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

Abstract

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 °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.

Highly Stable Oxide Thin-Film Transistor-Based Complementary Logic Circuits under X-ray Irradiation

“…Among them, a-Si TFTs have been widely used for flat-panel X-ray detectors; however, they suffer from low carrier mobility, thereby necessitating a large TFT with a large parasitic data line capacitance, which increases electronic noise and reduces the pixel fill factor. , The carrier mobility of the LTPS TFT is approximately 100 times higher than that of the a-Si TFTs; therefore, their use increases the pixel fill factor. However, LTPS TFTs exhibited a large degradation of the device performance in terms of threshold voltage ( V TH ) and subthreshold swing ( SS ) even under X-ray irradiation with low total doses of 0.87–4.35 Gy ascribed to the permanent damage to the crystalline structure of LTPS induced by X-ray radiation. − Recently, oxide TFTs have attracted considerable attention as backplanes for various flat-panel displays owing to their high carrier mobility, low processing temperature, and high uniformity. − Till date, only a few studies have examined the radiation tolerance of n-type oxide TFTs under X-ray irradiation. − However, the research results on the effects of X-ray irradiation on p-type oxide TFTs and oxide-TFT-based complementary metal–oxide–semiconductor (CMOS) logic circuits have not yet been reported. CMOS logic circuits using both n-type and p-type transistors exhibit rail-to-rail output voltage swings and high noise margins; , therefore, they can enhance the electrical performance of digital systems compared with n-type logic circuits.…”

Effects of X-ray Irradiation on the Noise Behavior of Amorphous Indium-Gallium-Zinc-Oxide TFTs

“…Therefore, the accumulation of positive trapped charges results in a negative shift of V th . 15,16 Despite the obvious change in V th , the degradation of S.S. is insignificant, which means that interface states do not generate at the channel/insulator interface owing to X-ray irradiation.…”