A semiconductor light emitting device comprises a substrate, an n-type semiconductor layer formed on the substrate, a p-type semiconductor layer formed on a portion of a surface of the n-type semiconductor layer, an electrode for applying a reverse biasing voltage to the PN junction to cause an avalanche breakdown and an area formed in a portion of the PN junction. The p-type semiconductor layers forms a planar type PN junction with the n-type semiconductor layer. The area formed in a portion of the PN junction has a lower break down voltage than that of other area.