前瞻銦鎵鋅氧薄膜電晶體應用於閘極驅動陣列基板技術之自我加熱效應物理機制研究

Abstract

近年來，主動層為透明氧化物半導體的薄膜電晶體備受關注，例如氧化鋅與非結晶態銦鎵鋅氧化物(a-IGZO)，因為其具有應用在平板、可撓式面板與透明顯示器上的巨大潛力。在次世代顯示器工業發展中，由於a-IGZO薄膜電晶體擁有很好的均勻性、高載子遷移率以及低溫製程等優勢，因此被廣泛地研究。此外，a-IGZO薄膜電晶體也可應用在閘極驅動陣列的技術上，但是操作於高電壓與高電流的情況下將會導致元件的劣化，因此本論文探討了a-IGZO薄膜電晶體於實際操作時可能面臨之自我加熱效應。 第一部分探討了a-IGZO薄膜電晶體通道寬度與汲極偏壓對臨界電壓飄移的異常現象，非汲極偏壓導致源極位能障下降的效應(DIBL)，臨界電壓而是隨著汲極偏壓上升而增加，且隨著通道寬度增加，臨界電壓飄移更嚴重，此外，自我加熱效應在大通道寬度與大汲極偏壓操作下會更顯著，我們利用Fast I-V的量測方式驗證了異常的臨界電壓劣化現象是由自我加熱效應所導致。 實驗第二部分探討了通道尺寸與開啟態電流劣化的關係，此開啟態電流的劣化現象是由於元件操作在高汲極電流時，引發自我加熱效應而造成電子捕獲，使臨界電壓上升，造成開啟態電流下降。我們利用Pulse ID-VD的量測方式，可以知道自我加熱效應導致開啟態電流產生劣化的有效加熱與散熱時間。 第三部份分別探討了自我加熱電應力實驗於不同頻率及電極結構下的劣化情形，實驗結果顯示，隨著頻率降低與通道寬度增加，臨界電壓的飄移量也隨著增加，因為IGZO材料的熱傳導係數很低，加熱時間愈長，熱愈容易累積在通道。除此之外，使用分離的U型電極結構可增加散熱效率，較小的單位U型通道寬度也可使通道中的熱更容易散失，降低自我加熱效應所造成的劣化。

Thin film transistors (TFTs) with active layers composed of transparent oxide-based semiconductors, such as ZnO and amorphous InGaZnO (a-IGZO), have attracted much attention recently due to their considerable potential applications in flat, flexible and transparent displays. In particular, a-IGZO TFTs have been widely investigated for the next generation display industry owing to their good uniformity, high mobility(10~100 cm2/V-s), and room temperature processing. Moreover, a-IGZO TFTs can also be used for gate driver on array (GOA) technology. However, the operation voltage and/or current lead to device degradation. Therefore, the effects of self-heating in a-IGZO TFT to cause threshold-voltage (VTH) shift are investigated in this work. Abnormal channel width-dependent VTH variation in a-IGZO TFT is investigated. Unlike drain-induced barrier lowering, VTH increases with increasing drain voltage. Furthermore, the wider channel, the larger VTH can be observed. The self-heating effect is more pronounced in wider channel devices and for larger operating drain bias. Fast I-V measurement is utilized to demonstrate the self-heating induced abnormal channel width-dependent VTH variation. The dimensional dependence on-current degradation behaviors is also investigated. This on-current degradation phenomenon is dominated by self-heating enhanced VTH shift caused by electron trapping at high drain current. Pulse ID-VD measurement method is used to determine the effective heating and cooling time by changing the peak and base time, respectively. Frequency and channel structure modulation dependent VTH shift during self-heating stress are investigated. Self-heating stress causes more serious VTH shift than positive gate-bias stress since drain current enhance more electron-trapping. VTH shift increases with decreasing frequency and increasing channel width. Heat can be effectively dissipated using a separated U-shaped electrode, and with smaller channel width per unit.