Browsing by Author "Wu, Chia-Chuan"

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A Novel Structural Design Serving as a Stress Relief Layer for Flexible LTPS TFTs
(2019-01-01)
Wang, Yu-Xuan
;
Chang, Ting-Chang
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Huang, Shin-Ping
;
Tai, Mao-Chou
;
Zheng, Yu-Zhe
In this work a novel device structure aiming to suppress the degradations under bending in flexible devices is demonstrated, named as a wing-shape TFT. Investigations of the electrical characteristics and reliabilities are supported by simulations, theoretical calculations, and structural discussions. First, through both electrical and mechanical stress simulation, the regions of degradation are confirmed to occur at the corner of the gate insulator. After long term mechanical stress, behaviors including an abnormal hump, serious threshold voltage (Vth) shifts, on-current increase, and off-current decrease are observed. By extending the active layer, also serving as a stress relief layer, removes the degradation regions away from the main channel. Therefore, the reliability of the devices is dramatically improved. The geometries of the wings in wing-shape TFTs are also in consideration. From results, with different wing lengths, a similar improvement of the reliability is shown, but the on current is enhanced when the wing length is extended. These phenomena are realized as a result of the difference in the effective width. The proposed wing-shape LTPS TFT has its potential and benefits for future flexible displays.
A Novel Structure to Reduce Degradation Under Mechanical Bending in Foldable Low Temperature Polysilicon TFTs Fabricated on Polyimide
(2020-05-01)
Wang, Yu-Xuan
;
Chang, Ting-Chang
;
Huang, Shin-Ping
;
Tai, Mao-Chou
;
Zheng, Yu-Zhe
In this study, a novel structural device, named as a wing-shaped TFT, is proposed and is demonstrated to prevent fast deterioration under mechanical bending in foldable devices. After long-term mechanical bending, standard low-temperature polysilicon TFTs exhibit instability behaviors such as sub-channel formation, serious threshold voltage shifts, an on-current (I-ON) increase, and an off-current (I-OFF) decrease. These phenomena are believed to be due to additional oxide traps along the channel-width edges of the gate insulator after bending. In order to suppress this degradation, a wing-shaped structure is proposed. By extending the active layer, degradation regions are shifted away from the main channel. Therefore, the reliability of the devices is enhanced. The transfer characteristics show enhanced electrical behavior after being held for 48 hours in a static bent condition, with an 88% drop in threshold voltage shift as well as a 55% decrease in subthreshold swing degradation, proving better durability of the foldable TFT to bending stress for devices with these wings. Time-dependent transfer characteristics, a COMSOL simulation, and device geometry are discussed to support our findings.
Abnormal C-V Hump Effect Induced by Hot Carriers in Gate Length-Dependent p-Type LTPS TFTs
(2019-11-01)
Huang, Shin-Ping
;
Chen, Po-Hsun
;
Tsao, Yu-Ching
;
Chen, Hong-Chih
;
Zheng, Yu-Zhe
We investigate the abnormal current-voltage (C-V) hump effect of p-type low-temperature polysilicon (LTPS) thin-film transistors (TFTs) which have undergone high current operations. Experimental results indicate localized electron trapping in the gate insulator (GI), which is carried out near the drain. The ON-current ( ${I}_{ \mathrm{\scriptscriptstyle ON}}$ ) enhancement is due to the reduction of effective length, and the OFF-current ( ${I}_{ \mathrm{\scriptscriptstyle OFF}}$ ) decrease as the electron tunneling path distance increases. These can be observed after hot carrier stress in current characteristics. The C-V measurements demonstrate that the threshold voltage ( ${V}_{\text {th}}$ ) shift is associated with the gate length. In addition, capacitance-voltage measurements also show that this localized trapping region remains the same in length, regardless of channel length. Hence, a model is proposed to explain how the electric field, which is gate length-dependent, affects the source side of the device, and then lowers the source barrier height. This leads to bulk leakage, which causes the subthreshold swing degradation at device scale down.
Abnormal hysteresis formation in hump region after positive gate bias stress in low-temperature poly-silicon thin film transistors
(2020-09-30)
Tu, Hong-Yi
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Chang, Ting-Chang
;
Tsao, Yu-Ching
;
Tai, Mao-Chou
;
Tsai, Yu-Lin
Degradation in low-temperature polycrystalline-silicon thin-film transistors after electrical stress was thoroughly investigated in this work. Main channel degradation, abnormal hump generation and hysteresis appearing in the hump region can be observed after positive bias stress. Furthermore, the difference in subthreshold swing (SS) values between forward/reverse sweep is observed. The electron trapping into the gate insulator (GI) dominates the main degradation and the hump generation. Additionally, the difference in SS values which appears in the hump region is attributed to the interface traps and the hysteresis is caused by electron trapping/detrapping into GI.
