경제적 단백질 생산을 위한 무세포 단백질 합성 시스템의 개발

권진호 忠南大學校 大學院 2016 국내석사

Conventional cell-based protein production methods are time- and labor intensive because they require complicated procedures from the preparation of recombinant DNA to protein purification. On the other hand, cell-free protein synthesis system does not involve these multiple steps and thus can express analyze target proteins within a day. In addition, the reaction conditions for protein synthesis can be easily manipulated for better expression and folding of target proteins. However, despite the versatility of cell-free protein synthesis, the limitations associated with high cost of materials have prevented its use for large-scale protein production. The major high cost fractions in CFPS mixture are attributed to the preparations of cell-extract and template DNA, nucleotide and energy sources for ATP-regeneration. In this study, we tried to develop more economical cell-free protein synthesis systems. First of all, generally used nucleoside triphosphate (NTP) was replaced with nucleoside monophosphates (NMP), which costs less than 21 fold than NTP. In addition, cell mass for extract preparation was prepared with high cell-density fermentation using a semi-defined media. When the superfolder green fluorescence protein (sfGFP) was expressed in a cell-free synthesis system employing with NMP and high-density cell extract (HD extract), yield of protein expression was almost comparable to the original system using NTP and regular cell extract. As a result, the cost for production of mg protein in CP system was reduced about 23%. However, significant loss of protein productivity was observed when the energy source for ATP regeneration was changed from CP to glucose. It appeared that the long cultivation time with slow growth rate in the semi-defined media affected the translational activity of resulting cell extract. In addition, cost-effective materials were also successfully used in the continuous-exchange cell-free system (CECF) when CP was used as an energy source for ATP regeneration, which further justifies the use of these low-cost materials for large scale production of recombinant proteins.

혼합영양 조건에서 석탄발전 배가스를 이용한 바이오디젤용 고지질 미세조류의 생산

김보화 忠南大學校 大學院 2016 국내석사

In recent years, microalgae have attracted much global attention for their potentials as biodiesel feedstock. However, microalgae-based biodiesel production, when considered for the commercial scale, remains challenging such as reduction of cost and improvement of lipid productivity. Industrial exhaust flue-gases are an inexpensive and rich source of CO2 (~14 % v/v) that can be utilized to facilitate microalgal culture. On the other hand, several microalgae can grow mixotrophically by utilizing organic carbons along with light, thereby yielding high lipid productivity. Exploiting such a mixotrophic cultivation system with a suitable microalga would open up opportunities to utilize waste organics as carbon sources and at the same time, help to bring down cultivation costs. Meanwhile, overgrowth of associative bacterial community is a major concern in a mixotrophic culture. In this study, 30 local strains of Chlorella were evaluated in photobioreactors for coal-fired flue-gas application under mixotrophic condition. Three strains (M082, M134, and KR-1) were sequentially selected based on cell growth, lipid content, and fatty acid composition under autotrophic and mixotrophic conditions. Interestingly, KR-1 demonstrated, along with maximal lipid content indoors and high adaptability to actual flue-gas (CO2, 13%). Furthermore, the biodiesel properties of the produced fatty acids could meet important international standards under all of the tested culture conditions. Finally, Chlorella sp. KR-1 was selected and evaluated for its feasibility of the utilization of flue gas under various mixotrophic culture conditions. When the culture was mediated by flue gas, highest biomass (0.8 g cells/L/d) and FAME (fatty acid methyl esters) productivity (121 mg/L/d) were achieved in the mixotrophic mode with 5 g/L glucose, 5 mM nitrate, and a flow rate of 0.2 vvm. Lastly, we report the use of Mg-APTES, known to have selective biocidal effects, for improving the growth of Chlorella sp. KR-1 while simultaneously controlling the bacterial populations. The concentration of 0.1 g Mg-APTES/L improved the growth of KR-1 by 28%, while the bacterial concentration was inhibited by 59% when compared with the untreated control. Further, the density gradient gel electrophoresis (DGGE) revealed bacterial consortium were restricted with the increase of Mg-APTES concentrations. Thus, it was concluded that KR-1 could be considered as one of potential strains for biodiesel production and the result of its optimal culture condition and understanding selective effects of Mg-APTES would greatly benefit microalgal mass cultivation systems.

