A method is described for locating the interface boundary between the space-charge-neutral regions in semiconductor devices. This is based on a two-dimensional S.O.R. solution of Poisson's equation for complete depletion within the space-charge region and Laplace's equation within the neutral region. Applications of this 2-dimensional analytical technique are considered, and particular reference is made to the junction field-effect transistor and the metal-oxide-silicon transistor.