P‐30: Thermally Activated and Field‐Enhanced Diffusion of Dopants in a‐InGaZnO TFTs Under Circuit Operations and Correlation to Device Stabilities

Abstract

The spatial distribution of the donor dopant and the source‐drain resistance (RSD) in oxide Thin Film Transistor (TFT) for display backplane is usually considered to be determined by the device structure and the process. This work shows that the dopant concentration of lateral distribution of thermal equilibrium carrier concentration n0(y) may vary dynamically depending on the operation conditions during circuit operation and thermally‐activated and field‐enhanced diffusion of dopants in InGaZnO (IGZO) was found as a physical cause. In actual Organic Light‐Emitting Diode (OLED) backplane circuit operation, the reliability degradation conditions of IGZO TFTs were summarized into three conditions: Negative Bias Temperature Stress (NBTS), Constant Voltage Stress (CVS), and Drain‐to‐Source Voltage (VDS)‐sweep, and as a result of analyzing TFT device reliability, dopant diffusion occurred as the temperature increased, and the lateral field increased, resulting in RSD increase and channel resistance decrease. This causes degradation of device characteristics accumulated during circuit operation and shows various degradation patterns. It is connected to the existing Positive Bias Stress (PBS) and Negative Bias Stress (NBS) degradation mechanisms and other hot carrier degradation and self‐heating. In particular, it has been reported that dopant diffusion and source‐drain series resistance (RSD) change occur under high VGS, high VDS, and short channel conditions, resulting in permanent failure of the IGZO TFT channel by forming positive feedback.