Functional expression of phospholipase A1 in cell-based and cell-free protein synthesis systems

임혜진 忠南大學校 大學院 2016 국내석사

Phospholipase A1 (EC 3.1.1.32) is an enzyme that hydrolyzes phospholipids into a fatty acid and lysophospholipid. Because fatty acid and lysophospholipid are able to be useful raw materials, it has been performed many studies on how to decompose the phospholipid. Furthermore, because phospholipase have function to hydrolyze phospholipid, phospholipase has potential industrial applications in such fields as bioenergy, food, and pharmaceutical industry. An enzyme is an environmentally friendly product so study of large quantity production for using catalyst in many industries has been increased. The lipolytic activity of phospholipase, however, causes inhibition of cell growth and hinders its production in the conventional cell-based expression methods. Because it does not require membrane integrity for protein synthesis and can control necessary components of expression system, cell-free protein synthesis system using cell lysates offers an alternative route to the production of this cell lytic enzyme. In this study, we attempted to express the phospholipase A1 gene cloned from the chromosomal DNA of Serratia sp. MK1 either in E. coli cells or a cell-free synthesis system derived from the lysate of the E. coli cells. It was found that the phospholipase A1 could be efficiently produced in the cell-free protein synthesis system with substantially higher productivity compared to the recombinant E. coli cells. We expect that the cell-free approach can provide a solution for production of many other enzymes that interfere with the growth and viability of host cells.

Modeling and Characterization of Electrical Parameters in Field-Effect Transistors through Optoelectronic Measurements

김준엽 국민대학교 일반대학원 2019 국내석사

본 논문에서는 광/전기적 실험과 모델링 과정을 통해 전계효과 트랜지스터의 전기적 특성 지표를 분석하였다. 제안된 기술은 소자의 채널 물질과 구조의 관계없이 다양한 소자에 적용 가능하다. 특히, 추출한 재결합 시간과 기생 저항은 소자의 동작 특성과 다양한 어플리케이션 활용 측면에서 중요한 특성 지표이다. 먼저, 이 연구는 InGaAs MISFET에서 광/전기적 측정을 통해 특정 재결합 시간을 분석하였다. 재결합 시간은 다양한 애플리케이션 및 소자의 성능 지표로 사용될 수 있기 때문에 모델링 및 특성 분석하는 일은 중요한 연구의 의미를 가지고 있다. 추출된 재결합 시간은 photoconductive와 photovoltaic효과를 고려한 연구이다. 게이트/드레인 전압과 어보브 밴드갭 빛을 인가한 상태에서 생성된 전자와 정공은 드레인 전류에 기여를 하게 된다. 특히, 수직 방향 전계 의해 기판 내에 축적되는 전하는 photovoltaic 영향에 관여한다. 따라서, 이러한 photovoltaic영향을 시간에 의존적인 문턱전압 수식에 반영하여 광/전기적 추출 기술을 개발하였다. 이 기술은 기판의 전하 저장 위치와 관여된 특정 재결합 시간을 분석하는 좋은 도구가 될 수 있다. 다음으로, 기생 저항은 미세화된 제조공정과 동작상 발생할 수 있는 전기적인 특성 저하에 관여된 특성 지표이다. 따라서, 우리는 MISFET 소자에서 포괄적인 전기적 측정 기반 기생 저항 추출 기술을 제안한다. 이 기술은 인가전압에 따른 채널의 저항을 조절하는 방법, 같은 공정 내 게이트 길이를 바꾸며 저항을 분리해내는 방법 외 기생 저항을 추출하는 전기적 방법들을 조합한 포괄적인 방법으로써, 본 연구의 핵심인 인가전압의 의존성을 고려하여 소스/드레인의 비대칭적으로 존재하는 기생 저항을 분리추출하였다. 따라서, 이 기술은 제조공정과 레이아웃 디자인의 분석적인 도구가 될 수 있는 중요한 연구적 의미를 가진다. We characterized the electrical parameters in field-effect transistors through optoelectronic measurements and modeling process. Proposed methods are applicable for multiple device regardless of channel material. Especially, Extracted characteristic lifetime and parasitic resistance are important for operation of device and several application. First, we investigate the characteristic lifetime with optoelecronic measurements in InGaAs MISFET. Modeling and Characterization of lifetime is very important for that it can be utilized the performance parameter of multiple device and application. In this work, extracted lifetime are considered of photoconductive and photovoltaic effect. Under the gate/drain bias with photo-illumination, generated electrons and holes are contributed to transient-drain current. Especially, stored substrate-charges by vertical field formed by gate bias corresponds to photovoltaic effect. Thus, we report a optoelectronic technique with photovoltaic effect by time-dependent threshold voltage model. This technique can be used for an analysis-tool for the storage-site of substrate and the characteristic of lifetime. Second, Parasitic resistances are indispensable due to fabrication process and electrical degradation of the operation. Therefore, we report fully current-based method for the extraction of parasitic resistances in MISFET. This method are combined with several integrated extraction technique with channel resistance method (CRM), transfer length method (TLM), dual-sweep combinational transconductance technique (DSCT), open drain method (ODM), and parasitic junction current method (PJCM). This method also considers any possibility for the asymmety and bias-dependency of source and drain resistance. Thus, it is very important for analysis and design of fabrication process and layout.

