Method of manufacturing a planar semiconductor device having a guard ring structure

A method of manufacturing a class of semiconductor devices of the planar type comprising a substrate of a first conductivity type having a high doping concentration c.sub.o, a first epitaxial surface layer of said first conductivity type and having a lower doping concentration c<c.sub.o, a surface adjacent zone of the second conductivity type opposite to the first conductivity type and forming with said epitaxial surface layer a main planar pn junction having, over the major part of its area, a junction depth x.sub.j, and a structure of floating guard rings of said second conductivity type, between said substrate and said first epitaxial layer there being provided a second epitaxial layer having a doping concentration c' such that c.sub.o >c'>c, each device of the class being defined by a particular maximum operating voltage V, all the devices of the class having the same structure of guard rings, said structure of guard rings having been determined for a reference device not belonging to the class, constituted by a single epitaxial layer having a thickness e.sub.o, a doping concentration c and a maximum operating voltage V.sub.m exceeding the maximum operating voltage V of all the devices of the class, the sum of the thickness e of the epitaxial surface layer and the thickness e' of the second epitaxial layer of all the devices of the class being at most equal to the thickness e.sub.o, said method comprising the steps of calculating, for said reference device, in the case of a planar structure, the thickness e.sub.o and the concentration c of the single epitaxial layer in such a manner that the breakdown voltage through the layer is at least equal to the maximum operating voltage V.sub.m and the resistance R.sub.on in the "on" state is a minimum, providing the particular structure of floating guard rings associated with the main pn junction having a depth x.sub.j, and providing, for each component of the class, the epitaxial surface layer and the second epitaxial layer of thicknesses e and e', respectively, while maintaining e+e'=e.sub.o, and providing said second epitaxial layer with said doping concentration c' such that the breakdown voltage is at least equal to the maximum operating voltage V and the resistance R.sub.on in the "on" state is a minimum.