Avalanche multiplication in amorphous selenium and its utilization in imaging

Amorphous selenium (a-Se) is a well known photoconductor that is currently used in X-ray image detectors and HARP video tubes. Recent advances have made it practical to operate a-Se at extremely high electric fields F, where avalanche multiplication occurs. At sufficiently high fields, the effective quantum efficiency (η∗) (or the overall yield) of the photoconductor can be increased several orders of magnitude above unity and renders avalanche a-Se photoconductors a prospective alternative to vacuum photomultiplier tubes (PMTs) and silicon avalanche photodiodes (APDs). In this work we report our study of η∗ and charge transport for a-Se avalanche photoconductors with different photoconductive layer thicknesses in wide range of F. Our study shows that a-Se is able to produce a gain of ∼1000 with a rise time of ∼1 ns, both of which clearly point to the potential (or realized) use of this photoconductor in a variety of imaging applications. Furthermore, our work supports the validity of the so-called modified Lucky drift model to explain the nature of impact ionization and avalanche in this material.