An analytical model of the three-dimensional (3-D) effect due to the lateral radius of the main junction and width of the ring junction on voltage and edge peak field profiles for the planar junction with a single floating electrical field limiting ring structure have been proposed for the first time. From this model analysis, the influence on the voltage and edge peak field of the reverse voltage, 3-D factors such as lateral curvature of the main junction and width of the ring junction, space distance between the main junction and ring junction and junction depth are analyzed, and then the expressions for predicting the optimal space and maximum breakdown voltage art: also obtained. The analytical results are in agreement with the previous numerical analysis.