In this thesis, the major discussion is the small angle method in PN junction. The small angle model comes from 90° junction type of a full circular junction. For breakdown analysis, the result from the small angle method is the same as that from the original 90°junction method. The small angle method is verified for its validity by the test with different radii and different angles. The simulation result is not accepted if the angle is too small and the number of square meshes is insufficient. Finally, the comparison on the breakdown voltage and the CPU time between two methods shows that the small angle method is excellent in reducing the CPU time without increasing the programming complexity.