재조합 단백질의 발현량 조절과 용해도 증가를 위한 유전자 변이체의 고속스크리닝 기술

문병진 忠南大學校 大學院 2021 국내석사

Recombinant proteins often fail to express in soluble forms during heterologous expression, which complicates the downstream processing of the proteins. Fusing a target protein with a solubility-enhancing partner is a common approach to tackle the issue of insoluble expression of recombinant proteins. Many otherwise highly insoluble proteins have been expressed as soluble fusion proteins with a number of fusion partners including maltose binding protein (MBP) and thioredoxin A (TrxA). However, only a few established fusion partners are currently available; thus, the development of novel fusion partners is necessary to enable proficient expression and analysis of rapidly increasing protein-coding sequences. Because the nature of initial codons next to the start codon has a crucial effect on the expression efficiency of the downstream genes, N-terminal fusion partners can also influence the translational efficiency as well as the solubility of the fusion protein. By combining the solubility-enhancing property of fusion partners with initial codon-dependent control of translational efficiency, we envisioned that it might be possible to control the relative expression levels of soluble recombinant proteins via engineering the initial codons of the fusion partners. Since it is not predictable how the changes in initial codons will affect the translational efficiency, selection of initial codons optimal for the targeted expression level requires exhaustive expression studies of different gene constructs. While the conventional cell-based expression methods are not fitted to this task due to the limited throughput, in this study, we conducted large scale cell-free expression screening of solubility-enhancing fusion partners whose initial codons have been randomized. We used MBP and TrxA as model fusion partners, whose initial codons were randomized and screened for targeted expression levels of superfolder green fluorescence protein (sfGFP). We were able to select a number of sets of initial codons that resulted in low, medium, and high expression levels of sfGFP fluorescence. Selected codon sets also provided similar patterns of expression when the fusion partners were added to other proteins, including the large subunit of methanol dehydrogenase (MxaF), and 5-aminovalerate aminotransferase (DavT). These results indicate that the present solubility-enhancing partners can be engineered into translation modulators that enable tuned expression of soluble recombinant proteins.

