1. Introduction

  Recently, amorphous oxide semiconductors (AOSs) have attracted considerable attention as novel candidates for channel materials of thin-film transistors (TFTs). Compared to traditional amorphous semiconductors such as a-Si and organic TFT materials, AOS based TFTs have clearly superior performances because of their unique features (high performance, low process temperature, and transparency).

Fig. 1. Transparent, flexible TFT sheet

Fig. 2. Bottom-gate TFT structure

 

2. Properties of AOS TFTs

 Various kinds of oxide semiconductors have been reported in the last few years. However, ZnO was the most frequently reported for a long time. But the polycrystalline phase and grain boundaries of the ZnO material deteriorate the performance, uniformity and long-term stability of the TFT. For this reason, the amorphous materials (a-IGZO, especially) are widely studied in recent time. The below table contains several advantages such as good uniformity, relatively easier manufacturing, low temperature processing, mid-range mobility and transparency in the visible region.

In the AOS TFTs, it can be expected that certain conditions or environments may lead to potential instabilities such as threshold voltage shift (VT).

Fig. 3. Mobility and carrier concentration as functions

 of chemical composition

a-Si

LTPS

AOS

Mobility(cm2/Vs)

<1

50~150

10~20

Leakage

VT Stability

(?)

VT Uniformity

L.T. Reliability

VT > 5 V

VT < 0.5 V

(?)

Process Temp.

~ 250

250 ~

~ 250

# of masks

4~5

5~11

4~5

etc

Expensive

Transparency

Table. 1. A comparison of various TFT channel materials

 

3. VT instabilities in AOS TFT.

  Light exposure (1) and Bias stressing (2) are the major causes of the AOS TFTs instability. These two different conditions have their own mechanisms in instability process of VT shift. And they can operate separately. In order to prevent or minimize these problems, many methods are studied in manufacturing process

Fig. 4. A brief progresses of VT instability.

Fig.5. The evolution of transfer curves as an applied gate stresses(positive / negative stress) under 4500-nits light exposure conditions for 5000second.

 

4. Our research

  The goal of this study is to find the mechanisms of defects and derive the improvements ideas in AOS TFTs.