Active Matrix Organic Light-Emitting Displays: Novel Amorphous Silicon Thin-Film Transistors and Pixel Electrode Circuits by Hojin Lee A dissertation submitted in partial fulfillment of the requirements for the degree of Doctor of Philosophy (Electrical Engineering) in The University of Michigan 2008 Doctoral Committee: Professor Jerzy Kanicki, Chair Professor Stephen R. Forrest Professor Khalil Najafi Assistant Professor Jinsang Kim _____Hojin Lee____ © 2008 All Rights Reserved This thesis is dedicated to my beautiful wife, Boyoung, my lovely son, Ian, and to my supporting parents ii ACKNOLEDGEMENTS First and foremost I would like to thank Professor Jerzy Kanicki, my research advisor and doctoral committee chair, for his unswerving support. From my first year in Michigan to the end of my course toward Ph.D., he has been an excellent guardian throughout my time on this interesting and exciting project in his research group. I am sincerely grateful for his encouragement and wisdom that steered me to grow as a scientist and expert from one humble student. I also sincerely thank to my doctoral committee members: Prof. Khalil Najafi, Prof. Stephen Forrest, and Prof. Jinsang Kim for their time and effort in helping me to complete my thesis work. I cannot express the depth of my gratitude to Dr. Juhn S. Yoo and Dr. In-Je Chung at LG Philips LCD Research and Development Center in Korea. A large portion of the success of my thesis work is due to their world best a-Si:H TFT fabrication technologies. Special thanks are in order for Dr. Juhn S. Yoo for his helpfulness and kind advice in pixel circuit simulation and modeling. I am also grateful to Dr. Yen-Chung Lin at Sitronix Corp. in Taiwan for his support in the pixel circuit fabrication and useful iii comments on our experimental results. I also would like to thank my current and previous group members: Aaron Johnson, Alex Kuo, Charlene Chen, Geonwook Yoo, Richard Fung, Patrick Chuang, Dr. Michael Hamilton, Dr. Patrick Shea, and Dr. Shujen Lee. It would be nearly impossible for me to reach this point without their help, support, and encouragement. Especially I would like to thank Aaron Johnson and Alex Kuo for their 4-year long friendship and useful discussions on my researches as well as their voluntary effort in helping me to understand young American culture and language. Finally, I would like to extend my sincere gratitude to all my family and friends for their love, belief, and support. Especially, I can not imagine this work would be possible without my lovely wife, Boyoung. With her unconditional love and support, I could get through all difficulties during my Ph.D. And my cute son Ian; he always gives me wonderful smile to relieve all my worrying. This thesis is dedicated to them. iv TABLE OF CONTENTS DEDICATION..............................................................................................................................ii ACKNOLEDGEMENTS ............................................................................................................iii LIST OF TABLES.....................................................................................................................viii LIST OF FIGURES .....................................................................................................................ix CHAPTER 1. INTRODUCTION......................................................................................................................1 1.1 MOTIVATION AND OVERVIEW .................................................................................................1 1.2 ORGANIZATION OF DISSERTATION ........................................................................................18 2. CURRENT-SCALING A-SI:H TFT PIXEL ELECTRODE CIRCUIT FOR AM-OLEDS: ELECTRICAL PROPERTIES AND STABILITY ...................................................................24 2.1 INTRODUCTION .....................................................................................................................24 2.2 FABRICATION OF PIXEL ELECTRODE CIRCUITS.....................................................................25 2.3 OPERATION OF THE FABRICATED CURRENT-SCALING PIXEL ELECTRODE CIRCUITS ...........27 2.4 PIXEL ELECTRODE CIRCUIT MEASUREMENT DETAILS .........................................................29 2.5 ELECTRICAL PROPERTIES OF THE CURRENT-SCALING PIXEL ELECTRODE CIRCUIT ............32 2.6 COMPARISON WITH OTHER PIXEL ELECTRODE CIRCUITS ....................................................36 2.7 ELECTRICAL STABILITY OF THE FABRICATED PIXEL ELECTRODE CIRCUIT ..........................37 2.7.1 a-Si:H TFT Stability Measurement ................................................................................37 2.7.2 Pixel Electrode Circuit Stability