Physical Parameter-based Current / Capacitance Models of Solution Processed Polymer Thin Film Transistors and Its Application to Circuit Design

장재만 국민대학교 대학원 2015 국내박사

It has become essential to explore systematic and efficient design methods on rapidly growing demands of various circuits and system, which utilizes high- performance based polymer thin film transistor (polymer-TFT). On account of such an increasing demand for high-performance polymer-TFTs, it should be important to establish a specific model of current-voltage (I-V) and capacitance-voltage (C-V) delineating the relationship of the polymer-TFT device characteristics and the condition of a fabrication process as well as the material properties of polymer-based semiconductors. Thus, in this thesis we have established an analytical model for I-V and C-V, which is based on the subgap density-of-states (DOS) of the high-performance polymer semiconducting materials. In a process of the establishment, three p-type polymers-, such as P3HT, PQT-12, and thiophene and thiazole based semiconducting polymers developed in Samsung Advanced Institute of Technology (SAIT) including PQTBTz-C12, P8T2Z-C12 and P(8T2Z-co-6T2Z)-12, were employed to fabricate TFTs along with inkjet-printing process. The mobility of polymer-TFTs was observed to be 0.03~0.72 cm2/Vs which is comparable to those of small-molecule organic TFTs (0.1~1 cm2/Vs). For the physical parameter-based I-V and C-V models, we postulated the subgap DOS as representative physical parameter which can be affected by the polymer material and/or the fabrication process details. In addition, concrete techniques for extracting values of DOS parameters from the measured electrical characteristics of polymer-TFTs were provided. In detail, the donor-like DOS (gD(E)) near the highest occupied molecular orbital (HOMO), i.e., valence band maximum EV, was extracted by using the frequency-dependence of C-V characteristics in the polymer-TFT. The acceptor-like tail DOS (gTA(E)) near the lowest unoccupied molecular orbital (LUMO), i.e., conduction band minimum EC, was also extracted from the characteristics of generation-recombination current. In addition, we extracted the shallow acceptor-like DOS (gSA(E)) near the HOMO by combining a numerical I-V simulation with two techniques, i.e., the multi-frequency C-V and the generation-recombination current spectroscopies. In this way, we extracted the DOS over an entire energy level in the bandgap of polymer semiconductors, and its validity has been thoroughly examined through technology computer-adied design (TCAD) simulation. Typically, the characteristic parameter delineating the subgap-DOS of high-performance p-type P(8T2Z-co-6T2Z)-12 polymer semiconductor was extracted to be NTD=1.01018 cm−3•eV−1, NDD=4.01016 cm−3•eV−1, kTTD=0.04 eV, kTDD=0.28 eV (donor-like subgap-DOS), NTA = 2.01020 [cm−3V−1], NSA = 6.81015 [cm−3V−1], kTTA = 0.095 [eV], kTSA = 0.18 [eV], and