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Sungju Choi,Hyeongjung Kim,Chunhyung Jo,Hyun-Suk Kim,Sung-Jin Choi,Dong Myong Kim,Park, Jozeph,Dae Hwan Kim IEEE 2015 IEEE electron device letters Vol.36 No.7
<P>Thin-film transistors using In-Ga-Zn-O (IGZO) semiconductors were evaluated under current stress by applying positive voltages to the gate and drain electrodes. Initially, the transfer characteristics exhibit identical threshold voltages (V-T) when the source and drain electrodes are interchanged during measurement (forward and reverse V-DS sweep). However, as stress time increases, larger shifts in V-T are observed under forward V-DS sweep than under reverse V-DS sweep conditions. Subgap states analyses based on the photoresponse of capacitance-voltage (C-V) curves suggest that local annihilation of donor-like traps occurs near the drain electrode. Hump-like features are clearly observed in the C-V curves collected between the drain and gate electrodes, while they do not appear in the C-V data obtained between the source and the gate. Based on the above, a local charge trapping model is introduced in order to interpret the device degradation. In this model, the major carrier electrons are trapped more abundantly near the source electrode due to the presence of a Schottky junction between IGZO and the source/drain electrodes.</P>
Sungju Choi,Youngjin Kang,Jonghwa Kim,Jungmok Kim,Sung-Jin Choi,Dong Myong Kim,Ho-Young Cha,Hyungtak Kim,Dae Hwan Kim 대한전자공학회 2015 Journal of semiconductor technology and science Vol.15 No.5
It is essential to acquire an accurate and simple technique for extracting the interface trap density (Dit) in order to characterize the normally-off gate-recessed AlGaN/GaN hetero field-effect transistors (HFETs) because they can undergo interface trap generation induced by the etch damage in each interfacial layer provoking the degradation of device performance as well as serious instability. Here, the frequency-dependent capacitance-voltage (C-V) method (FDCM) is proposed as a simple and fast technique for extracting Dit and demonstrated in normally-off gate-recessed AlGaN/GaN HFETs. The FDCM is found to be not only simpler than the conductance method along with the same precision, but also much useful for a simple C-V model for AlGaN/GaN HFETs because it identifies frequency-independent and biasdependent capacitance components.
Choi, Sungju,Kim, Seohyeon,Jang, Jungkyu,Ahn, Gumho,Jang, Jun Tae,Yoon, Jinsu,Park, Tae Jung,Park, Byung-Gook,Kim, Dong Myong,Choi, Sung-Jin,Lee, Seung Min,Kim, Eun Young,Mo, Hyun Sun,Kim, Dae Hwan Elsevier Sequoia 2019 Sensors and actuators. B Chemical Vol.296 No.-
<P><B>Abstract</B></P> <P>In this study, we implement an artificial synapse and neuron in a single platform by combining a silicon nanowire (SiNW) ion-sensitive field-effect transistor (ISFET), an indium-gallium-zinc-oxide (IGZO) memristor, and a voltage-controlled oscillator (VCO). The chemical and electrical operations of the synapse are emulated using the pH sensor operation of the ISFET and long-term potentiation/short-term plasticity of the IGZO memristor, respectively. The concentration of hydrogen ions in an electrolyte is successfully transformed via a VCO-based neuron into modulation of synaptic strength, i.e., the current of the memristor. It mimics the strength of the synaptic connection modulated by the concentration of the neurotransmitter. Thus, the chemical-electrical signal conversion in chemical synapses is clearly demonstrated. Furthermore, the proposed artificial platform can discriminate the chemical synapse from the electrical synapse and the path of the neuro-signal propagation and that of memorization/update of synaptic strength. This can potentially provide a new insight into the principles of brain-inspired computing that can overcome the bottleneck of the state-of-the-art von-Neumann computing systems.</P>
Sungju Choi,Juntae Jang,Hara Kang,Ju Heyuck Baeck,Jong Uk Bae,Kwon-Shik Park,Soo Young Yoon,In Byeong Kang,Dong Myong Kim,Sung-Jin Choi,Yong-Sung Kim,Oh, Saeroonter,Dae Hwan Kim IEEE 2017 IEEE electron device letters Vol.38 No.5
