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

Abstract

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...

NBTI Degradation in LTPS TFTs Under Mechanical Tensile Strain

“…This result indicates that the diffusion-controlled electrochemical reaction is the principal degradation mechanism for this NBTI stress [11]. Moreover, these results are similar to those of previous papers on LTPS TFTs [12]. This indicates that interface states and grain boundary traps were generated during NBTI stress.…”