PhD Thesis Presentation
Bridged-Grain Small Grain Low Temperature Polycrystalline Silicon Thin-Film Transistors for Active Matrix Displays

by Ms Wei ZHOU

 :  14 Aug 2013 (Wed)
 :  2pm
Venue  :  Room 2463, 2/F (Lifts 25-26), HKUST

Examination Committee
Prof Helen C M SHEN, CSE/HKUST (Chairman)
Prof Hoi Sing KWOK, ECE/HKUST (Thesis Supervisor)
Prof Min Koo HAN, Department of Electrical and Computer Engineering, Seoul National University (External Examiner)
Prof Zhiyong FAN, ECE/HKUST
A new structure (dubbed bridged-grain, or BG for short) for thin-film transistors (TFTs), exhibiting the benefits but not the drawbacks of both the short-channel and multi-junction effects, is proposed and demonstrated. A series of short channels is formed through selective doping of the channel using a grating-shaped layer as a mask. This grating can be formed simply using a low-cost nanoimprinting process. All characteristics such as threshold voltage (Vth), pseudo subthreshold slope (SS), on-off current ratio and field-effect mobility are improved. The physics of this device structure is studied in detail. The fabrication process is optimized for small grain low temperature polycrystalline silicon (LTPS) TFTs, including those based on solid phase crystallization (SPC) and metal induced crystallization (MIC). A prototype of 4-inch active matrix organic light emitting diode (OLED) display adopting BG MIC TFTs is demonstrated, showing the potential for mass production and commercialization of BG-TFT based active matrix displays.
Application of high-κ dielectrics in LTPS TFTs can greatly reduce the Vth and sharpen the SS. However, the leakage current due to the high drain electric field is much more serious than in those using SiO2 as gate dielectrics. The combined application of high- κ dielectrics and BG in LTPS TFTs is explored. It is considered as an optimal combination for realization of high performance small grain TFTs with good uniformity, which is highly desirable for driving high resolution displays including those based on OLED.
To simplify the fabrication process, a new device structure is designed, making use of the anisotropic conductivity of BG lines. In the new design, the shape of the active island is modified so that the current flows along the BG lines in the source/drain region. As a result, one implantation step is saved, and the measurements of the fabricated devices show no problems in source/drain contact.
*** ALL ARE WELCOME !! ***