# Reduction of Bias and Light Instability of Mixed Oxide Thin-Film Transistors
## 4 Discussion
### Fabrication Process Optimization  
The variations in performance and theories from research group to research group are closely related to the sensitivity of mixed oxide semiconductors to air or chemicals. Exposure to air should be avoided, not only after device fabrication (Figure 1f), but also, more importantly, during the fabrication process itself (Figure 1b). If a device is fabricated with neither an etch stopper nor a passivation layer, the device stability will improve with increasing a-IGZO thickness. Thick a-IGZO layers will have a self-passivation effect by the intrinsic a-IGZO, even though the surface region is affected by moisture or oxygen from ambient air; that is, the thicker the a-IGZO layer, the farther away the front channel accumulation layer (bottom surface of the semiconductor layer) is from the desorption and adsorption properties occurring at the top surface. More importantly, if the vacuum is broken between the deposition of the a-IGZO and the etch stopper, in cases where the two processes have to be performed in two different chambers, significant contamination to the a-IGZO layer will occur during the transfer from the a-IGZO deposition chamber to the etch-stopper deposition chamber. This is very important, but often overlooked. In the case of the devices presented herein, a cluster deposition tool that allows consecutive deposition of the gate insulator, active layer, and etch stopper without breaking vacuum is used, thereby passivating the a-IGZO layer throughout the whole fabrication process. Therefore, we emphasize that due to fabrication process optimization, intrinsic, rather than process-related instability mechanisms, were detectable in this study. Figure 12: Effect of monochromatic light on the performance of amorphous indium-gallium zinc-oxide (a-IGZO) thin-film transistors (TFTs) (**a**) before and (**b**) after application of negative-gate bias stress combined with visible light illumination (NBIS). Device structure and the type of dielectrics used can also be a source of the differences in the trends observed with varying semiconductor thickness. For instance, the stability of inverted, staggered devices without a passivation layer is more likely to improve with increasing semiconductor thickness because the thicker the semiconductor, the further away the front channel (bottom surface of the semiconductor layer) is from the absorption/desorption processes occurring at the top surface [8]. In this report, the root cause of these conflicting theories is not only attributed to the possibility of varying the types and/or ratios of the components making up a mixed oxide semiconductor, but it is mainly attributed to fabrication process variations, given the sensitivity of mixed oxide semiconductors to wet etchants, gasses, or air exposure. Therefore, we show here that under an optimized fabrication process, which involves the passivation of the mixed oxide semiconductor in situ, employing very thin mixed oxide semiconductors completely suppresses the NBIS-induced instability. It is, thus, reasonable to conclude that under the optimized fabrication process presented herein, bulk, rather than interfacial defects, are the main source of bias and light instability in mixed oxide TFTs. It is important to note that the self-passivation effect of thick semiconductor layers is not useful in real applications, such as display panels, where TFTs are passivated/encapsulated by materials with very low water-vapor-transmission-rates. As good NBIS stability is of utmost importance, TFTs with thin semiconductor layers are thus more desirable. However, in TFTs with thin active layers, there is an NBIS stability versus on-state current trade-off. This can be minimized by the employment of large channel widths, if the intended application allows.