Measurement..............................................................40 2.8 CONCLUSION ........................................................................................................................44 3. NOVEL CURRENT-SCALING CURRENT-MIRROR A-SI:H TFT PIXEL ELECTRODE CIRCUIT WITH CASCADE CAPACITOR FOR AM-OLEDS ...................48 3.1 INTRODUCTION .....................................................................................................................48 3.2 OPERATION OF THE PROPOSED CURRENT-SCALING PIXEL ELECTRODE CIRCUIT ................49 3.3 DEVICE PARAMETER EXTRACTION.......................................................................................51 3.4 SIMULATED ELECTRICAL PROPERTIES OF THE PROPOSED PIXEL ELECTRODE CIRCUIT.......53 v 3.5 COMPARISON WITH OTHER PIXEL ELECTRODE CIRCUITS ....................................................58 3.6 INFLUENCE OF THRESHOLD VOLTAGE VARIATION ...............................................................59 3.7 INFLUENCE OF DEVICE SPATIAL MISMATCH AND TEMPERATURE.........................................62 3.8 POSSIBLE AM-OLED DISPLAY BY THE PROPOSED PIXEL ELECTRODE CIRCUIT..................65 3.9 CONCLUSION ........................................................................................................................67 4. ASYMMETRIC ELECTRICAL PROPERTIES OF CORBINO A-SI:H TFTS AND THEIR APPLICATIONS TO FLAT PANEL DISPLAYS........................................................70 4.1 INTRODUCTION .....................................................................................................................70 4.2 CORBINO A-SI:H TFT FABRICATION .....................................................................................72 4.3 EXPERIMENTAL RESULTS......................................................................................................74 4.4 DISCUSSIONS OF A-SI:H TFT GEOMETRY EFFECT................................................................76 4.5 DEVICE PARAMETERS EXTRACTION .....................................................................................84 4.6 POSSIBLE APPLICATIONS OF CORBINO A-SI:H TFTS ............................................................90 4.7 CONCLUSIONS.......................................................................................................................92 5. HEXAGONAL A-SI:H TFTS, A NEW ADVANCED TECHNOLOGY FOR FLAT PANEL DISPLAYS......................................................................................................................95 5.1 INTRODUCTION .....................................................................................................................95 5.2 MULTIPLE A-SI:H HEXAGONAL TFT STRUCTURE AND FABRICATION ..................................96 5.3 EXPERIMENTAL RESULTS AND DISCUSSIONS......................................................................100 5.3.1 Electrical Characteristics of Multiple a-Si:H HEX-TFTs............................................100 5.3.2 Electrical Stabilities and Pixel Area Occupation of the Multiple a-Si:H HEX-TFTs ...111 5.4 CONCLUSIONS..................................................................................................................... 118 6. DYNAMIC MEASUREMENTS OF NORMAL AND CORBINO A-SI:H TFTS FOR AM- OLEDS........................................................................................................................................121 6.1 INTRODUCTION ...................................................................................................................121 6.2 A-SI:H TFTS FABRICATION.............................................................................................123 6.2.1 Top-Gate a-Si:H TFTs..................................................................................................123 6.2.2 Bottom-Gate a-Si:H TFTs............................................................................................124 6.3 PIXEL OPRATION, EXPERIMENTAL SET-UP, AND MEASUREMENTS ...................128 6.4 EXPERIMENTAL RESULTS ............................................................................................130 6.5 DISCUSSIONS ..................................................................................................................138 6.5.1 Charging Characteristics of the a-Si:H TFTs..............................................................138 6.5.2 Feed-Though Voltage of the a-Si:H TFTs ....................................................................141 6.6 INFLUENCE OF CORBINO A-SI:H TFTS ON OLED CURRENT .................................146 vi 6.7 CONCLUSION ..................................................................................................................150 7. CONCLUSIONS AND FUTURE WORK ...........................................................................154