in a bipolar transistor , around the border line of the surface of a base region formed on a semiconductor substrate is formed a base electrode having a constant width of less than one micron and made of polycrystalline silicon . an island shaped emitter region is formed in the base region and an emitter electrode is formed on the surface of the emitter region . the emitter electrode is electrically isolated from the base electrode by an insulating film extending between the periphery of the emitter region and the base electrode .