Design of a-IGZO Thin film transistors

Abstract

Thin film transistors (TFT) have received great attention due to their applications in flat planed display, e-paper and flexible electronics. a-IGZO based TFTs have shown superior performance as far the stability is concerned. However, some issues remain to be treated properly. In this thesis, numerical simulation (by SILVACO ATLAS) was carried out to investigated three problems. The first, the effect of Hydrogen contamination of a-IGZO was investigated. It was found that donor defects near the valance band has no effect, while the mobility degradation induces a degradation in the TFT performance. Acceptor defects states near valance band is the reason of positive Vth shift. It is therefore concluded that near valance band defects are not donor defects but acceptor defects with a Gaussian distribution which can also degrade the mobility. The second, is the effect of different gate dielectrics. Four different insulators (SiO2 Si3N4, Al2O3 and HfO2) are examined. It is found that the output performance is significantly enhanced with high relative permittivity of the insulator. The HfO2 gate insulator gives the best performance: lower threshold voltage 0.23V and subthreshold 0.09 Vdec-1, and higher field effect mobility 13.73 cm2 s-1 V-1, on current and Ion/Ioff ratio 2.81 × 10−6 A,5.06 × 1012 respectively. Therefore, The HfO2 gate insulator showed high stability compared with other gate insulators materials. The third is a comparison of simulation to measurement of ultra-thin channel on the TFT performance. The thinner channel layer was found to have a better performance than the thicker channel layer. The 4 nm-thick, ultra-thin a-IGZO TFT exhibited high saturation mobility (7.56 cm2 V−1 s−1), low threshold voltage (2.73 V), a small value of sub threshold swing (0.22 Vdec−1) and a high on/off ratio (1.77 × 108). It was also noticed that the threshold voltage (Vth) shifts negatively as the thickness increases. The 4 nm long channel TFT shows more stability under NIBS and PBS while 16 nm have a strong degradation under NIBS and PBS