This paper is dedicated to the investigation of stoichiometric defect semiconductors, allowing extremely high vacancy concentrations of 1021 cm-3, determining their unusual physical properties. The material studied, Hg3In2Te6, is of high interest for creation of efficient photonic structures: self-calibrating photodiodes, high-speed photodiodes, multi-element photodiodes with improved sensitivity, as well as optical filters for the spectral ranges of 2-28μm. Measurements of photoconductivity confirmed high sensitivity of this material in wide spectral ranges (λ=0.35-1.85 μm), including the areas of sensitivity for CdS, CdSe, GaAs, Si and Ge. In comparison with the latter, Hg3In2Te6 is characterized with the lowest melting temperature (983K) that allows reduction of energy consumption during the synthesis process. The experimental results prove high photoconductive quantum yield for Hg3In2Te6 at hν = 0.74 - 3.5 eV. For the sake of comparison, we are also presenting the investigation results concerning CdIn2Te4 crystals. In general, defect semiconductors are ideal for creation of surface-barrier structures and hetero-junctions because of low surface state concentration and atmospheric oxygen absorption rate.