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When bias is applied to a mercuric iodide detector with planar contacts, placed in a light-tight enclosure, the leakage
current initially increases with increasing bias, as can be expected for a semiconductor detector. At some value of the
bias however, the current starts to decrease sharply, and reaches very low values when the bias is further increased. This
phenomenon of negative differential conductivity has been observed previously in high-bandgap semiconductors with
deep electron and hole traps. This combination of traps can provide an effective recombination channel which reduces
the number of free electrons and holes at high electric fields.
It is suggested in this paper that mercuric iodide contains a set of traps with the required properties in the form of doubly
ionized mercury interstitials and iodine vacancies. These traps are incorporated in the single crystals by the processing
methods used to obtain the pure material and by the presence of mercurous iodide in the synthesized material.
The main advantage of this behavior is the apparent low leakage current of the detectors at high fields (1000 V/mm) at
ambient temperatures. The disadvantage is that carriers generated by ionizing radiation will also be subject to this
recombination process, so that complete charge collection may be impaired. Additional details of this hypothesis will be
presented for discussion.
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