Bias Stress Stability of Asymmetric Source-Drain a-Si:H Thin Film Transistors
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0910-A22-02
Bias Stress Stability of Asymmetric Source-Drain a-Si:H Thin Film Transistors Kwang-Sub Shin, Jae-Hoon Lee, Won-kyu Lee, Sang-Geun Park, and Min-Koo Han School of Electrical Engineering, Seoul National University, San 56-1, Sillim-dong, Gwanak-gu, Seoul, 151-742, Seoul, Korea, Republic of
ABSTRACT The threshold voltage (VT) degradation of asymmetric source-drain a-Si:H TFTs due to the electrical stress has been investigated. In the absence of a drain bias (VG=15V, VD=0V), the threshold voltage (VT) shifts of asymmetric TFTs were similar to that of symmetric TFT. However, in the presence of drain bias (VG=15V, VD=20V), the VT shifts of asymmetric TFTs were less than symmetric TFT. The VT shifts of ‘L’ and ‘J’ shaped TFT were 0.29V, 0.24V respectively, while the VT shift of ‘I’ shaped TFT was 0.42V. The less VT degradation of the asymmetric source-drain a-Si:H TFT compared with the symmetric TFT may be explained by the defect creation model. Since the actual drain width of asymmetric TFT is longer than symmetric TFT at the same W/L ratio, the charge depletion due to the drain bias is larger than that of the asymmetric TFT. Due to the less carrier concentration in the channel, the asymmetric a-Si:H TFT shows the less VT degradation compared with the symmetric TFT. INTRODUCTION Hydrogenated amorphous silicon (a-Si:H) TFTs have been widely used as switching elements for low cost, large-size AMLCD panels due to their high uniformity and wellestablished manufacturing base. Recently, a-Si:H TFTs are being employed as a current source as well as a switching device in AMOLED applications [1], [2], [3]. The a-Si:H TFTs with a large on-off ratio and low off-state current are suitable for LCD applications, while the TFTs with a high driving capability and electrical stability are suitable to control OLEDs. However, the degradation of a-Si:H TFTs due to the electrical stress has been a critical issue for a-Si:H TFT based AMOLEDs [4]. Especially, the threshold voltage degradation of a-Si:H TFTs under drain bias stress is very important, since the current source in AMOLED pixel arrays has a common constant voltage source line (VDD) at the drain terminal [5]. Besides, the TFT used for a current source should occupy a very small area for high aperture ratio and fine resolution. The purpose of this paper is to report the electrical stability of asymmetric source-drain a-Si:H TFTs from the viewpoint of the current source for AMOLEDs. Our experimental results showed that the asymmetric source-drain a-Si:H TFT is more reliable than the symmetric TFT. Under the gate bias stress only (VG=15V, VD=0V), the threshold voltage (VT) shifts did not vary significantly with the structure, while, under the gate and drain bias stress (VG=15V, VD=20V), the VT shifts of asymmetric TFTs were less than that of symmetric TFT.
DEVICE STRUCTURE AND EXPERIMENT The asymmetric source-drain TFTs have been employed as switching devices in AMLCDs. They can not only suppress the feed-through voltage, but also reduce the transistor size. Two types o
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