ESA= 0.34 [eV] (acceptor-like subgap-DOS). Next, the bias stress-induced instability is the mosta challenging issue for commercializing polymer-TFT based circuits and systems. The extracted subgap-DOS was conjugated to investigate the instability mechanism on the variation of threshold voltage (VT) under the bias stress. In the case of applying the positive bias stress to polymer-TFTs without the passivation layer, the VT was shifted to the positive voltage direction and the gSA(E) was increased with the increase of stress time. On the contrary, no significant change of VT was observed when we applied the positive bias stress to polymer-TFTs with the passivation layer. It was found that the positive bias stress-induced VT shift can be attributed to moving towards into the gate insulator film of OH-, which is attributed to the chemical interaction and migration of the oxygen- and/or water-related species in atmosphere, followed by the increase of gSA(E). As a consequence, it was quantitatively analyzed that the change of VT caused by the charge trapping into gate insulator turned out to be about 85 % and that caused by the DOS change to be nearly 15 %. Whereas the negative bias stress-induced change of VT was the movement to the negative voltage direction regardless of the presence of passivation layer. No change of subgap-DOS was observed under the negative bias stress. It suggested that the negative bias stress-induced instability of polymer-TFTs is dominated by the accumulated holes trapping into the gate insulator. Generalization of the proposed methodology would be realized by in-depth study of various conditions, such as the existence of passivation, bias, temperature, TFT structure, and atmospheric environment. Further work on the instability awareness by quantitatively analyzing the instability mechanism will contribute to the promise of mass production of polymer-TFT based electronics. In addition, to reflect an understanding of the extracted subgap-DOS and instability mechanisms in the physical parameter-based circuit design, we were able to establish the analytical I-V and C-V models by transforming the extracted subgap-DOS into the carrier concentration and charge amount. Their accuracy was verified through incorporating them into the software process improvement and capability determination (SPICE) and comparing the simulated performance with the measured one in the polymer-TFT based inverters. The result indicated the average error rate was within 5 % along with the propagation delay of 15 ms. In the process of establishing models, it was found that the core of describing features of the high mobility polymer-TFTs is including the Schottky contact and the Poole-Frenkel mobility. Universality of the proposed model was confirmed by applying it to cases of a variety of material and/or temperature. Thus, the material-device-circuit co-design platform has been provided based on the solution processed high-performance polymer-TFTs, which would be verified through the actual circuit design. Finally, static random access memory (SRAM) cell was designed and integrated by using the established platform for the sub 고성능 고분자 박막트랜지스터를 기반으로 다양한 회로 및 시스템에 대한 요구가 증가할수록, 체계적이고 효율적인 설계 방법이 요구된다. 