Negative threshold voltage shift for LTPS TFTs under x-ray irradiation and gate bias

In this paper, the behavior of the low-temperature polycrystalline-silicon (LTPS) thin film transistors (TFTs) during x-ray irradiation and gate bias voltage (V-G) simultaneously is analyzed. Both n-type and p-type LTPS TFTs show negative shifts of threshold voltage under same dose of x-ray irradiation, regardless of the V-G polarity, while the field effect mobility of n-type LTPS TFT keeps fairly well. The degradation of subthreshold swing is attributed to the interface states, which can be repaired by 300 degrees C annealing. A model is proposed to explain the results for different V-G, and verified by changing the thickness of the gate oxide. More irradiation-induced holes are trapped by the far defects owing to the electric field. This study can be helpful to develop more stable devices or circuits for the application of x-ray image sensors.