Abstract The penetration depth of low-energy electrons into ZnS was investigated using the device structure combined with CaS : Ce emitting layer and ZnS buffer layer. The luminance of the film with ZnS was different from that of the film without a ZnS buffer layer. The properties depend on the thickness of ZnS and the energy of incident electrons at the film surface. The penetration depths obtained in this experiment were larger than the depth calculated by Feldman's equation. The difference between the experimental result and the analysis of Feldman is caused by a difference between the anode voltage and the actual excitation voltage.