[en] Dependence of the short-circuit photocurrent on the voltage V_g applied to the gate of the 6H-SiC planar field-effect transistor is studied. The negative differential photoconductivity appeared at a certain value of V_g; the parameters of this photoconductivity corresponded to those of the Wannier-Stark ladders in the natural 6H-SiC super lattice. At the same value of V_g, a fairly abrupt decrease to zero of the source-drain current I_sd is observed, which is indicative of cutoff at the voltage that is much lower than the expected cutoff voltage for this structure. The effect is attributed to a decrease in mobility in the mode of the Wannier-Stark ladders, a decrease in the rate of ionization of the donor atoms, and a reduction in the screening of the field