<P>We propose an experimental method to decompose the positive gate-bias stress (PBS)induced threshold voltage shift (AVth) of amorphous InGaZnO (a-IGZO) thin-film transistors (TFTs) into the contributions of distinct degradation mechanisms. Topgate self-aligned coplanar structure TFTs are used for this letter. Stress-time-divided measurements, which combine the subgap density-of-states (DOS) extraction and the analysis on recovery characteristics, are performed to separate the AVth components. Change in excess oxygen (Oex)-related DOS is clearly observed, and AVth by PBS is quantitatively decomposed into the contributions of the active Oex, and the deep and shallow gate insulator traps. The quantitative decomposition of PBS-induced AVth provides physical insight and key guidelines for PBS stability optimization of a-IGZO TFTs.</P>
Sungju Choi,Hyeongjung Kim,Chunhyung Jo,Hyun-Suk Kim,Sung-Jin Choi,Dong Myong Kim,Park, Jozeph,Dae Hwan Kim IEEE 2015 IEEE electron device letters Vol.36 No.12
<P>Thin-film transistors using In-Ga-Zn-O (IGZO) semiconductors were evaluated under positive bias stress with different gate and drain voltages (VGS and VDS, respectively). The transfer characteristics with respect to stress time were examined, focusing on the threshold voltage (V-T) values obtained when the source and drain electrodes are interchanged during readout (forward and reverse VDS sweep). The V-T values shift toward either negative or positive values during stress, while transitions from negative to positive shifts are also observed. The negative V-T shift under positive bias stress is interpreted to occur by the generation of donor-like states related to ionized oxygen vacancies. On the other hand, positive V-T shifts result from the trapping of electrons near the IGZO/gate insulator interface. The transitions from negative to positive V-T shift are believed to result from the local electron trapping mechanism that gradually takes over donor-like state creation. From the experimental results and TCAD device simulation, it is suggested that a competition occurs between donor-like state creation and electron trapping. The relative magnitudes of the VGS and VDS fields determine which mechanism dominates, providing an analytical insight for the design of stable devices for driving transistors in AMOLED backplanes.</P>
Choi, Sungju,Kang, Youngjin,Kim, Jonghwa,Kim, Jungmok,Choi, Sung-Jin,Kim, Dong Myong,Cha, Ho-Young,Kim, Hyungtak,Kim, Dae Hwan The Institute of Electronics and Information Engin 2015 Journal of semiconductor technology and science Vol.15 No.5
It is essential to acquire an accurate and simple technique for extracting the interface trap density ($D_{it}$) in order to characterize the normally-off gate-recessed AlGaN/GaN hetero field-effect transistors (HFETs) because they can undergo interface trap generation induced by the etch damage in each interfacial layer provoking the degradation of device performance as well as serious instability. Here, the frequency-dependent capacitance-voltage (C-V) method (FDCM) is proposed as a simple and fast technique for extracting $D_{it}$ and demonstrated in normally-off gate-recessed AlGaN/GaN HFETs. The FDCM is found to be not only simpler than the conductance method along with the same precision, but also much useful for a simple C-V model for AlGaN/GaN HFETs because it identifies frequency-independent and bias-dependent capacitance components.
Selective acetate detection using functional carbon nanotube fiber
( Seung-ho Choi ),( Joon-seok Lee ),( Won-jun Choi ),( Sungju Lee ),( Hyeon Su Jeong ),( Seon-jin Choi ) 한국센서학회 2021 센서학회지 Vol.30 No.6
We developed a chemiresistive anion sensor using highly conductive carbon nanotube fibers (CNTFs) functionalized with anion receptors. Mechanically robust CNTFs were prepared via wet-spinning utilizing the nematic liquid crystal properties of CNTs in chlorosulfonic acid (CSA). For anion detection, polymeric receptors composed of dual-hydrogen bond donors, including thiourea 1, squaramide 2, and croconamide 3, were prepared and bonded non-covalently on the surface of the CNTFs. The binding affinities of the anion receptors were studied using UV-vis titrations. The results revealed that squaramide 2 exhibited the highest binding affinity toward AcO¯, followed by thiourea 1 and croconamide 3. This trend was consistent with the chemiresistive sensing responses toward AcO¯ using functional CNTFs. Selective anion sensing properties were observed that CNTFs functionalized with squaramide 2 exhibited a response of 1.08% toward 33.33 mM AcO¯, while negligible responses (<0.1%) were observed for other anions such as Cl¯, Br¯, and NO₃¯. The improved response was attributed to the internal charge transfer of dual-hydrogen bond donors owing to the deprotonation of the receptor upon the addition of AcO¯.
