This paper is dedicated to the investigation of stoichiometric defect semiconductors, allowing extremely high vacancy concentrations of 10<sup>21</sup> cm<sup>-3</sup>, determining their unusual physical properties. The material studied, Hg<sub>3</sub>In<sub>2</sub>Te<sub>6</sub>, 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, Hg<sub>3</sub>In<sub>2</sub>Te<sub>6</sub> 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 Hg<sub>3</sub>In<sub>2</sub>Te<sub>6</sub> at hν = 0.74 - 3.5 eV. For the sake of comparison, we are also presenting the investigation results concerning CdIn<sub>2</sub>Te<sub>4</sub> 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.