Toxic gas sensors using thin film transistor platform at low temperature

Thesis (S.M.)--Massachusetts Institute of Technology, Dept. of Materials Science and Engineering, 2009.Includes bibliographical references (leaves [71-73]).Semiconducting metal-oxides such as SnO₂, TiO₂, ZnO and WO₃ are commonly used for gas sensing in the form of thin film resistors (TFRs) given their high sensitivity to many vapor species, simple construction and capability for miniaturization. Furthermore, they are generally more stable than polymer-based gas sensors. However, unlike polymers, metal oxide gas sensors must typically be operated between 200-400°C to insure rapid kinetics. Another problem impacting TFR performance and reproducibility is related to poorly understood substrate-semiconductor film interactions. Space charges at this heterojunction are believed to influence chemisorption on the semiconductor-gas interface, but unfortunately, in an unpredictable manner. In this study, the feasibility of employing illumination and the thin film transistor (TFT) platform as a means of reducing operation temperature was investigated on ZnO based TFTs for gas sensors applications. Response to NO₂ is observed at significantly reduced temperature. Photoconductivity measurements, performed as a function of temperature on ZnO based TFRs, indicate that this results in a photon-induced desorption process. Also, transient changes in TFT channel conductance and transistor threshold voltage are obtained with application of gate bias, suggesting that TFTs offer additional control over chemisorption at the semiconductor-gas interface.by Yoonsil Jin.S.M

전류 센싱 피드백 시스템을 이용한 고안정성 산화물 TFT 쉬프트 레지스터의 설계 및 제작

학위논문 (박사)-- 서울대학교 대학원 : 전기·컴퓨터공학부, 2017. 2. 정덕균.Integration of shift registers on the glass panel allows the display to be thinner, lighter, and cheaper to produce, thanks to the reduction of the number of ICs for scanning horizontal lines. Circuits of the shift register employing n-type thin film transistors (TFTs), such as hydrogenated amorphous silicon (a-Si:H) and oxide TFTs, have been reported. Recently, oxide TFTs attract much attention due to their high mobility (5~10 cm2/V∙s) compared with that of a-Si:H TFT (0.8cm2/V∙s). However, oxide TFTs often suffer from severe degradation of the threshold voltage (VTH) against the temperature and electrical stress. In this paper, in order to compensate the instability of oxide TFTs in the shift register, an oxide TFT with double gates, which can control VTH by varying the top gate bias (VTG) is adopted. The top gate of the double-gate TFT can be fabricated in the same process for the pixel IZO (Indium Zinc Oxide) so that an additional process only for the top gate is not required. Adequate VTG is provided timely, adaptively to the gate of the oxide TFTs to stabilize the threshold voltage. The fabrication result shows that the proposed shift register using VTG set at an adapted value become stable at 100℃ whereas the conventional one is mal-functioning. The optimum VTG varies from product to product and changes continuously over the lifetime of the display. Therefore, the feedback driving system suitable for the proposed shift register is required to search the optimum VTG. The system has two main functionsthe first is to sense the current of shift register and the second is the searching algorithm for finding the optimum VTG. When the transistors are degraded by an external stress, the current of the whole shift registers is changed. The information about the VTH degradation in the shift register can be gathered via current sensing circuit. The sensed current is integrated to generate the output and is forwarded to an ADC. The binary-converted current of shift register is processed by the proposed algorithm in the digital domain for obtaining an optimum VTG and then the result is converted back to analog to generate VTG. The IC implementing such functions is fabricated in a 0.18 μm BCDMOS process. When the shift register current is measured on the conventional system with increasing temperature up to 80℃, it is increased to more than 10 times than that at the room temperature. However, the proposed feedback system keeps a highly stable (<13%) current level of shift register up to 80℃ with an optimized VTG.Abstracts i Table of Contents iii List of Tables v List of Figures vi Chapter 1 Introduction 1 1.1 Background 2 1.2 Outline 7 Chapter 2 Review of oxide-based TFT device and N-type TFT circuit design 8 2.1 Overview 9 2.1.1 Characteristics of Oxide TFT 9 2.2 Oxide-based TFT 14 2.2.1 Electrical characteristics of oxide-based TFT 14 2.2.2 Stability of oxide-based TFT 18 2.3 NMOS driving circuit 24 2.3.1 Bootstrapping driving circuit 24 2.3.2 Shift register with n-type TFT 28 Chapter 3 Proposed Oxide TFT Shift Register 37 3.1 Overview 38 3.2 Characteristic of Double Gate TFT 39 3.3 Design of New shift register 46 3.3.1 Simulation Result of Conventional shift register 46 3.3.2 New shift register using Double Gate TFT 51 3.3.3 Simulation Modeling of Double Gate TFT 58 3.3.4 Simulation and Experimental Result 61 Chapter 4 Real Time Current-Sensing Feedback Compensation System 71 4.1 Overview 72 4.2 System Architecture 74 4.3 Circuit Design 77 4.3.1 Current Sensing Block 77 4.3.2 ADC/DAC Block 85 4.4 Optimum Point Searching Algorithm 100 4.5 System Verification 106 Chapter 5 Summary 116 Appendix A SPICE models 118 Bibliography 120Docto