지문 퍼지볼트의 상관공격에 강인한 거짓 특징점 삽입 방법
최한나 ( Hanna Choi ),이성주 ( Sungju Lee ),정용화 ( Yongwha Chung ),최우용 ( Woo-yong Choi ),문대성 ( Daesung Moon ),문기영 ( Kiyoung Moon ) 한국정보처리학회 2009 한국정보처리학회 학술대회논문집 Vol.16 No.2
지문 템플릿(Fingerprint Template)을 보호하기 위해 암호학적 기법인 퍼지볼트(Fuzzy Vault)가 적용되었다. 퍼지볼트 기법은 지문으로 부터 추출되는 특징점을 은닉하기 위하여 지문 템플릿에 다수의 거짓 특징점을 “임의”로 삽입하는 방법이다. 그러나 최근 이러한 지문 퍼지볼트를 효과적으로 크래킹할 수 있는 상관공격(Correlation Attack)에 관한 연구가 발표되었는데, 이것은 동일한 지문으로부터 생성되는 두 개의 지문 템플릿을 획득함으로써 진짜와 거짓 특징점을 쉽게 구별하는 방법이다. 본 논문에서는 상관공격에 강인도록 지문 퍼지볼트를 생성하는 방법을 제안한다. 제안한 방법은 특징점의 각도 정보를 이용하여 거짓선분(Chaff Line)을 생성한 후 “규칙적”으로 거짓특징점을 삽입함으로써, 두 개 지문 템플릿을 획득하더라도 동일한 지문에 대해 삽입된 거짓 특징점의 위치와 각도가 유사하기 때문에 진짜와 거짓 특징점을 구별하기 어려워 상관공격을 피할 수 있다. 실험을 통하여 거짓 특징점을 규칙적으로 저장하는 방법을 적용함으로써 기존 방법의 인식 성능을 유지하면서, 상관 공격에 강인함을 확인하였다.
비밀분산 기법을 이용한 보안토큰 기반 지문 퍼지볼트의 보안성 향상 방법
최한나(Hanna Choi),이성주(Sungju Lee),문대성(Daesung Moon),최우용(Woo-Yong Choi),정용화(Yongwha Chung),반성범(Sung Bum Pan) 한국정보보호학회 2009 정보보호학회논문지 Vol.19 No.1
보안토큰 기반의 사용자 인증 시스템에서 사용자의 지문정보를 이용하는 방법이 대두되고 있다. 그러나 지문정보가 타인에게 도용된다면 패스워드와 달리 변경이 불가능하거나 제한적이기 때문에 사용자의 지문정보는 더욱 안전하게 보관되어야 한다. 이러한 문제를 해결하기 위한 방법 중 하나로 지문 퍼지볼트(Fuzzy Fingerprint Vault)가 보고되었다. 본 논문에서는 비밀분산 기법을 이용하여 지문 인식률의 성능저하 없이 보안토큰 기반 지문 퍼지볼트 시스템의 보안성을 향상시키는 방법을 제안한다. 즉, 퍼지볼트 이론이 적용된 사용자의 지문 템플릿을 지문 인식률과 보안성을 모두 고려하여 두 부분으로 나누어 각각 보안토큰과 서버에 저장한다. 또한, 퍼지볼트 이론을 지문에 적용하였을 때 발생하는 자동 정렬(Auto-Alignment) 문제는 기하학적 해싱 방법을 분산 적용하여 해결한다. 실험을 통하여 지문 인식률의 성능저하는 무시할 수준이고 보안성은 향상됨을 확인하였다. Recently, in the security token based authentication system, there is an increasing trend of using fingerprint for the token holder verification, instead of passwords. However, the security of the fingerprint data is particularly important as the possible compromise of the data will be permanent. In this paper, we propose an approach for secure fingerprint verification by distributing both the secret and the computation based on the fuzzy vault(a cryptographic construct which has been proposed for crypto-biometric systems). That is, a user fingerprint template which is applied to the fuzzy vault is divided into two parts, and each part is stored into a security token and a server, respectively. At distributing the fingerprint template, we consider both the security level and the verification accuracy. Then, the geometric hashing technique is applied to solve the fingerprint alignment problem, and this computation is also distributed over the combination of the security token and the server in the form of the challenge-response. Finally, the polynomial can be reconstructed from the accumulated real points from both the security token and the server. Based on the experimental results, we confirm that our proposed approach can perform the fuzzy vault-based fingerprint verification more securely on a combination of a security token and a server without significant degradation of the verification accuracy.
Direct comparison of ohmic contact properties between graphene and metal source/drain electrodes
Choi Minku,Park Chang Yong,Kim Sungju,Lee Young Tack 한국물리학회 2022 THE JOURNAL OF THE KOREAN PHYSICAL SOCIETY Vol.80 No.10
In this study, we directly compared the Ohmic contact properties between graphene and Ag source/drain (S/D) electrodes at the exactly same MoS2 FET device. In order to compare Ohmic contact properties of graphene and Ag electrodes on the same MoS2 nanofake, graphene S/D electrodes were fabricated on MoS2 active channel and the electrical properties were investigated as a frst group. After then, the graphene electrodes were fully covered by Ag S/D electrodes as a control group. Although graphene and Ag have similar workfunctions of~4.5 eV, the graphene S/D FET shows higher ON and lower OFF drain current (ION/IOFF of~ 106 ) than that of Ag S/D FET (ION/IOFF of~ 102 ), which comes from the Fermi energy level modulation efect of graphene. In addition, the non-classical asymmetric transistor was investigated by constructing the graphene as a source and Ag as s drain in MoS2 FET application, and it showed the gate tunable diode-like behavior and the excellent maximum photo-sensitivity of~ 105 under the green light (520 nm) as well.