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.