In this thesis, it transforms Poisson’s equation, electron continuity equation and hole continuity equation to equivalent circuit model for 1D and 2D numerical device simulation. This thesis uses the equivalent circuit model to simulate PNPN device. Firstly, this thesis builds 1D PNPN to quickly find the design parameters such as length and doping due to the fast calculation in 1D simulation. Secondly, this thesis develops quasi-2D simulation because the quasi-2D is very close to 1D simulation and it can be used to develop the complete 2D simulation for 2D PNPN design. This thesis compares the qusi-2D simulation result with that of the 1D simulation. Finally, this thesis develops the complete 2D modeling for 2D PNPN simulation, and it proposes some good suggestions for reducing the pain suffering during 2D PNPN simulation.