Photosensitivity of a metal-insulator-semiconductor field-effect transistor based on PbSnTe:In film with a composition close to the bands inversion

A prototype of a metal-insulator-semiconductor field-effect transistor based on PbSnTe:In/(111)BaF$_2$ film with an Al$_2$O$_3$ gate dielectric was designed for the first time. With the gate voltage applied in the range -7.7 $<U_{gate}<$ +7.7 V the relative modulation in the drain-source current $\Delta I_{ds}/I_{ds}$ attained near five-fold change at $T$ = 4.2 K. When illuminated with relatively low ($\sim$ 100 photon/s) fluxes, negative photoconductivity was detected accompanied with a decrease in $I_{ds}$ by $\sim$ 10$^4$ times and a simultaneous decrease in $\Delta I_{ds}$ by $\sim$ 10$^3$ times or even more. The estimated detectivity was about $\sim$ 7 $\cdot$ 10$^{16}$ cm $\cdot$ Hz$^{0.5}$ $\cdot$ W$^{-1}$ at a wavelength $\lambda$ about 25 micron with the accumulation time about 0.5 s. A qualitative model is discussed which assumes the existence of deep traps and a photo-capacitance effect.