Asymmetry Degradation Behavior Under Forward and Backward Overlap Dynamical Stress in Large-Size Amorphous InGaZnO TFT

In this work, the degradation behaviors for large size amorphous InGaZnO (a-IGZO) thin-film transistors (TFTs) under realistic waveforms in active driving circuit are revealed, and the corresponding degradation mechanisms are analyzed. When the back half of the drain pulse overlaps with the first half of the gate pulse (forward overlap), self-heating effect becomes the key factor for positive threshold voltage shift. When the front half of the drain pulse overlaps with the back half of the gate pulse (backward overlap), hot carrier effect takes the dominating role over self-heating. Under this circumstance, hot carrier effect could generate deep states, which are hard to recover, causing both positive voltage shift and ON-current deterioration. However, slightly increasing the pulse frequency leads to self-heating effect taking over again in backward overlap. To understand this phenomenon, a competition model between self-heating and hot carrier effect is proposed, which indicates the significant role played by the large W/L ratio of the a-IGZO driving transistor. The model might be suitable for degradation analysis of a-IGZO TFTs in gate driven on array (GOA) driving circuits application.