Dimension dependent immunity of X-ray irradiation on low-temperature polycrystalline-silicon TFTs

Abstract

Typically, each element in a large-area flat-panel X-ray image sensor consists of a photodetector and amorphous silicon (a-Si) thin-film transistor (TFT) switches. In order to reduce noise, increase sensor dynamic range, and increase carrying capacity, the low-temperature polycrystalline-silicon (LTPS) TFTs have been proposed as a candidate to replace the a-Si TFTs. However, there are concerns regarding the impact of X-ray radiation in LTPS-TFTs, and several studies have been conducted to inquire into the same. In this paper, we show that LTPS TFTs with small channel length (<2 µm) are almost immune to X-ray radiation.

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

“…Both types of LTPS TFTs show negative shift of threshold voltage (V th ) and degradation of subthreshold swing (SS) under same dose with increase of irradiation dose while the field effect mobility keeps fairly well, as shown in the inset of figure 2(a). These results are consistent to the other reports[11][12][13], in which the consequences were primarily attributed to the trapping holes in the gate oxide.In more detail, electron-hole pairs are generated in the gate oxide during irradiation, and some holes will be trapped and formed positive oxide trapped charges. Thus the accumulation of positive trapped charges results in a negative shift of V th[14][15][16].…”

“…The drain current gate voltage (I DS -V GS ) characteristics of the LTPS TFTs were measured in sweeping voltage by Keithley 4200 semiconductor parametric analyzer before and after the irradiation procedure. [11][12][13], in which the consequences were primarily attributed to the trapping holes in the gate oxide.…”

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

“…X-ray radiography has been widely used for a broad range of applications such as nondestructive inspection in industry and disease diagnosis in hospitals. − In X-ray diagnosis systems, many efforts have been made to achieve high-performance digital X-ray detectors with high resolution and stability to generate optimized image quality for accurate diagnosis. − A thin-film transistor (TFT) panel, which is one of the components of a digital X-ray detector, plays an important role in implementing high-quality images from the incoming electric signal produced by photodiodes and scintillators. Thus, developing highly stable TFT devices that can sustain their electrical performance under X-ray irradiation is crucial. ,,, In previous reports, several groups utilized TFTs with various channel materials, such as amorphous silicon (a-Si) TFTs ,− and low-temperature polycrystalline silicon (LTPS) TFTs, − for backplane pixels of digital X-ray detectors. 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.…”

The time response for the low-temperature poly-silicon thin-film transistors to x-ray irradiation pulse

“…Figure 2 shows the time response of drain current change (∆I D ) under the x-ray irradiation pulse with a period of 20 min and duty ratio of 50%. We can see that, with x-ray irradiation, the current gradually increases due to the threshold voltage shift (V th ) of LTPS TFT [16,17]; otherwise, without x-ray irradiation, the current decreases back.…”