A Study on the Degradation of In-Ga–Zn-O Thin-Film Transistors Under Current Stress by Local Variations in Density of States and Trapped Charge Distribution

Thin-film transistors using In-Ga-Zn-O (IGZO) semiconductors were evaluated under current stress by applying positive voltages to the gate and drain electrodes. Initially, the transfer characteristics exhibit identical threshold voltages (VT) when the source and drain electrodes are interchanged during measurement (forward and reverse VDS sweep). However, as stress time increases, larger shifts in VT are observed under forward VDS sweep than under reverse VDS sweep conditions. Subgap states analyses based on the photoresponse of capacitance-voltage (C-V) curves suggest that local annihilation of donor-like traps occurs near the drain electrode. Hump-like features are clearly observed in the C-V curves collected between the drain and gate electrodes, while they do not appear in the C-V data obtained between the source and the gate. Based on the above, a local charge trapping model is introduced in order to interpret the device degradation. In this model, the major carrier electrons are trapped more abundantly near the source electrode due to the presence of a Schottky junction between IGZO and the source/drain electrodes.