Optoelectronic Properties and Applications of 3-D Hybrid a-Si:H/ZnO Nanowire Structures

This Ph.D. dissertation presents the study and development of optoelectronic properties of 3-D hybrid zinc oxide nanowire (ZnO NW) and hydrogenated amorphous silicon (a-Si:H) core-shell structures and a demonstration of their application in infrared photodiodes. In addition, the demonstration of infrared photoconductors using the 3-D a-Si:H/ZnO NW structure, which are then integrated onto conventional back channel etched (BCE) a-Si:H thin film transistors (TFTs) for potential large-area sensor applications is also presented. A hybrid 3-D core-shell structure formed using ZnO NW cores and conformally coated a-Si:H thin film shell was found to be infrared sensitive for wavelengths up to 2.5 µm wavelength. Scattering and multiple reflection enabled by the 3-D morphology were found to enhance the effective thickness of the a-Si:H shell by ~3 orders of magnitude. With the enhanced effective thickness, defects within the a-Si:H material were associated with enabling infrared absorption, achieving up to 73% infrared absorption at 2.3 µm infrared wavelength for 500 nm coated a-Si:H film on 7 µm long ZnO NWs having a nanowire density of 4.3 × 107 NW/cm2. Comprehensive materials and device characteristics were studied to show a defect mediated infrared absorption process in the 3-D a-Si:H/ZnO NW material system. 3-D infrared photoconductors were fabricated afterwards at a temperature of ≤ 150°C using the infrared sensitive 3-D a-Si:H/ZnO NW hybrid material system. An intentional ‘NW gap’ was created between the edge of the NW array and the contacts of the infrared photoconductors to minimize parasitic conduction from conductive and connected NWs thereby reducing the dark current of the 3-D photoconductor. An ON/OFF ratio of 3.2 × 102 was achieved for 1 µm thick a-Si:H shell coating on 2.7 µm long ZnO NWs with a nanowire density of 3.9 × 108 NW/cm2 using 1.55 µm LED illumination. As an alternative infrared photodetector, 3-D infrared photodiodes were also fabricated using similar process conditions. Dark current as low as 1.6 × 10-9 A/cm2 was achieved for a diode with NW length vs a-Si:H thickness ratio of 1.5× and NW density of 6.1 × 107 NW/cm2 giving an infrared signal to noise ratio of 2.5 × 102 with 1.55 µm LED irradiation. The factors that influence the dark currents were studied and several optimizations were implemented. The top contact was optimized by replacing aluminum doped zinc oxide (AZO) top conducting oxide (TCO) with thinned-down, conductive gallium indium zinc oxide (GIZO) and p+ doped a-Si:H to minimize the window layer absorption and enhance the its infrared transmission. 3-D infrared photoconductors were also integrated onto a-Si:H BCE TFT at a process temperature ≤ 150°C. The process development and the effects of both the structure and the integration process flow were evaluated. A 3× signal to noise ratio due to infrared irradiation using a heat lamp was obtained for the integrated device with a photoconductor section that contains only 20% 3-D a-Si:H/ZnO NW structure

Amorphous In-Ga-Zn-O Thin Film Transistor for Future Optoelectronics.

