In this paper a comparative analysis of various field emission triode micro- and nanostructures is based on the study of static differential parameters, measured both from experimental data and from simulation results as volt-ampere characteristics. The modelling of electron trajectories is based on a current function that is analogous to one used in hydrodynamics. Finite-element method over a non-uniform mesh is used for electric field calculations and algorithms in Matlab and Matlab PDE Toolbox are implemented. It is shown field emission micro-, nanotriode has differential parameters, which significantly differ from those characteristics of usual vacuum lamps based on thermionic emission.