Negative Data Insertion Method for Suppressing Hysteresis of Polysilicon Thin-Film Transistors

Abstract

The electrical hysteresis of polysilicon thin-film transistors ͑poly-Si TFTs͒ employing a negative data insertion ͑NDI͒ has been investigated. It is known that the image sticking in active-matrix organic light emitting diode displays using the poly-Si TFT back plane was mainly caused by the hysteresis of driving TFT in a pixel. In this paper, we propose a type of NDI method using inverse data signals in order to suppress the hysteresis of poly-Si TFTs. Experimental results show that the NDI was very effective to reduce the hysteresis level of poly-Si TFTs induced by carrier trapping and detrapping.Active-matrix organic light-emitting diode ͑AMOLED͒ displays have attracted considerable attention among flat-panel displays due to high brightness with low power consumption, light weight, fast response time, wide viewing angle, and self-emissive characteristics. 1,2 Recently, low-temperature polysilicon thin-film transistor ͑poly-Si TFT͒-based AMOLED displays have been commercialized, presenting superiority of image quality. 3 Poly-Si TFTs have been preferred as a back plane of AMOLED displays due to their higher mobility and greater stability compared to hydrogenated amorphous silicon thin-film transistors ͑a-Si:H-TFTs͒. However, poly-Si TFT-based AMOLED displays suffer from the nonuniformity problem caused by irregular characteristics of laser crystallization. In order to overcome the nonuniformity of poly-Si TFTs and make the mass production possible, various kinds of compensation methods 4-6 and different crystallization methods are being introduced continuously. 7,8 Image sticking is another issue in poly-Si TFT-based AMOLED displays, which is known to be caused by the hysteresis of poly-Si TFTs according to different bias stress. 9 Variations in TFT characteristics by the hysteresis occur in different degrees for each pixel, causing each driving TFT in a display panel to have different threshold voltage values. Such an effect is often referred to as differential aging, which causes some undesirable visual defects, such as image sticking or residual image. 9 In order to solve this problem, there are several pixel circuits for driving organic light-emitting diode ͑OLED͒ current that compensates for the variation of TFT characteristics, some of which employ current programming, 4 time ratio gray scaling, 5 or threshold voltage compensating methods. 6 However, these methods require development of complicated source drive integrated circuits and also limit the aperture ratio of pixel due to the complexity of pixel circuit. 10 In this paper, we propose a type of negative data insertion ͑NDI͒ method capable of suppressing the hysteresis of poly-Si TFTs to enhance the image quality of AMOLED displays without the use of any complicated pixel circuit. TFT FabricationIn order to investigate the electrical hysteresis of poly-Si TFTs and the effect of NDI, top-gate p-channel TFTs were fabricated on SiO 2 ͑300 nm͒ stacked as a buffer layer on a glass substrate. A 50 nm thick a-Si film was deposited on the buffer layer by plas...

P‐16: A Novel Shift Register for Driving AMOLED Display

“…This is well known that the realization of an AMOLED display requires good uniformity poly-Si TFTs fabricated on a large glass substrate. Until now, threshold voltage (V th ) variation, VDD IR drop and image retention [3] are critical issues on AMOLED applications. For this reason, many techniques have been proposed to improve these issues, such as digital driving method [4], voltage programming method and current programming method [5].…”