Analysis of Negative Bias Temperature Instability Degradation in p-Type Low-Temperature Polycrystalline Silicon Thin-Film Transistors of Different Grain Sizes
(2019-11-01)
Tu, Hong-Yi
;
Tsao, Yu-Ching
;
Tai, Mao-Chou
;
Chang, Ting-Chang
;
Tsai, Yu-Lin
This letter investigates degradation after negative bias temperature instability (NBTI) stress applied to LTPS TFTs with different polycrystalline-silicon grain sizes. The initial characteristics of the LTPS TFTs are similar regardless of grain size; however, we observed a different degree of degradation after NBTI depending on grain size. In general, after NBTI, both grain boundary traps and interface traps were generated. We found that the degree of NBTI degradation is dominated by the concentration of grain boundary traps, which themselves are a result of the different grain sizes that occur due to excimer laser annealing energy. At initial, dangling bonds in the grain boundaries and at the interface are passivated by hydrogen atoms, hence the initial characteristics are similar. Since the large grain of poly-Si initially generates more dangling bonds in the grain boundaries, after NBTI, hydrogen depassivation generates more grain boundary traps and causes much more serious degradation in device performance.
Effect of ELA Energy Density on Self-Heating Stress in Low-Temperature Polycrystalline Silicon Thin-Film Transistors
(2020-08-01)
Huang, Shin-Ping
;
Chen, Hong-Chih
;
Chen, Po-Hsun
;
Zheng, Yu-Zhe
;
Chu, Ann-Kuo
The extent of the poly-silicon crystalline protrusion, a result of differences in excimer laser annealing (ELA), affects the performance and reliability of thin-film transistors (TFTs). This study investigates the degradation mechanism of the low-temperature polycrystalline silicon (LTPS) TFT devices with differences in crystalline protrusion under self-heating stress (SHS). Higher ELA energy will induce higher protrusion height in the interface between the poly-silicon and gate insulator (GI). This surface morphology leads to serious charge trapping into the GI layers; in contrast, the smallest degradation after SHS can be seen in the devices with the lowest protrusion height. This indicates that the degradation is caused by the surface morphology between the poly-Si and GI interface. In addition, the COMSOL simulation results confirm that the large electric field in the GI layer appears in the rough surface morphology devices; therefore, choosing the appropriate ELA energy of the poly-Si is beneficial for the applications of the driving TFT in organic light-emitting diode (OLED) display in the manufacturing industry.
Enhancement of Mechanical Bending Stress Endurance Using an Organic Trench Structure in Foldable Polycrystalline Silicon TFTs
(2020-05-01)
Zheng, Yu-Zhe
;
Huang, Shin-Ping
;
Chen, Po-Hsun
;
Chang, Ting-Chang
;
Tsai, Tsung-Ming
This letter proposes a new organic trench structure which can enhance the endurance of polycrystalline silicon thin film transistors (TFTs) under mechanical bending stress. It compares conventional structure TFTs to devices with an organic trench to examine reliability after undergoing 100,000 iterations of channel width-axis (100,000'WC) compression at R = 2mm. Due to the low Young's modulus of organic materials, stress while undergoing bending strain is desorbed to the buffer layer, which can lower the overall stress in TFTs. Furthermore, an optimal location and ideal length for such an organic trench is evaluated by mechanical bending simulation.
Impact of Dehydrogenation Annealing Process Temperature on Reliability of Polycrystalline Silicon Thin Film Transistors
(2019-10-01)
Huang, Shin-Ping
;
Chen, Po-Hsun
;
Chen, Hong-Chih
;
Zheng, Yu-Zhe
;
Chu, Ann-Kuo
This letter investigates variations in polycrystalline silicon thin film transistor (TFT) performance under illumination and negative bias temperature instability (NBTI) tests due to different dehydrogenation annealing temperatures during the fabrication process. The depth of the density of state (DOS) in polysilicon can be indirectly determined by the TFT response to light illumination, since dangling bonds act as recombination centers. The electrical characteristics of TFTs after undergoing NBTI can also be indicative of the bonding type of silicon atoms. By analyzing the results of these reliability tests, the type of DOS can be clarified, which is beneficial for realizing the relationship between performance and reliability.