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Analog Signal Processing Circuits in Organic Transistor Technology a Dissertation Submitted to the Department of Electrical Engi ANALOG SIGNAL PROCESSING CIRCUITS IN ORGANIC TRANSISTOR TECHNOLOGY A DISSERTATION SUBMITTED TO THE DEPARTMENT OF ELECTRICAL ENGINEERING AND THE COMMITTEE ON GRADUATE STUDIES OF STANFORD UNIVERSITY IN PARTIAL FULFILLMENT OF THE REQUIREMENTS FOR THE DEGREE OF DOCTOR OF PHILOSOPHY Wei Xiong October 2010 © 2011 by Wei Xiong. All Rights Reserved. Re-distributed by Stanford University under license with the author. This work is licensed under a Creative Commons Attribution- Noncommercial 3.0 United States License. http://creativecommons.org/licenses/by-nc/3.0/us/ This dissertation is online at: http://purl.stanford.edu/fk938sr9136 ii I certify that I have read this dissertation and that, in my opinion, it is fully adequate in scope and quality as a dissertation for the degree of Doctor of Philosophy. Boris Murmann, Primary Adviser I certify that I have read this dissertation and that, in my opinion, it is fully adequate in scope and quality as a dissertation for the degree of Doctor of Philosophy. Zhenan Bao I certify that I have read this dissertation and that, in my opinion, it is fully adequate in scope and quality as a dissertation for the degree of Doctor of Philosophy. Robert Dutton Approved for the Stanford University Committee on Graduate Studies. Patricia J. Gumport, Vice Provost Graduate Education This signature page was generated electronically upon submission of this dissertation in electronic format. An original signed hard copy of the signature page is on file in University Archives. iii Abstract Low-voltage organic thin-film transistors offer potential for many novel applications. Because organic transistors can be fabricated near room temperature, they allow integrated circuits to be made on flexible plastic substrates. This physical flexibility allows organic transistors to integrate with bendable organic displays, polymeric muscles, and conformal sensors. Additionally, organic semiconductors are inherently sensitive to specific molecules, making organic transistors naturally suited for chemical and biological sensors. In all these applications, data converters are the essential link between the digital processors and the analog media. However, because of the inherent non-uniformities in organic processing, organic analog circuits suffer from large variations that lead to inaccurate and unreliable data conversion. Dielectric leakage and large parasitic capacitances further limit the available design space. This dissertation describes the sources of these process handicaps and offers design techniques to counter them. By applying these techniques, this research demonstrated the world's first organic-transistor digital-to-analog converter and the first organic- transistor analog-to-digital converter. Both data converters operate at 3 V, 100 Hz and resolve 6 bits. Similar design methodology can be utilized in designing other organic- transistor based analog signal processing circuits. iv Acknowledgements First, I would like to thank Professor Boris Murmann for advising my doctoral research. I knew nothing about organic electronics, but Boris’s vision convinced me to enter this field. Through his guidance, I was able to build truly novel devices. I very much appreciate his generous support – never did I worry about lack of resources, and cherish the freedom he gave – to try things, to ruminate, and to twiddle thumbs every now and then. Most of all, I thank him for an enjoyable Ph.D. experience. I thank my co-advisor, Professor Zhenan Bao, for being a constant source of knowledge and support. Her lab had been instrumental in helping me finding my way around organic transistors. She offered valuable advices on both organic electronics and on the general science during the long shared drives to conferences, for which I am greatly thankful. I thank Professor Alberto Salleo for his help from Day 1 to Day 1800. He was always available whenever I needed enlightenment on organic materials (or Italy). On the first day, he helped me understand the basic polymer conduction; and on the 1800th day, he advised me on career opportunities. In between, he sent me papers, offered his lab equipment, and showed me how to properly measure organic transistors. I thank him for all the help and for chairing my oral defense committee. v I also thank Professor Bob Dutton for serving on my oral and reading committees, and for showing me the genesis of some of the semiconductor theories, and how they can apply to the organic transistors. I am tremendously grateful to my collaborators, Drs. Hagen Klauk and Ute Zschieschang of the Max Planck Institute for Solid State Research in Stuttgart, Germany, for the fabrication of the organic transistors and circuits. Their work on developing high-quality organic transistors was instrumental in achieving the circuit performance cited in this dissertation. Especially, I thank Hagen for his experience and optimism, and Ute for her dedication in making sure every transistor worked as best as it could. In addition to fabrication at the Max Planck Institute, much of the earliest fabrication was performed and advised by Dr. Jim Kruger of Stanford Nano Fab. Almost everything I knew about silicon fabrication I learned from Jim. These knowledge would be constructive in developing the organic circuit processes. I’m very grateful for Jim’s help. I would like to thank the current and former members of Murmann Group and Bao Group for useful discussions. In particular, I thank Alex Guo, who helped with much of FPGA programming, Yoonyoung Chung, Echere Iroaga, Jason Hu, Clay Daigle, Jim Salvia, Justin Kim, Pedram Lajevardi, Manar El-Chammas, Noam Dolev, Alireza Dastgheib, Prastoo Nikaeen, Colin Reese, and Tony Sokolov. On the personal side, I would like to thank my parents – for everything. They always strived to achieve a better life for our family. It is through their hard work and vi dedication that I am in the position to complete this Ph.D. I will always remember their love, care and inspiration. I also thank my brother for his love and support (and free Google food). He took care many of our family tasks so I could focus on my career and school. I could not ask for a better sibling. I want to thank my grandparents – who took turns rearing me when my parents were studying abroad. It’s often said that the Ph.D. is an exercise in perseverance. Whatever perseverance I had, I inherited from them. Finally, my deepest gratitude goes to my beautiful wife Cheryl. She put up with some truly atrocious schedules – dinner at 10pm, not home until 3am; on more than a few occasions our dates ended in the lab. She may not care for the difference between a transistor and a capacitor, but her hugs and smiles made all the difference in the world when things were going badly – and organic transistors went badly often. I thank her being there for me, always. vii Table of Contents Abstract ........................................................................................................................ iv Acknowledgements ....................................................................................................... v Table of Contents ....................................................................................................... viii List of Tables ................................................................................................................. x List of Figures .............................................................................................................. xi 1. Introduction .............................................................................................................. 1 1.1. Motivation ................................................................................................................................... 1 1.2. Background on Organic Semiconductors and Organic Transistors ...................................... 4 1.3. Research Goals ........................................................................................................................... 9 1.4. Organization ............................................................................................................................. 13 2. Organic Transistor Fabrication ............................................................................ 14 2.1. Fabrication at the Stanford Nano Fab ................................................................................... 14 2.2. Fabrication at the Max Planck Institute ................................................................................ 19 2.3. Special Circuit Layout Considerations................................................................................... 25 3. Effects of Organic Device Non-idealities on Analog Circuits ............................. 28 3.1. Device Mismatch ...................................................................................................................... 28 3.2. Large Parasitic Capacitances .................................................................................................. 33 3.3. Dielectric Leakage .................................................................................................................... 34 3.4. Modeling Inaccuracies ............................................................................................................. 37 4. Design of Organic DAC and ADC ........................................................................ 40 4.1. Design Limitations due to the OTFT Process .......................................................................
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