Internal Compensation Pixel Circuit for X-Ray Detector with on a-Igzo Thin-Film Transistors and Perovskite Materials
Authors: Lee Jang Hoo , Kim Youngjin , Park Hye Kang , Moon Seung Jae , Wei Lei , Bae Byung Seong 
Published: December 2024

Flat Panel X-ray Detectors (FPXDs) have various applications across diverse fields, including medical imaging, industrial inspection, and security checks. Based on their pixel-driving mechanisms, these detectors are divided into Passive Pixel Sensors (PPS) and Active Pixel Sensors (APS). While PPSs feature a simple structure, converting signals to voltage outside of the pixel, they encounter drawbacks such as crosstalk and high noise levels. To solve these issues, APSs were introduced [1]. Despite their more intricate configuration, requiring much more transistors than PPSs, APSs offer advantages in overcoming the drawbacks mentioned above. X-ray detection methods are divided into indirect conversion and direct conversion based on how X-ray energy is transformed into electrical signals. Indirect conversion methods utilize a scintillator to convert X-ray energy into light, which is then converted back into electrical signals through transistors. However, this approach involves a trade-off between the thickness of the scintillator and spatial resolution. While thicker scintillators enhance X-ray absorption efficiency, they also lead to decreased spatial resolution due to optical crosstalk. In contrast, direct conversion methods employ a photoconductor to directly convert X-ray energy into electrical signals.
Modern X-ray imaging technology demands high resolution and sensitivity at low dose rates. To meet these demands, research focuses on enhancing the performance of photoconductors and designing pixel circuits [2, 3]. The conventional APS n-type pixel circuit is composed of three transistors and one storage capacitor. This design minimizes the area of instances in the pixel and enables a wider light-collecting area. However, shifts in the threshold voltage of the amplification transistor can cause image distortion. Typically, this issue is addressed using external compensation. Nevertheless, external compensation methods have the drawback of requiring reconfiguration if the TFT model changes and need complex ICs.
The proposed pixel circuit in this paper has internal compensation method. This method could simplify external ICs for external compensation and memory used for storing the threshold voltages. The RPI LEVEL =35 oxide TFT model was employed for simulations to compare and validate the process of compensating threshold voltage in the proposed circuit. Figure 1 depicts simulation results comparing the conventional pixel circuit with the proposed design. While the conventional pixel circuit showed an error rate exceeding 20% for ±1V threshold voltage shifts, the proposed circuit maintained an error rate below 1%, demonstrating successful threshold voltage compensation. Future research will focus on fabricating pixel arrays and conducting measurements to verify whether uniform images can be obtained under identical X-ray doses, validating the efficacy of threshold voltage compensation. These efforts are crucial for enhancing image quality in X-ray detectors.

Acknowledgement

This work was supported under the framework of international cooperation program managed by the National Research Foundation of Korea (Project No., FY2023).

References

[1] Fan, C. L., Lin, W. Y. and Liu, S. Y., “An Active-Matrix Organic Light-Emitting Diode Pixel Circuit Featuring Mobility Compensation for Portable Applications.”, Micromachines., Vol. 14(9), p. 1785 (2023).
[2] Scott, C. C., Farrier, M., Li, Y., Laxer, S., Ravi, P., Kenesei, P., Wojcik, M. J., Miceli, A. and Karim, K. S., “High-energy micrometre-scale pixel direct conversion X-ray detector”, J. Synchrotron Rad., Vol. 28, pp. 1081-1089 (2021).
[3] Wang, K., Ou, H., and Chen, J., "Dual-gate photosensitive thin-film transistor-based active pixel sensor for indirect-conversion X-ray imaging.", IEEE Trans. Electron Devices, Vol. 62(9), pp. 2894-2899 (2015).

Figure 1