Advancing thin-film transistors and display circuits

Active-matrix organic light-emitting diode (AMOLED) on plastic substrates has recently attracted the attention of many researchers. It uses thin-film transistors that exhibit high performance under mechanical and electrical stresses such as the low-temperature polysilicon types. Unfortunately, they are prone to various challenges like nonuniformity in the material and high manufacturing cost that limit their use. Therefore, scientists have been looking for alternatives for manufacturing highly robust and low-cost AMOLED and have identified amorphous oxide semiconductor (AOS) as possible solution owing to their high mobility, low-temperature process and low manufacturing cost.

Among the available amorphous oxide semiconductor, amorphous indium-gallium-zinc oxide (a-IGZO) is highly popular. Thin-film transistors-based a-IGZO are widely used in commercial products like liquid crystal display. They use structures including etch-stopper, coplanar and back-channel etched. However, etch-stopper structure exhibit high parasitic capacitance and low drain currents that hinder the realization of high-frequency display devices.

Kyung Hee University researchers: Suhui Lee (PhD candidate), Yuanfeng Chen, Jaekwon Jeon, Chanju Park and led by Professor Jin Jang developed a new thin-film transistor structure that exhibits low overlap capacitance without having to scarify the drain current. They utilized the back-channel etched structure and formed the stripe patterned source/drain electrodes to minimize the gate-to-drain overlap capacitance. They investigated the spreading currents in the amorphous indium-gallium-zinc oxide thin-film transistors with different stripe widths and open space widths. Their work is published in the research journal, Advanced Electronics Materials.

The research team observed that using stripe-patterned source/drain electrodes made it possible to reduce the parasitic capacitance of the thin-film transistors by one-third of the initial conventional structure. For instance, due to the reduction in the overlap capacitance, the oscillating frequency of the ring oscillator made of thin-film transistors with 10 µm open space width stripe source/drain was 2.5 times as compared to that made of the conventional thin-film transistor. Furthermore, they confirmed the existence of the spreading currents. This was attributed to the constant drain currents of the amorphous indium-gallium-zinc oxide thin-film transistors as the open space width changes from 0-10 µm.

The study by Professor Jin Jang and his research team successfully developed a new technology based on the current spreading concept that uses stripe-patterned source/drain electrodes to significantly reduce the overlap capacitance without having to scarify the drain currents. Additionally, the technology has numerous benefits including high current stability attributed to the reduced source/drain surface area that enables easier dissipation of heat. Consequently, it resulted in a significant improvement in the mechanical stability of back-channel etched thin-film transistors thus preventing crack generations. Therefore, the Kyung Hee University technology will help advance the design and development of robust thin-film transistors and display circuits.