따라서, 고분자 반도체의 재료적 특성 및 제조 공정 상의 조건들과 소자특성의 관계를 명확히 기술하면서도, 효율과 정확도를 모두 담보하는 전류-전압 및 커패시턴스-전압 모델 수립이 매우 시급한 과제이다. 이에 본 논문에서는 최신 고성능 고분자 반도체의 밴드갭 내 상태밀도함수를 기반으로 해석적 전류-전압 및 커패시턴스-전압 모델을 수립하였다. 그 과정에서 상용화된 세 가지 종류의 고분자 반도체 재료(P3HT, PQT-12, PQTBTz-C12)와, 최근 개발된 고성능/고신뢰성 고분자 반도체 물질 두 가지 종류(P8T2Z-C12, P(8T2Z-co-6T2Z)-12)를 사용 하였으며, 고분자 재료는 인쇄 공정을 이용하여 박막트랜지스터로 제작되었다. 사용된 고분자 반도체 박막트랜지스터들의 이동도는 0.03 ~ 0.72 [cm2/Vs] 로 단분자 반도체 박막트랜지스터의 이동도 (0.1 ~ 1 [cm2/Vs])에 상응하는 값을 가지는 고이동도 p형 반도체이다. 전류-전압 및 커패시턴스-전압 모델을 수립하기 위하여, 대표적인 물성 변수인 밴드갭 내 상태밀도함수를 상정하였다. 또한, 제작된 소자의 전기적 특성의 측정치로부터 밴드갭 내 상태밀도함수를 추출할 수 있는 방법을 명확히 제시하였다. 구체적으로는, 박막트랜지스터의 커패시턴스-전압 특성의 주파수의존성을 이용하여 밴드갭 내 최고준위 점유 분자궤도 (highest occupied molecular orbital; HOMO) 또는 가전자대역 최상위 레벨 (valence band maximum level; EV) 근처의 도너형 상태밀도함수를 추출하였으며, 생성-재결합 전류특성으로부터 최저준위 비점유 분자궤도 (lowest unoccupied molecular orbital; LUMO) 또는 전도대역 최하위 레벨 (conduction band minimum level; EC) 근방 억셉터형 상태밀도함수를 추출하였다. 또한 수치해석적 전류-전압 모델을 이용하여 최고준위 점유 분자궤도 근처의 억셉터형 상태밀도함수를 추출함으로써 밴드갭 내 전체 에너지 영역에 대한 상태밀도함수를 추출할 수 있었으며, 그 타당성을 TCAD (technology computer-aided design) 및 XRD (x-ray diffraction) 등을 이용하여 검증하였다. 본 논문에서 회로 설계에 사용된 고성능/고신뢰성 고분자 반도체인 P(8T2Z-co-6T2Z)-12 를 기준으로 밴드갭 내 상태밀도함수를 기술하는 특성 파라미터는 다음과 같이 도너형 상태밀도 함수 파라미터 : NTD = 1.01018 [cm−3eV−1], NDD = 4.01016 [cm−3eV−1], kTTD = 0.04 [eV], kTDD = 0.28 [eV], 그리고, 억셉터형 상태밀도함수 파라미터 : NTA = 2.01020 [cm−3eV−1], NSA = 6.81015 [cm−3eV−1], kTTA = 0.095 [eV], kTSA = 0.18 [eV], ESA= 0.34 [eV] 로 추출되었다. 한편, 고분자 박막트랜지스터의 불안정성은 산업화를 이루기 위하여 해결해야하는 가장 큰 이슈 중 하나이다. 추출된 밴드갭 내 상태밀도함수를 바이어스 스트레스 하에서의 임계전압 (threshold voltage)의 변화 원인을 분석하는데 활용하였다. 보호층 (passivation layer) 유/무에 따라, 양 전압/음 전압 스트레스 조건에 따라 불안정성을 분석한 결과, 양 전압 스트레스 하에서 보호층이 없는 소자의 경우 임계전압이 양의 방향으로 크게 증가했고, 측정으로부터 추출한 밴드갭 내 최고준위 비점유 분자점 근처의 억셉터형 상태밀도함수가 스트레스 시간에 따라 증가하는 현상이 확연히 관찰 되었으며, 보호층이 있는 소자의 경우 임계전압의 변화가 관측되지 않았다. 임계전압의 변화 요인을 크게 게이트 절연막 내부에서 일어나는 현상과 활성층에서 일어나는 현상으로 구분하여 분석을 진행한 결과, 보호층이 없는 소자의 경우, 고분자 반도체와 공기중의 산소, 수분 등의 화학 반응으로 생성될 수 있는 OH- 이온이 양 전압에 의해 생성된 전계의 영향으로 게이트 절연막 내부로의 이동하고, 그 일부는 활성층 내부에서 최고준위 비점유 분자점 근처의 억셉터형 상태밀도함수를 증가시켜 임계전압이 양의 방향으로 증가하는 요인으로 작용함을 발견했다. 양 전압 스트레스에 따른 임계전압의 변화중 게이트 절연막 내부에서 일어나는 현상에 의한 변화가 약 85 %, 억셉터형 상태밀도함수의 증가에 의한 변화가 약 15 % 로 나타났다. 반면, 음 전압 스트레스 조건에서는 보호층 유/무에 상관없이 스트레스에 따라 임계전압이 음의 방향으로 증가하고, 밴드갭 내 상태밀도함수의 변화가 거의 일어나지 않았다. 이 결과를 통해, 음 전압 스트레스 조건에서 임계전압 변화의 주된 요인은 축적된 홀이 게이트 절연막으로 트래핑되는 현상임을 규명하였다. 본 논문에서 사용한 불안정성 분석에 대한 방법을 적용하여 보호층 유/무, 전압 조건, 온도, 소자 구조 등 여러 조건에서 좀더 구체적인 실험결과를 바탕으로 불안정성성 메커니즘을 정량적으로 분석하고, 신뢰성 예측에 대한 정량 모델을 세운다면, 고분자 박막트랜지스터의 상용화에 기여할 수 있을 것으로 기대된다. 또한, 수립된 밴드갭 내 상태밀도함수의 추출법과 신뢰성에 대한 이해를 고분자 박막트랜지스터 기반 회로 설계에 반영하기 위해, 추출된 상태밀도함수를 캐리어 농도 및 전하량으로 변형함으로써 해석적 전류-전압 및 커패시턴스-

