An electronic device is provided with a display such as a liquid crystal display mounted in an electronic device housing. The display may be provided with a backlight unit. The backlight unit may include radioluminescent backlight structures. The radioluminescent backlight structures include an ionizing radiation source that produces beta particles or other ionizing radiation. The radioluminescent backlight structures also include a phosphor that produces light in response to being struck by the ionizing radiation. The light produced by the phosphor passes through liquid crystal display layers and serves as backlight for the display. Photoluminescent material and light-emitting diodes may also be used to produce backlight for the display. The display backlight may have a light guide plate that is provided with light from the radioluminescent light source or other light source structures.