- 學位論文
氫化非晶矽薄膜電晶體的負偏壓溫度不穩定性劣化之研究
Research on the Degradation of Negative Bias Temperature Instability in a-Si:H TFTs
摘要
氫化非晶矽薄膜電晶體(a-Si:H TFTs)已廣泛的應用於平面顯示器中，做為畫素開關的重要元件，然而，隨著陣列上之電晶體數量不斷的增加，或更先進的電路應用中，氫化非晶矽薄膜電晶體會進一步作為驅動元件，這意味的電性將會受到更多因素的影響，其可靠度問題，於負偏壓溫度不穩定性(NBTI)仍有探討的空間，而劣化機制也有待確立，這也是本研究的動機。 本研究針對負偏壓溫度不穩定性對氫化非晶矽博膜電晶體所造成的影響進行研究，其中探討了臨界電壓、次臨界擺幅、開狀態電流、閉狀態電流與載子遷移率，並計算介面狀態與氧化層陷入的變化量，最後探討其所造成的機制。 研究結果發現，在高溫及高壓下，元件的劣化較為嚴重，主要表現在次臨界擺幅增加1.44V/decade、關閉電流增加180.5%和載子遷移率下降50.83%，但由於臨界電壓的偏移，最嚴重的下降達16.82V，造成導通電流增加200.35%，此部份可說是元件性能的提升。臨界電壓的下降主要是由氮化矽閘極介電層之電洞攫取所造成的，而次臨界擺幅增大的原因被歸咎於非晶矽主動層中懸鍵數量的增加所導致的。從劣化模型匹配的結果得知，臨界電壓的偏移與時間呈現冪次定律的關係。
並列摘要
Hydrogenated amorphous silicon thin film transistors (a-Si:H TFTs) have been widely used as pixel switching element in flat panel displays. However, due to the higher density of transistors on array and for the more advanced circuit applications, a-Si:H TFTs are used as driving devices. This means that the device electrical properties would be influence by more factors. For reliability concerns, it still has space to explore under negative bias temperature instability (NBTI), and the degradation mechanism has yet to be established. That is the motivation of the study. In this study, I investigate the NBTI impact for the a-Si:H TFTs, and discuss the threshold voltage, sub-threshold swing, on-state current, off-state current and effective mobility, and calculate the interface states and oxide trapped charges. Finally, the degradation mechanisms are also analyzed. From the experimental results, it is found that the a-Si:H TFTs exhibit more serious degradation with the enhancement of temperatures and negative stress voltages. The main features of the degradation are sub-threshold swing increase 1.44V/decade, off-state current increases 180.5% and effective mobility decrease 50.83%. Nevertheless, the on-state current increases 200.35% due to the threshold voltage shift decrease 16.82V. For this, the device performance can be said that it is improved. Mechanisms responsible for the threshold voltage shift are the holes trapping in the silicon nitride gate insulator, and an increase in the sub-threshold swing is attributed to the generation of additional dangling bonds. From the degradation model matching results, the threshold voltage shift has a power law dependent on stress time.