**4. Conclusions**

Buried-channel TFTs with multiple-stacked channel layers including an a-IGZO:N film (front-channel layer), an a-IGZO film (buried-channel layer), and an a-IGZO:N film (back-channel layer) were fabricated and measured in this work. Compared with those of the conventional a-IGZO TFT, the better electrical performance (e.g., smaller SS value, larger μFE, and optimum *V*TH) as well as the improved stability properties (e.g., smaller *V*TH shifts during PBS, NBS, and NBIS tests) were obtained for the buried-channel device. The a-IGZO:N film used as the front-channel layer in the buried-channel structure could reduce both the interface trap density and the bulk-layer deep state density, leading to a smaller SS value (0.8 V/dec) in comparison with the conventional a-IGZO TFT device (SS = 1.3 V/dec). The non-degraded μFE (5.8 cm2V−1s−1) for the buried-channel TFT might be attributed to its three conduction channels, including the main channel at the interface between the channel layer and the dielectric layer, the first sub-channel formed at the bottom a-IGZO:N/a-IGZO interface, and the second sub-channel formed at the top a-IGZO/a-IGZO:N interface. In addition, the improved bias-stress and NBIS stability of the buried-channel TFT compared with those of the conventional a-IGZO TFT might be mainly due to the suppression of the trap states in the bulk channel layers, the reduction in the defects at the front-channel interface, and the passivation effect created by using the a-IGZO:N film as the back-channel layer.

**Author Contributions:** Y.Z. and H.X. fabricated and measured all the a-IGZO TFTs. Y.Z. and C.D. designed the experiments and contributed to the theoretical explanations. The manuscript was written and revised by all the authors.

**Funding:** This work was supported by Natural Science Foundation of China (Grant No. 61474075).

**Conflicts of Interest:** The authors declare no conflict of interest.
