Analysis of a-InGaZnO TFT Threshold Voltage Instability and Mobility Boosting by Current Stress at a Cryogenic Temperature

Device characteristics of amorphous InGaZnO thin film transistors (a-IGZO TFTs) at a cryogenic temperature ( ${T}$ ) are analyzed including device instability after applying current stress. Threshold voltage instability ( $\Delta {V}_{\mathrm{T}}$ ) mechanisms are analyzed before and after applying stress in the two TFTs with different oxygen flow rates in the formation of a-IGZO active layers (O-poor and O-rich TFTs). $\Delta {V}_{\mathrm{T}}$ of the a-IGZO TFTs at 77K are reduced less than 50% of that at 300K, because charge trapping into the gate insulator stack and the donor creation in the active layer are T- dependent processes. After applying stress at 77K, the O-poor TFT has better stability than the O-rich TFT, but the O-rich TFT exhibits a mobility boosting ( $\times 1.7$ ) related to donor creation, which can increase the operating speed and reduce the operating voltage at a cryogenic ${T}$ operation.