Asymmetric Electrical Properties of Corbino a-Si:H TFTs

Hojin Lee, Juhn S. Yoo, and Jerzy Kanicki

Since Corbino disk was first reported by M. Corbino in 1911, this disk with inner and outer concentric ring contacts has been generally used in magneto-resistance measurement, and more recently has also been adopted for organic thin-film transistor (TFT) structures. In hydrogenated amorphous silicon (a-Si:H) TFT, so called, annular shape electrode was first introduced in 1996 to provide a reduced gate-to-source capacitance and a smaller photo-current that are important for active-matrix liquid crystal displays (AM-LCDs). In this project, we analyzed Corbino a-Si:H TFTs asymmetric electrical characteristics by investigating effects of the drain bias polarity on Corbino TFTs electrical properties. We also studied the a-Si:H TFT geometric effect on the extraction of key device electrical parameters such as sub-threshold slope, field-effect mobility, and threshold voltage, which are important for AM-LCDs and active-matrix organic light-emitting devices (AM-OLEDs).
The Corbino a-Si:H TFT is consisting of circle-shape inner electrode (radius R1= 12μm) and ring-shaped outer electrode (inner radius R2=18μm) as in the figure above. Bottom gate electrode is large enough to cover the entire area of device outer and inner electrodes. The Corbino a-Si:H TFT was fabricated using the normal AM-LCD 5-photomask process steps. Due to unique Corbino disk geometry, when source is connected to outer ring electrode, ON-current is 2 times higher, and OFF-current is 10 times lower than when source is applied to inner circle electrode at high drain voltages (>10V). Accordingly, sub-threshold swing is also smaller while threshold voltage and field-effect mobility remain same for both cases. We also found that the Corbino a-Si:H TFT might not be adequate device as a switching TFT for AM-LCD due to the asymmetric OFF-current behavior; the leakage current would vary depending on the drain bias condition. However, at the same time, thanks to its enhanced ON- and OFF- current ratio and possibly electrical stability, Corbino a-Si:H TFT can be a good candidate as a driving TFT for top-emitting anode AM-OLEDs. The goal of this project is to understand the geometric effect on a-Si:H TFT electrical characteristics and to optimize the device structure to achieve highest device electrical performances and stability. The long-term goals include the development of pixel electrode circuits for AM-OLEDs based on this research. This project is supported by LG Philips LCD Research & Development Center, Korea.

H. Lee, J. S. Yoo, C- D. Kim, I- J. Chung, and J. Kanicki, “Asymmetric Electrical Properties of Corbino a-Si:H TFTs and Concepts of Their Applications to Flat Panel Displays,” IEEE Trans. on Electron Devices, Vol. 54, 2007.

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