New transport and low‐frequency current oscillation phenomena have been observed in a photoexcited Bi12GeO20single crystal in the temperature range 77–110 K. These phenomena can be explained on the basis of the relaxation semiconductor regime where the dielectric relaxation time exceeds the minority‐carrier lifetime. Experiments strongly suggest that sublinear dependencies in the current‐voltage characteristic and the current oscillation in the higher bias range are general features of the relaxation regime.