In this thesis, we try to derive the analytical equations of the potential, the electric field, and the depletion width in a three-dimensional spherical pn junction. The analytical equations for spherical pn junction is more complex than that for 1D pn junction. However, the depletion width can be obtained by a simple calculation. We compare the analytical results with the results from 3D numerical simulation. With these analytical equations, we can quickly investigate the dependence of the depletion width, the maximum electric field, and the breakdown voltage on the radius of a spherical pn junction. Similarly, the dependence of the depletion width, the maximum electric field, and the breakdown voltage on the doping of a spherical pn junction can be obtained quickly.