Current conduction mechanisms have been studied for three representative films, namely, silicon-rich, nearly stoichiometric and nitrogen-rich silicon nitride films, prepared by rf glow-discharge decomposition of silane and ammonia with nitrogen dilution. Ohmic conduction has been observed for all the films at low electric fields. The dominance of Poole-Frenkel conduction at intermediate fields and Fowler-Nordheim conduction at high fields has been observed both for the nitrogen-rich and the nearly stoichiometric films. However, for the silicon-rich films, the Poole-Frenkel conduction mechanism dominates both for the intermediate as well as the higher fields. This study indicates that the silicon-rich films have the highest density of traps and the nitrogen-rich films have the lowest, which may be ascribed to the effect of nitrogen dilution.