The goal of this investigation is to determine the theoretical limit of the efficiency of planar junction termination. In order to gain general statements, the p-n junction geometry is simplified, and the total two-dimensional electrostatic problem is subdivided into two tasks, which are executed subsequently. At first, the ideal potential distribution along the surface minimizing the ionization integrals within the semiconductor bulk is calculated by an optimization procedure. Afterwards, ideal structures concerning the variation of lateral doping concentration and the geometry of field plates are determined, assuring the optimum potential distribution inside the semiconductor.