AbstractThe versatile algorithms of two‐dimensional device simulation programs are studied in detail. We have separated the device simulator which is independent of the device structure and the user's analysis purpose, and the preprocessor that specifically deals with user interface. Moreover, the algorithms of the device simulator become the discretization by information obtained at the nodes only, electrode current definition at the nodes of electrodes, initial guess using two‐dimensional numerical depletion approximation, and the iterative operation of the device simulation by step design. Using these algorithms we developed a two‐dimensional device simulator UNISAS, and confirmed the validity of the versatile algorithm using an example of NMOS. Step design is used for the analysis of the avalanche phenomenon of NMOS. Solving for the two carriers, we have explained easily the flow of electrons and holes in the avalanche breakdown phenomenon. Moreover, by changing only input data, we have easily analyzed the five‐ga