyunwoo,Kim, Tae Geun,Shin, Changhwan Elsevier 2017 APPLIED SURFACE SCIENCE - Vol.407 No.-
<P><B>Abstract</B></P> <P>A topological insulator (TI) is a new kind of material that exhibits unique electronic properties owing to its topological surface state (TSS). Previous studies focused on the transport properties of the TSS, since it can be used as the active channel layer in metal-oxide-semiconductor field-effect transistors (MOSFETs). However, a TI with a negative quantum capacitance (QC) effect can be used in the gate stack of MOSFETs, thereby facilitating the creation of ultra-low power electronics. Therefore, it is important to study the physics behind the QC in TIs in the absence of any external magnetic field, at room temperature. We fabricated a simple capacitor structure using a TI (TI-capacitor: Au-TI-SiO<SUB>2</SUB>-Si), which shows clear evidence of QC at room temperature. In the capacitance-voltage (C-V) measurement, the total capacitance of the TI-capacitor increases in the accumulation regime, since QC is the dominant capacitive component in the series capacitor model (i.e., C<SUB>T</SUB> <SUP>−1</SUP> =C<SUB>Q</SUB> <SUP>−1</SUP> +C<SUB>SiO2</SUB> <SUP>−1</SUP>). Based on the QC model of the two-dimensional electron systems, we quantitatively calculated the QC, and observed that the simulated C-V curve theoretically supports the conclusion that the QC of the TI-capacitor is originated from electron–electron interaction in the two-dimensional surface state of the TI.</P> <P><B>Highlights</B></P> <P> <UL> <LI> The quantum capacitance in topological insulator (TI) at room temperature is directly revealed. </LI> <LI> The physical origin of quantum capacitance, the two dimensional surface state of TI, is experimentally validated. </LI> <LI> Theoretically calculated results of ideal quantum capacitance can well predict the experimental data. </LI> </UL> </P>
Comparison of Electrical Properties and Thermal Degradation of RGB-Organic Light Emitting Diodes
Choi, Pyungho,Park, Hyunae,Kim, Hyunwoo,Kim, Sangsoo,Choi, Byoungdeog American Scientific Publishers 2016 Journal of Nanoscience and Nanotechnology Vol.16 No.10
<P>The electrical properties and thermal degradation of red (R), green (G), and blue (B) organic-light-emitting-diodes (OLEDs) were characterized using the current-voltage (I-V) method. We investigated the temperature-dependent electrical degradation by determining the ideality factor n, series resistance r(s), and barrier height phi(B). The turn-on voltage for the R-OLED drastically decreased as temperature increased, whereas the others gradually decreased. Among the three types of OLEDs tested, the R-OLED showed the lowest current values under forward bias conditions as well as a severe temperature-dependency beyond the quasi-neutral region (QNR). The temperature-dependent variations of n and rs were largest for the R-OLED compared with the G-and B-OLEDs. This means that the R-OLED is the most easily degraded under thermal stress compared with the G-and B-OLEDs.</P>
Large-Scale Analysis of Remote Code Injection Attacks in Android Apps
Choi, Hyunwoo,Kim, Yongdae Hindawi Limited 2018 Security and communication networks Vol.2018 No.-
<P>It is pretty well known that insecure code updating procedures for Android allow remote code injection attack. However, other than codes, there are many resources in Android that have to be updated, such as temporary files, images, databases, and configurations (XML and JSON). Security of update procedures for these resources is largely unknown. This paper investigates general conditions for remote code injection attacks on these resources. Using this, we design and implement a static detection tool that automatically identifies apps that meet these conditions. We apply the detection tool to a large dataset comprising 9,054 apps, from three different types of datasets: official market, third-party market, and preinstalled apps. As a result, 97 apps were found to be potentially vulnerable, with 53 confirmed as vulnerable to remote code injection attacks.</P>
Choi, Pyungho,Kim, Dongsoo,Kim, Sangsub,Kim, Hyunwoo,Choi, Byoungdeog American Scientific Publishers 2016 Journal of Nanoscience and Nanotechnology Vol.16 No.10
<P>In this study, we characterized the interface and oxide charge generation in p-MOSFETs under negative bias temperature stress (NBTS). Thin (2.5 nm) and thick (6 nm) gate oxide MOSFETs were utilized to induce direct and Fowler-Nordheim (FN) tunneling, respectively. The threshold voltage and subthreshold swing in the thick oxide MOSFET was more significantly affected by NBTS than that of the thin oxide MOSFETs. The direct-current current-voltage (DCIV) method was implemented to investigate changes in trapped charges at the SiO2/Si interface. The change in oxide charges in the SiO2 bulk was obtained from the midgap voltage shift. The interface and oxide charges are predominantly affected by FN tunneling with less impact from direct tunneling, because of not only the hole-induced impact ionization at the SiO2/Si interface, but also the larger number of broken hydrogen atoms from the Si-H bonds, which are induced by the high applied gate bias. We conclude that devices' electrical performance can be significantly degraded when the MOSFETs are predominantly affected by FN tunneling rather than by direct tunneling under NBTS.</P>
Choi, Hyunwoo,Park, June,Shim, Jae Won,Shin, Changhwan Elsevier 2019 APPLIED SURFACE SCIENCE - Vol.463 No.-
<P><B>Abstract</B></P> <P>As an effort to overcome the physical limitation of scaling down the size of transistors and to implement ultra-low-power consumption in integrated circuits, the use of negative quantum capacitance has attracted significant attention, because it can simply enhance the gate capacitance in field-effect transistors. Among various two-dimensional materials, topological insulators (TI) have immense potential to achieve the effect of negative quantum capacitance, because they can be included in the gate stack of conventional metal oxide semiconductor (MOS) field-effect transistors. In order to verify their advantages for various gate capacitance values, we have fabricated a MOS capacitor with a TI. At room temperature, the capacitance enhancement (i.e., total gate capacitance higher than the theoretically estimated geometric capacitance) of the capacitor was experimentally observed near the depletion region. The effect of negative quantum capacitance, which is caused by the electron system on the surface of TI, demonstrates that the gate capacitance can increase in the range of frequency, i.e., 50–100 kHz.</P> <P><B>Highlights</B></P> <P> <UL> <LI> The impact of PVA film’s capacitance on the quantum capacitance of TI is understood. </LI> <LI> Positive/negative quantum capacitance from the 2DEG at the surface of TI was revealed. </LI> <LI> Theoretical study of carrier density at 2D surface state was matched to experiment. </LI> </UL> </P>
Choi, Hyunwoo,Shin, Jaemin,Shin, Changhwan The Electrochemical Society 2017 ECS journal of solid state science and technology Vol.6 No.7
<P>Thin film transistors (TFTs) with anatase-TiO2 channel material deposited on an SiO2/Si substrate by atomic layer deposition are fabricated to investigate the effect of the source/drain (S/D) metal work function on the electrical characteristics of the TFTs. S/D materials such as aluminum, silver, and gold are selected for this study. From the measured current-voltage characteristic curves in the dark and at room temperature, it is demonstrated that the anatase TiO2-based TFT with Ag S/D material shows the highest on/off-current ratio (similar to 10(5)), field effect mobility in the saturation regime (mu(sat) = 5.17 cm(2)V(-1)s(-1)), and saturation current (I-DS,I-sat similar to 0.47 mA). (C) 2017 The Electrochemical Society. All rights reserved.</P>