생태지수를 이용한 굴 양식장의 지속가능성 평가 모델링

김진호 부경대학교 2019 국내박사

Since shellfish culture does not require the addition of artificial food supplement, the growth and productivity of shellfish depend on the supply ability of food, and the filtration ability of shellfish. The supply ability of food is determined by the exchange rate of seawater and the growth rate of phytoplankton. So it determines the amount of shellfish that can be accommodated by the shellfish culture area. In this study, we studied the sustainability of the oyster farm using ecological indices clearance efficiency and food limitation. Clearance efficiency results show that the inner area of Jaran Bay has the lowest environmental sustainability. Food limitation results show that oyster farms are currently unsustainable in four of eight sectors. Based on the results of ecological indices calculations, we simulated the growth and production of the oyster by reducing stocking density and moving to the offshore farm. As a result, we found that the yield was higher in the case of the farm was moved. These results suggest that it is more effective to apply a different reduction rate for different areas rather than using the same reduction rate to the whole area for oyster aquaculture sustainability. It also indicates that moving the farm is more efficient in terms of yield than reducing stocking density.

Extraction Technique for Laterally Distributed Grain Boundary Location in Low Temperature Polycrystalline Silicon Thin-Film Transistors

안제엽 국민대학교 대학원 2015 국내석사

As a convenient novel technique, we employed the characteristics of gate-to-drain and gate-to-source capacitance-voltage to obtain the grain boundary location in polycrystalline silicon (Poly-Si) thin-film transistors (TFTs). When the channel is more conductive by the gate bias, the effective channel length become increased and the change of capacitance is modulated by the trapped charges at the grain boundary trap. This causes irregular decrease or increase in the measured C-V characteristics. Therefore, it can be said that the grain boundary location can be detected by the correlation between the effective channel length and the grain boundary location. It should be noted that the location of extracted grain boundary is consistent with the grain boundary position, which was determined through the simulation. Also, two grain boundaries in the channel are extracted by using the gate-to-source and the gate-to-drain C-V characteristics. It was observed that the extracted location of the grain boundary from the drain edge of channel turned out to be correct. Hence, it should be considered that our proposed technique is substantially effective for developing the large and robust grain poly-Si TFTs as well as for the characterizing the device reliability.

Sub-Bandgap Photonic C-V Technique for Extraction of Intrinsic Density-of-States over the Bandgap in AOS TFTs with Bias-Dependent Channel Conduction Factor

최현준 국민대학교 대학원 2014 국내석사

We proposed a novel single-scan monochromatic photonic capacitance-voltage (MPCV) technique for simultaneous extraction of both valence- and conduction-band tail states over the bandgap (EV<E<EC) based on only experimental data by using a sub-bandgap (hν=Eph≈2.6eV<Eg) optical source in amorphous Indium-Gallium-Zinc-Oxide (a-IGZO) thin film transistors (TFTs). In the proposed technique, we applied two different equivalent circuit models for the photo-responsive carriers excited from donor- and acceptor-like states (gD(E) and gA(E)) as the subgap density-of-states (DOS) under depletion (VG<VFB) region and accumulation (VG>VFB) region. We also proposed a channel conduction factor (α(VG)) for empirical modeling of the gate-bias dependent effective channel length (Leff(VG)) for consistent capacitance-voltage (C-V) characterization of the intrinsic subgap DOS over the bandgap with the MPCV technique. We define Leff(VG) through the product of α(VG) and the metallurgical channel length. In the characterization, in addition to Leff(VG), we also considered the capacitance caused by the overlapped region of the source (Lov,S) and drain (Lov,D) with the gate. We confirm that the gate bias-dependent channel conductivity effect is significant in the subgap DOS close to the valence band tail due to the low conductivity of the channel.

