Dependence of Photosensitive Effect on the Defects Created by DC Stress for LTPS TFTs

This work investigates negative bias temperature instability ͑NBTI͒ in low temperature polycrystalline silicon thin film transistors ͑LTPS TFTs͒ in a darkened and in an illuminated environment of different light intensities. Experimental results reveal that the generations of interface state density ͑N it ͒ are identical under various illuminated intensity NBTI stresses. Nevertheless, the degradation of the grain boundary trap ͑N trap ͒ under illumination was more significant than that for the darkened environment, with degradation increasing as illumination intensity increases. This phenomenon is mainly caused by the extra number of holes generated during the illuminated NBTI stress. The increased N trap degradation leads to an increase in the darkened environment leakage current. This indicates that more traps are generated in the drain junction region than from carrier tunneling via the trap, resulting in leakage current. Conversely, an increase in N trap degradation results in a decrease in the photoleakage current. This indicates that the number of recombination centers increases in the poly-Si bulk, affecting photosensitivity in LTPS TFTs.Low temperature polycrystalline silicon thin film transistors ͑LTPS TFTs͒ have been widely investigated for flat-panel applications, such as for active matrix liquid crystal displays ͑AMLCDs͒ and active matrix organic light emitting diodes. 1-3 Compared to amorphous silicon ͑a-Si͒ TFTs, LTPS TFTs have a higher electron mobility and driving current. Consequently, the LTPS TFTs can integrate both the pixel array and peripheral circuits on the same glass substrate to realize a system-on-panel ͑SOP͒ display. 4,5 Because driving circuits for LTPS TFTs are designed using the complementary metal oxide semiconductor inverter structure, the reliability of the p-channel LTPS TFTs ͓understood by such measurements as negative bias temperature instability ͑NBTI͔͒ has been an important problem and has been widely investigated and discussed. Previous studies, however, have investigated NBTI only in darkened environments 6-8 although LTPS TFTs are usually operated in illuminated environments on AMLCDs or SOP displays. Therefore, NBTI under illumination is an important issue for predicting LTPS TFT degradation. Both the illumination effect in LTPS TFTs 9,10 as well as the reliability of a-Si TFTs under illumination have previously been studied. 11 However, NBTI degradation in p-channel LTPS TFTs under illumination has not been adequately clarified.This work studies the NBTI effect of p-channel LTPS TFTs under various illumination intensities and temperatures. By analyzing the interface state density ͑N it ͒ and the grain boundary trap density ͑N trap ͒ after NBTI stresses, the results indicated that only the generation of N trap after illuminated NBTI stress is more pronounced due to the excess light-induced holes. The darkened leakage and the photoleakage currents after different NBTI stresses were also investigated. ExperimentalThe p-channel LTPS TFTs were fabricated on a glass substrate...

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“…This is because the increase in N trap actually functions to increase the number and availability of recombination sites. 19 …”

Section: J30mentioning

confidence: 99%

Degradation of Low Temperature Polycrystalline Silicon Thin Film Transistors under Negative Bias Temperature Instability Stress with Illumination Effect

Lin¹,

Chen²,

Chang

et al. 2010

J. Electrochem. Soc.

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“…InGaZnO 4 bulk with a 3.2-eV energy bandgap. These results demonstrate that the carrier in the source-offset region of a-InGaZnO 4 can be increased by trap-assisted photogeneration of electron-hole pairs under blue light [28], [32]- [33]. In addition, the electrical characteristic curve of the same device is measured under UV illumination.…”

Section: Methodsmentioning

confidence: 67%

Ultrahigh Sensitivity Self-Amplification Phototransistor Achieved by Automatic Energy Band Lowering Behavior

Chen

Chang

Tai

et al. 2014

IEEE Trans. Electron Devices

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In this paper, amorphous InGaZnO 4 thin-film transistors with an asymmetric structure exhibit ultraviolet (UV) light sensing property. At the offset region near the drain electrode, the extended active layer plays the role of a resistor. However, the ON-state current is obviously reduced with increasing offset length at the offset region near the source electrode and the characteristics cannot be turned ON when offset length is over 4 µm. After exposure to UV light, photogenerated holes-induce source barrier lowering and then amplifying the photocurrent response. Therefore, the devices in this paper reach a high-ON/OFF ratio of UV sensitivity to 10 6 .

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“…However, LTPS TFTs are not perfect. There are many defects in LTPS TFTs, which are the originals of many stability problems on displays [1,2]. Illumination Mura in AMOLED display could be observed by naked eye after several hours' illumination on localized display area.…”

Section: Introductionmentioning

confidence: 99%

P‐32: Investigation on Mechanism of Illumination Mura in AMOLED Display with LTPS‐TFT Backplane after Long Term Localized Illumination

Guo¹,

Gai²,

Li³

et al. 2022

Symp Digest of Tech Papers

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Low‐temperature polycrystalline silicon (LTPS) thin film transistors have been widely used in AMOLED display applications due to the high mobility and the capability of realizing peripheral and active matrix circuits on glass. Though LTPS TFTs exhibited relatively good electrical performance and stability, illumination Mura on AMOLED display with LTPS backplane could be observed by naked eye after several hours’ illumination on localized display area. The Mura boundary separates the areas with and without illumination when the AMOLED display is lighted on without pattern. The electrical instability of LTPS TFT was proved to be the main underlying cause. The DTFT threshold voltage compensation effect and SS variation in 7T1C pixel circuit has certain relationship with the illumination Mura detail.

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Dependence of Photosensitive Effect on the Defects Created by DC Stress for LTPS TFTs

Cited by 17 publications

References 4 publications

Degradation of Low Temperature Polycrystalline Silicon Thin Film Transistors under Negative Bias Temperature Instability Stress with Illumination Effect

Degradation of Low Temperature Polycrystalline Silicon Thin Film Transistors under Negative Bias Temperature Instability Stress with Illumination Effect

Ultrahigh Sensitivity Self-Amplification Phototransistor Achieved by Automatic Energy Band Lowering Behavior

P‐32: Investigation on Mechanism of Illumination Mura in AMOLED Display with LTPS‐TFT Backplane after Long Term Localized Illumination

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