After initial report of its potential use for flexible/large area electronics, amorphous In-Ga-Zn-O (a-IGZO) is now emerging worldwide as a new semiconductor for next gen-eration thin-film transistor (TFT) flat panel displays and imagers. This dissertation work examines in detail the basic properties and physics of the a-IGZO TFTs, including the photofield-effect, numerical simulations, electrical instability and noise characteristics. Our a-IGZO TFTs have following electrical performance: field-effect mobility (μeff) of 7~12.3 cm2V-1s-1, threshold voltage of 1~3V, subthreshold swing of 130~420mV/decade and on/off current ratio over 108. Aluminum and titanium are both suitable for source/drain (S/D) electrodes with the contact resistivity (rC) lower than 10-3Ω-cm2. The active layer thickness was also found to have a major impact on S/D series resistance. To accurately model the TFT current-voltage (I/V) properties, a gate-to-source voltage dependent μeff model is proposed. Light wavelength and intensity dependent photo-responses were studied. The a-IGZO TFT is stable under visible light illumination (460~660nm). TFT off-state drain current starts to increase when the photon energy is higher than its band-gap (~3.05eV); and we observed a high UV-photocurrent conversion efficiency. In addition, the a-IGZO mid-gap density-of-states (DOS) was extracted and is more than an order of magnitude lower than the values of hydrogenated amorphous silicon (a-Si:H). The DOS model for a-IGZO was then developed. In this model, the donor-like states are proposed to be associated with oxygen vacancy in a-IGZO. We showed through numerical simulation that the a-IGZO TFT has a very sharp conduction band-tail slope (Ea=13meV). The impacts of rC and DOS on TFT electrical properties were also studied. Bias-temperature-stress (BTS) induced electrical instability was investigated. Our re-sults suggest that the observed shifts in TFT I/V curves are primarily due to channel charge injection/trapping. The validity of using stretched-exponential model in simulating the time, voltage and temperature dependences of BTS data was demonstrated for a-IGZO TFTs. Finally, the TFT low frequency noise properties were examined. The 1/f noise is the dominant source in a-IGZO TFT and can be modeled by Hooge mobility fluctuation theory. The a-IGZO has a lower Hooge’s parameter than a-Si:H and may be better used in imaging applications.Ph.D.Electrical EngineeringUniversity of Michigan, Horace H. Rackham School of Graduate Studieshttp://deepblue.lib.umich.edu/bitstream/2027.42/75856/1/tcfung_1.pd

Miniaturized Silicon Photodetectors

Silicon (Si) technologies provide an excellent platform for the design of microsystems where photonic and microelectronic functionalities are monolithically integrated on the same substrate. In recent years, a variety of passive and active Si photonic devices have been developed, and among them, photodetectors have attracted particular interest from the scientific community. Si photodiodes are typically designed to operate at visible wavelengths, but, unfortunately, their employment in the infrared (IR) range is limited due to the neglectable Si absorption over 1100 nm, even though the use of germanium (Ge) grown on Si has historically allowed operations to be extended up to 1550 nm. In recent years, significant progress has been achieved both by improving the performance of Si-based photodetectors in the visible range and by extending their operation to infrared wavelengths. Near-infrared (NIR) SiGe photodetectors have been demonstrated to have a “zero change” CMOS process flow, while the investigation of new effects and structures has shown that an all-Si approach could be a viable option to construct devices comparable with Ge technology. In addition, the capability to integrate new emerging 2D and 3D materials with Si, together with the capability of manufacturing devices at the nanometric scale, has led to the development of new device families with unexpected performance. Accordingly, this Special Issue of Micromachines seeks to showcase research papers, short communications, and review articles that show the most recent advances in the field of silicon photodetectors and their respective applications

Interpretation and Regulation of Electronic Defects in IGZO TFTs Through Materials & Processes

