# Reduction of Bias and Light Instability of Mixed Oxide Thin-Film Transistors
## 4 Discussion
### Effect of Channel Thickness on NBIS Stability  
The NBIS instability originates from the presence of gap states near the \(E_{\rm V}\), which play the role of absorption sites for visible light. Thermal desorption spectra (TDS) obtained by [PERSON] et al. indicated that these defects may include trap states, which originate from the incorporation of the weakly bonded oxygen commonly known as oxygen vacancies [10; 22; 23]. Upon illumination, a neutral oxygen vacancy (\(V_{\rm O}\)) releases one or two electrons to the conduction band (\(E_{\rm C}\)), forming its ionized states, \(V_{\rm O}\)+ or \(V_{\rm O}\)\({}^{2+}\), which are located in the vicinity of the mobility edge (Figure 8a). The negative \(V_{\rm GS}\) applied during NBIS pushes the Fermi level (\(E_{\rm F}\)) toward the \(E_{\rm V}\), decreasing the ionized vacancies' formation enthalpy, \(\Delta H\), which is given by the term \(-q(E_{\rm F}-E_{\rm V})\), \(q\) being the defect charge state [25]. Creation of additional defects is also possible due to the decrease in \(\Delta H\). Note that in an oxygen-deficient material, the shift  
Figure 7: Amorphous indium-gallium zinc-oxide (a-IGZO) semiconductor thickness (\(t_{\rm IGZO}\)) dependency of total trap density (\(N_{\rm T}=N_{\rm int}+t_{\rm IGZO}\times N_{\rm bulk}\)), where \(N_{\rm int}\) is the area density of the shallow traps close to the Fermi level (\(E_{\rm F}\)) at the gate-insulator/semiconductor interface and \(N_{\rm bulk}\) is the volume density of shallow traps in the bulk of the a-IGZO semiconductor. of \(E_{\rm F}\) toward \(E_{\rm V}\) may result in a negative \(\Delta H\) and spontaneous defect formation [25]. The energy distribution of the defects formed after application of NBIS should be very close to \(E_{\rm F}\) because the SS increases after application of NBIS (Figure 5). Existing and created ionized oxygen vacancies can also drift toward the gate insulator interface, contributing to a buildup of a positive charge at the gate-insulator/active-layer interface. Note that trapped/accumulated positive charge may provide a positive bias effect on the n-type a-IGZO channel, consistent with the negative \(\Delta V_{\rm ON}\) after NBIS. Figure 8: Effect of negative-gate bias stress combined with visible light illumination (NBIS). (**a**–**e**) Thin-film transistor (TFT) current-voltage (\(I\)-\(V\)) characteristics before and after NBIS for varying amorphous indium-gallium zinc-oxide thickness (\(t_{\rm IGZO}\)). Turn-on voltage (\(V_{\rm ON}\)) dependence on NBIS Time. Channel width \(W=2000\) μm and channel length \(L=10\) μm. The NBIS-induced negative turn-on voltage (\(V_{\rm ON}\)) shift increases with increasing \(t_{\rm IGZO}\). NBIS is accomplished by holding the TFT gate voltage at \(-20\) V for 10,000 s, while biasing the source and drain electrodes at zero volts, under white light illumination (9000 nit). When the neutral \(V_{\text{O}}\) states lose electrons to form the ionized \(V_{\text{O}}{}^{+}\) and \(V_{\text{O}}{}^{2+}\), they undergo outward structural relaxation, suggesting higher energy for \(V_{\text{O}}{}^{+}\) and \(V_{\text{O}}{}^{2+}\) compared to the neutral state (\(V_{\text{O}}\)). Therefore, shallow states increase, while the deep states decrease, as illustrated in Figure 8b-e. The magnitude of the NBIS-induced negative \(\Delta V_{\text{TH}}\) thus depends on the total number of ionized donors, because events occurring at the semiconductor/gate-insulator interface govern TFT operation. Figure 9 shows the DOS extracted from the combined analysis of \(I\)-\(V\) and \(C\)-\(V\) characteristics before and after application of NBIS. After NBIS, the DOS shows the formation of a broad peak between \(E\)-\(E_{\text{C}}\) of 0.2 eV and 1.0 eV, which increases in height and width with increasing \(t_{\text{IGZO}}\).