Current source using emitter region as base region isolation structure

A current source, comprising:

• a substrate;
• a base region of a first doping type, formed in the substrate;
• an emitter region of a second doping type, formed in the substrate and surrounding the base region, the emitter region comprising a deep-well portion and an extending portion, the deep-well portion situated beneath the base region, the extending portion laterally surrounding the base region, the extending portion joined at a bottom thereof to the deep-well portion, the extending portion being flush at a top thereof with a top surface of the substrate;
• a first collector region of the second doping type, formed in the base region in the substrate;
• at least one second collector region of the second doping type, formed in the base region in the substrate;
• a base contact region of the first doping type formed in the base region in the substrate, the base contact region and the first collector region being electrically connected;
• wherein each of the base contact region and the first collector region extends to the top surface of the substrate and is covered by a metal silicide so as to be electrically connected through the metal silicide.