Abnormal Behavior of Threshold Voltage Shift in Bias-Stressed a-Si:H Thin Film Transistor under Extremely High Intensity Illumination

Abstract

We report on the unusual behavior of threshold voltage turnaround in a hydrogenated amorphous silicon (a-Si:H) thin film transistor (TFT) when biased under extremely high intensity illumination. The threshold voltage shift changes from negative to positive gate bias direction after ∼30 min of bias stress even when the negative gate bias stress is applied under high intensity illumination (>400 000 Cd/cm(2)), which has not been observed in low intensity (∼6000 Cd/cm(2)). This behavior is more pronounced in a low work function gate metal structure (Al: 4.1-4.3 eV), compared to the high work function of Cu (4.5-5.1 eV). Also this is mainly observed in shorter wavelength of high photon energy illumination. However, this behavior is effectively prohibited by embedding the high energy band gap (∼8.6 eV) of SiOx in the gate insulator layer. These imply that this behavior could be originated from the injection of electrons from gate electrode, transported and trapped in the electron trap sites of the SiNx/a-Si:H interface, which causes the shift of threshold voltage toward positive gate bias direction. The results reported here can be applicable to the large-sized outdoor displays which are usually exposed to the extremely high intensity illumination.

P‐16: Turnaround Phenomenon of Threshold Voltage Shifts in Bias‐Stressed a‐Si:H Thin‐Film Transistor under Extremely High Intensity Illumination

“…In contrast, when the a-Si:H TFT was under lower intensity illumination (6000 cd/cm 2 ) stress, no abnormal behavior of Vth was observed (Figure 1e). Generally, with respect to the Vth turnaround phenomenon occurring under high-intensity illumination during negative gate bias, there are two possible sources of trapped charge in a-Si:H TFTs, including increase in electron trapping or decrease in hole trapping [6][7] . According to previous reports, electrons can be injected into the gate insulator under high intensity negative gate bias-stress, the number of injected electrons is expected to be higher under high-intensity illumination.…”

“…These electrons were then trapped at the SiNx/a-Si:H interface. 6 TFTs with various compositions of gate insulator were fabricated in order to alleviate the abnormal Vth turnaround phenomenon. Results reveal that TFT based on Si-rich gate insulator has better stability under high intensity illumination due to superior SiNx/a-Si:H interface performance, while Vth did not turnaround in high-intensity negative gate bias-stress.…”

P‐1.5: Influence of Bandgap of SiNx Gate Insulator on the Degradation Behavior of Amorphous Silicon Thin‐Film Transistors under Reliability Test

“…Although a-Si:H TFTs exhibit excellent characteristics such as good switching characteristics, large-area deposition capability, good large-area uniformity and low processing temperature, they also suffer from instability issues of threshold voltage shift under electrical bias and illumination stresses and the low field effect mobility (< 1cm 2 /Vs) [4]- [6]. In order to improve the stability of a-Si:H TFTs, there have been numerous studies about the mechanisms of degradation behaviors of device performance under different stress conditions [7]- [13]. However, the degradation characteristics of a-Si:H TFTs under reliability (RA) test or long-term usage in AMLCD panel applications were seldom reported.…”

32.4: Abnormal Turnaround Phenomenon of Threshold Voltage Shifts in Bias‐Stressed a‐Si:H Thin Film Transistor under Extremely High Intensity Illumination

“…These electrons were then trapped at the SiNx/a-Si:H interface. 6 TFTs with various compositions of gate insulator interface layer were fabricated in order to alleviate the abnormal Vth turnaround phenomenon. Results reveal that TFT based on Si-rich gate insulator interface layer has better stability under high intensity illumination due to superior SiNx/a-Si:H interface performance, while Vth did not turnaround in high-intensity negative gate bias-stress.…”