We introduce three different geometries of gated field emitter devices and their simulation results. Each geometry is defined as tip-on-post, flat cathode, and bottleneck, respectively. The optimized cathode height and width of tip-on-post with best emission characteristics are reported. The calculated electric field and emission current are compared at different aspect ratios of tip-on-post cathode. We demonstrate the feasibility of a diamond thin film for triode geometry application with less leakage current. The calculation of the electric field of a flat cathode is determined by variable geometric parameters. Additionally, the feasibility of the bottleneck type of field emission cathode is studied in this simulation.