Amorphous and polycrystalline photoconductors for direct conversion flat panel x-ray image sensors

Area of Science:

• Materials Science
• Medical Imaging Physics
• Semiconductor Physics

Background:

• Amorphous and polycrystalline semiconductors are increasingly researched for x-ray photoconductors in flat panel x-ray imagers (FPXIs).
• Ideal FPXI photoconductors require specific properties for large-area applications, including high charge collection efficiency and low dark current.

Purpose of the Study:

• To review the requirements for ideal large-area x-ray photoconductors in FPXIs.
• To discuss the performance and limitations of current amorphous and polycrystalline semiconductor materials.
• To highlight advancements in amorphous selenium (a-Se) and explore potential of polycrystalline alternatives.

Main Methods:

• Review of essential requirements for x-ray photoconductors.
• Discussion of material properties: large area deposition, charge collection efficiency, sensitivity, DQE, MTF, and dark current.
• Analysis of charge trapping effects and dark current mitigation strategies.

Main Results:

• Stabilized amorphous selenium (a-Se) is commercialized and excels in mammography due to high detective quantum efficiency (DQE).
• Dark current in a-Se was reduced significantly using blocking layers, meeting critical constraints.
• Polycrystalline photoconductors like HgI(2) and PbO show commercialization potential.
• Avalanche multiplication in a-Se enabled the HARP video-tube, with solid-state versions promising quantum-noise-limited imaging.

Conclusions:

• Amorphous selenium (a-Se) is a leading material for FPXIs, particularly in mammography, with ongoing improvements in dark current reduction.
• Polycrystalline semiconductors offer promising alternatives for future x-ray imaging applications.
• Advancements in detector technology, including avalanche multiplication and active pixel sensors, are pushing the boundaries of x-ray imaging performance.