The terminal voltage of the PN junction forms a certain potential barrier. When a positive bias voltage is applied, the barrier drops and most carriers in the P and N regions spread to each other. Since the mobility of the Electron mobility is much higher than that of the hole, a large number of electrons diffuse into the p region, constituting an injection of a small number of carriers into the p region. These electrons compound with holes in the valence band, and the resulting energy is released as light energy. This is how the PN junction emits light.