A diffused junction capacitor having two P N junctions, one in the semiconductor substrate and one in an epitaxial layer thereon and exhibiting high capacitance per unit area. A method for forming such a capacitor makes use of the fact that the outdiffusion rate for boron is much faster than the outdiffusion rate for arsenic, whereby, for instance, boron and arsenic diffused into the surface of a semiconductor wafer can, after the growth of an N epitaxial layer, be diffused into the N epitaxial layer. Since the boron outdiffuses much faster, it will cover a larger area than the arsenic outdiffusion. This will, in turn, result in two P -N junctions, one in the substrate and one in the N epitaxial layer.