1. A method of manufacturing a semiconductor device comprising the steps of providing a semiconductor layer with a main surface, forming a first semiconductor region making a PN junction with said semiconductor layer, said PN junction reaching said main surface, forming an insulating layer having a hole for exposing at least a part of said first semiconductor region on said main surface, introducing impurities having the same conductivity type as said first semiconductor region in said semiconductor layer through said hole of said insulating layer to form a second semiconductor region having a depth which is shallower than the depth of said first semiconductor region, said second semiconductor region making contact with said first semiconductor region, and forming a metal-semiconductor alloy layer electrically connected to said second semiconductor region.