We present the form of the electron concentration during the transition from a “perfect” square well to a system of “two separated heterojunctions”, in Selectively Doped Double Heterojunction structures. We also present a cartography of the surface Ns = Ns(Nd,Ds), that is, of the dependence of the sheet electron concentration, Ns, from the doping concentration, Nd and the spacer thickness, Ds. We project the iso-Ns curves on the (Nd,Ds) plane. The iso-Ns map allows us to choose the most suitable combination of the (Nd,Ds) values to obtain the desired Ns.