Fully I-V-Based Technique for Extraction of Subgap Density-of-States and Separated Parasitic Resistances in Amorphous Thin Film Transistors

전성우 국민대학교 대학원 2014 국내석사

We report a unified subthreshold coupling factor technique for a simultaneous extraction of the surface potential (ψS) and the subgap density-of-states (DOS: g(E)) over the bandgap in amorphous oxide semiconductor (AOS) thin film transistors (TFTs). It is fully based on the experimental gate bias-dependent coupling factor (m(VGS)) under subthreshold region. Through the proposed technique only with current-voltage data under subthreshold operation, a unified extraction of the subgap DOS with a consistent mapping of the gate bias (VGS) to the subgap energy is obtained. Applying to amorphous IGZO TFTs, g(E) was obtained to be a superposition of two exponential functions with NTA [eV-1∙cm-3] and kTTA [eV] for the tail states while NDA [eV-1∙cm-3] and kTDA [eV] for the deep states. Additionally, for separate extraction of the parasitic source and drain resistances (RS and RD) in AOS TFTs, we report a combinational transconductance technique by combining forward and reverse transfer characteristics. In the proposed technique, VGS-dependent total resistance (RTOT(VGS)) and degradation of the transconductance due to the parasitic resistance at the source terminal during the bi-directional forward and reverse characterization are employed. Applying the proposed technique to AOS TFTs with various combinations of channel length (L) and width (W), we successfully separated RS and RD. A model for the W- and L- dependence of the extracted parasitic resistance is also provided. Through a unified subthreshold coupling factor technique and combinational transconductance technique with only current-voltage (I-V) characteristics, we extracted the subgap DOS and the separated RS and RD by employing a subthreshold coupling factor and transconductance, respectively. Though the proposed fully I-V-based technique, simultaneous characterization of both the subgap DOS and the parasitic resistances (RS and RD) were accomplished from only transfer curve (IDS-VGS) characteristics in AOS TFTs. 본 논문에서는 산화물 박막 트랜지스터의 전류-전압(I-V) 특성을 이용하여 표면 전위와 밴드갭 내 상태 밀도 및 기생저항을 동시에 추출하는 방법을 제안했다. 먼저, 밴드갭 내 상태밀도가 반영되는 문턱전압 이전 영역(subthreshold drain current)의 커플링 계수(coupling factor)에 기초하여 측정된 전류만으로 표면 전위 및 밴드갭 내 상태밀도를 추출하는 방법을 제안하였다. 이는 밴드갭 내 에너지의 함수로 분포하는 상태밀도를 단 하나의 I-V 데이터로부터 추출하기 때문에 광 또는 열 등의 영향에 의하여 소자의 전기적 성질이 변하는 문제로 인한 신뢰성이 떨어지는 문제점을 개선한다. 그리고 수학적인 계산의 반복 없이 빠르고 정확하게 산화물 박막 트랜지스터의 밴드갭 내 상태밀도를 추출할 수 있다. 또한, saturation 및 linear 영역의 I-V 데이터를 이용하여 산화물 박막 트랜지스터의 소스와 드레인 기생저항을 분리 추출하는 방법을 제안했다. 이 때, 소스와 드레인 역할을 서로 바꿔가며 측정한 I-V 데이터에서 기생저항에 의해 감소하는 saturation 영역의 transconductances와 linear 영역에서 게이트 전압에 의존하는 직렬 전체 저항을 통하여 소스 및 드레인 기생저항을 각각 추출했다. 이 방법은 독립적인 두 측정인 capacitance-voltage(C-V)와 I-V 특성을 동시에 이용하지 않고 I-V 특성만을 고려하기 때문에 보다 정확한 기생저항을 추출하는 장점이 있다. 따라서 본 연구에서 제안한 방법은 독립적이거나 소자에 스트레스를 주는 측정 없이 동일한 I-V 특성만을 이용하여 산화물 박막 트랜지스터의 표면 전위와 밴드갭 내 상태밀도 그리고 기생저항을 추출할 수 있다. 또한, 게이트 면적이 작은 소자에서 변동 현상이 있는 C-V 특성과 달리 I-V 특성은 작은 게이트 면적을 가지는 소자에서도 안정적으로 측정되는 장점을 가진다. 이러한 장점들을 통해 제안된 방법이 장차 산화물 박막 트랜지스터의 물질, 공정, 구조 등을 최적화하는데 매우 유용한 방법론이 될 것이라 기대한다.