The recent rise in the market for consumer electronics has fueled extensive research in the field of display. Thin-Film Transistors (TFTs) are used as active matrix switching devices for flat panel displays such as LCD and OLED. The following investigation involves an amorphous metal-oxide semiconductor that has the potential for improved performance over current technology, while maintaining high manufacturability. Indium-Gallium-Zinc-Oxide (IGZO) is a semiconductor material which is at the onset of commercialization. The low-temperature large-area deposition compatibility of IGZO makes it an attractive technology from a manufacturing standpoint, with an electron mobility that is 10 times higher than current amorphous silicon technology. The stability of IGZO TFTs continues to be a challenge due to the presence of defect states and problems associated with interface passivation. The goal of this dissertation is to further the understanding of the role of defect states in IGZO, and investigate materials and processes needed to regulate defects to the level at which the associated influence on device operation is controlled. The relationships between processes associated with IGZO TFT operation including IGZO sputter deposition, annealing conditions and back-channel passivation are established through process experimentation, materials analysis, electrical characterization, and modeling of electronic properties and transistor behavior. Each of these components has been essential in formulating and testing several hypotheses on the mechanisms involved, and directing efforts towards achieving the goal. Key accomplishments and quantified results are summarized as follows: • XPS analysis identified differences in oxygen vacancies in samples before and after oxidizing ambient annealing at 400 °C, showing a drop in relative integrated area of the O 1s peak from 32% to 19%, which experimentally translates to over a thousand fold decrease in the channel free electron concentration. • Transport behavior at cryogenic temperatures identified variable range hopping as the electron transport mechanism at temperature below 130 K, whereas at temperature greater than 130 K, the current vs temperature response followed an Arrhenius relationship consistent with extended state transport. • Refinement of an IGZO material model for TCAD simulation, which consists of oxygen vacancy donors providing an integrated space charge concentration NVO = +5e15 cm-3, and acceptor-like band-tail states with a total integrated ionized concentration of NTA = -2e18 cm-3. An intrinsic electron mobility was established to be Un = 12.7 cm2/V∙s. • A SPICE-compatible 2D on-state operation model for IGZO TFTs has been developed which includes the integration of drain-impressed deionization of band-tail states and results in a 2D modification of free channel charge. The model provides an exceptional match to measured data and TCAD simulation, with model parameters for channel mobility (Uch = 12 cm2/V∙s) and threshold voltage (VT = 0.14 V) having a close match to TCAD analogs. • TCAD material and device models for bottom-gate and double-gate TFT configurations have been developed which depict the role of defect states on device operation, as well as provide insight and support of a presented hypothesis on DIBL like device behavior associated with back-channel interface trap inhomogeneity. This phenomenon has been named Trap Associated Barrier Lowering (TABL). • A process integration scheme has been developed that includes IGZO back-channel passivation with PECVD SiO2, furnace annealing in O2 at 400 °C, and a thin capping layer of alumina deposited via atomic layer deposition. This process supports device stability when subjected to negative and positive bias stress conditions, and thermal stability up to 140 °C. It also enables TFT operation at short channel lengths (Leff ~ 3 µm) with steep subthreshold characteristics (SS ~ 120 mV/dec). The details of these contributions in the interpretation and regulation of electronic defect states in IGZO TFTs is presented, along with the support of device characteristics that are among the best reported in the literature. Additional material on a complementary technology which utilizes flash-lamp annealing of amorphous silicon will also be described. Flash-Lamp Annealed Polycrystalline Silicon (FLAPS) has realized n-channel and p-channel TFTs with promising results, and may provide an option for future applications with the highest performance demands. IGZO is rapidly emerging as the candidate to replace a-Si:H and address the performance needs of display products produced by large panel manufacturing

Fabrication and characterization of thin-film transistors with organic heterostructure of pentacene and PTCDI-C13

This final thesis aims to advance research on the use of organic materials to produce devices such as OTFTs (Organic Thin-Film Transistors) or solar cells. Therefore, several OTFTs were performed to investigate the behavior of two organic semiconductors: pentacene and N,N′-ditridecyl-3,4,9,10-perylenetetracarboxylic diimide (commonly called PTCDI-C13). They were used alone or together to form some OTFT samples (simple and with heterostructure). Good electrical characteristics were found by measuring. Regarding multilayer OTFTs, ambipolar behavior were found, what is very interesting. The innovation of this project lies in the fact that all the manufacturing steps were performed at low temperature, including the dielectric (around 150°C) composed of Al2O3. It should be noted that ambipolar devices have never been carried out at less temperature. Finally, the samples were submitted to the lighting of different LEDs, chosen according to the wavelengths absorbed by each semiconductor. The principal aim of this final thesis part is to see if it can be interesting to use these materials in solar cells and should be seen as an introduction for further study