Amorphous and Polycrystalline Photoconductors for Direct Conversion Flat Panel X-Ray Image Sensors

Sensors

Safa Kasap³  Joel B. Frey³  George Belev³  Olivier Tousignant⁵  Habib Mani⁵  Jonathan Greenspan⁵  Luc Laperriere⁵  Oleksandr Bubon²  Alla Reznik²  Giovanni DeCrescenzo⁴  Karim S. Karim¹ 

[1] Department of Electrical and Computer Engineering, University of Waterloo, Waterloo, ON, N2L 3G1, Canada; E-Mail:;Department of Physics, Lakehead University, 955 Oliver Road, Thunder Bay, ON, P7B 5E1, Canada; E-Mails:;Department of Electrical and Computer Engineering, University of Saskatchewan, Saskatoon, SK, S7N 5A9, Canada; E-Mails:;Thunder Bay Regional Research Institute, 980 Oliver Road, Thunder Bay, ON, P7B 6V4, Canada; E-Mails:;Anrad Corporation, 4950 rue Lévy, Saint-Laurent, QC, H4R 2P1, Canada; E-Mails:

关键词: x-ray image sensor;    detector;    direct conversion;    x-ray photoconductor;   

DOI  :  10.3390/s110505112

来源: mdpi

【 摘 要 】

In the last ten to fifteen years there has been much research in using amorphous and polycrystalline semiconductors as x-ray photoconductors in various x-ray image sensor applications, most notably in flat panel x-ray imagers (FPXIs). We first outline the essential requirements for an ideal large area photoconductor for use in a FPXI, and discuss how some of the current amorphous and polycrystalline semiconductors fulfill these requirements. At present, only stabilized amorphous selenium (doped and alloyed a-Se) has been commercialized, and FPXIs based on a-Se are particularly suitable for mammography, operating at the ideal limit of high detective quantum efficiency (DQE). Further, these FPXIs can also be used in real-time, and have already been used in such applications as tomosynthesis. We discuss some of the important attributes of amorphous and polycrystalline x-ray photoconductors such as their large area deposition ability, charge collection efficiency, x-ray sensitivity, DQE, modulation transfer function (MTF) and the importance of the dark current. We show the importance of charge trapping in limiting not only the sensitivity but also the resolution of these detectors. Limitations on the maximum acceptable dark current and the corresponding charge collection efficiency jointly impose a practical constraint that many photoconductors fail to satisfy. We discuss the case of a-Se in which the dark current was brought down by three orders of magnitude by the use of special blocking layers to satisfy the dark current constraint. There are also a number of polycrystalline photoconductors, HgI₂ and PbO being good examples, that show potential for commercialization in the same way that multilayer stabilized a-Se x-ray photoconductors were developed for commercial applications. We highlight the unique nature of avalanche multiplication in a-Se and how it has led to the development of the commercial HARP video-tube. An all solid state version of the HARP has been recently demonstrated with excellent avalanche gains; the latter is expected to lead to a number of novel imaging device applications that would be quantum noise limited. While passive pixel sensors use one TFT (thin film transistor) as a switch at the pixel, active pixel sensors (APSs) have two or more transistors and provide gain at the pixel level. The advantages of APS based x-ray imagers are also discussed with examples.

【 授权许可】

CC BY   
© 2011 by the authors; licensee MDPI, Basel, Switzerland.