The saturation behavior of field emission strips is investigated by using a finite-difference time-domain (FDTD) particle-in-cell (PIC) simulation. The field-emission process is described quantum mechanically by the Fowler-Nordheim (FN) equation. Space charge effects are automatically included in the FDTD PIC simulations. The simulation results have confirmed that the saturation is caused by the space charge effects. A self-consistent parallel plane (SCPP) model has been developed for fitting the nonlinear FN plots of field emission strips. Within the framework of an effective work function approximation and energy conservation, the FN and Poisson's equations are solved self-consistently in the SCPP model. It is demonstrated that the nonlinear FN plots of the field emission strips can be fitted by the SCPP model very well. The electric properties of the field emission strips can be characterized by the effective work function and the effective emission area. In addition, the field emission of Spindt-type cathodes may be possibly fitted with the SCPP model as well as the field emission strips. (c) 2008 American Vacuum Society.