We have experimentally and theoretically investigated negative differential conductance regions in the current-voltage characteristic of undoped semiconductor superlattice surrounded by barriers and illuminated by laser light. The negative differential conductances are nonlinear and show bistable behavior as a function of the applied voltage. These phenomena are quantitatively described by a self-consistent analysis of the field-dependent Wannier-Stark absorption and accumulation of photocarriers in the presence of barriers at the superlattice borders.