由於金屬氧化物半導體有許多優越的特性，所以近年來已經成為相當熱門的研究領域。例如材料本身在可見光下是透明的，並可以在低溫下沉積製造且擁有相當高的載子移動率。因此，金屬氧化物將於下一新時代的光電元件應用上扮演著重要的角色。
雖然金屬氧化物薄膜電晶體有許多良好的優點，然而它的電性卻會受到環境的影響，例如氧氣、水氣與可見光。在本篇論文中，我們會討論電晶體在不同氣氛下閘極正偏壓的可靠度。然而，在顯示器的應用上，施加在畫素開關上負偏壓的時間比起正偏壓還要來得久。因此元件在閘極負偏壓的可靠度是值得被研究的。同時，元件在當作液晶的畫素開關時，會受到背照光的影響，故元件在可見光下的可靠度亦是相當重要的。
實驗結果顯示非晶態銦鎵鋅氧化物薄膜電晶體的電性會受到氧氣與水氣的影響。我們提出當元件施加閘極正偏壓時，若有水氣分子的存在，會有更多的氧氣分子可吸附在元件的表面，而造成電性的改變；然而，當元件處於閘極負偏壓下，金屬氧化物薄膜電晶體中的缺陷會與水氣結合，導致元件起始電壓與次臨界擺幅大量劣化。此外，吸附在非晶態銦鎵鋅氧化物表面的氧氣分子可藉由可見光的照射而脫附，導致特性曲線大量的偏移。

In recent years, amorphous oxide semiconductors have been studied due to their superior characteristics, such as transparent property, high electron mobility exceeding 10 cm2/V•s, and can be fabricated on plastic substrates at low temperatures. According to these advantages, a-IGZO thin-film transistors are promising as next-generation electronic devices.
Although a-IGZO TFTs have such unique properties, the electrical performances are strongly dependent on its environment such as oxygen, water and visible light. In this study, the electrical characteristics of a-IGZO TFTs under positive bias stress with different ambient gases have been discussed. In particular, the total duration of the negative gate bias applied on the switching transistor is larger than that of the positive gate bias in display application. Therefore, the electrical stability under negative bias stress is vital to investigate. Moreover, a-IGZO TFT regarded as a panel switch may be exposed to visible light for the application of liquid crystal display. The electrical stability under illumination of visible light is also important to study.
Experiment results show that device characteristics are affected under water-containing oxygen ambience. We indicates that the existence of water molecules can assist more oxygen to adsorb on the a-IGZO surface than the case without water assisting. That cause the variation of transfer curve under positive bias stress. However, the degradations in subthreshold swing and threshold voltage are caused by the state-related adsorption of water molecules under negative bias stress. Furthermore, adsorbed oxygen on the surface of a-IGZO can be desorbed by illumination of visible light, leading to large variation in transfer curve.

Thesis oral examination committee members approval sheet.....................................i
Acknowledgement...........................................................................................................ii
Chinese abstract.............................................................................................................iii
English abstract..............................................................................................................iv
Figure caption................................................................................................................vii
Table caption..................................................................................................................xii
Chapter 1 - Introduction.................................................................................................1
1.1 Overview......................................................................................................................1
1.2 Introduction of oxide semiconductor...........................................................................2
1.3 Motivation...................................................................................................................4
Chapter 2 - Device fabrication and electrical characterization...................................7
2.1 Device fabrication........................................................................................................7
2.2 Electrical characteristics..............................................................................................7
2.2.1 The I-V transfer characteristics................................................................................7
2.2.2 The C-V transfer characteristics.............................................................................10
Chapter 3 - Instruments and device parameter extraction.......................................14
3.1 Instruments and measurement setup.........................................................................14
3.1.1 Instruments.............................................................................................................14
3.1.2 Set up instruments for I-V.......................................................................................15
3.2 Device parameter extraction......................................................................................15
3.2.1 Determination of the threshold voltage..................................................................16
3.2.2 Determination of the field-effect mobility..............................................................17
3.2.3 Determination of the subthreshold swing...............................................................17
3.2.4 Determination of on/off current ratio....................................................................18
Chapter 4 - Results and Discussion.............................................................................21
4.1 Environment-dependent instability of a-IGZO TFTs under positive gate bias stress...............................................................................................................................21
4.2 Negative bias-induced adsorption of moisture in a-IGZO TFTs..............................26
4.3 Photofield-effect in a-IGZO TFTs under Different Environment............................32
Chapter 5 - Conclusion................................................................................................62
Reference.......................................................................................................................64

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