Metal-nitride-silicon capacitors were used to explore the stability of the electrical properties of silicon nitride films of various stoichiometries. Silicon-rich silicon nitride films were found to display a large and symmetric hysteresis loop in the capacitance-voltage curve, a large flat-band voltage shift approximately symmetric with respect to the polarity of the voltage under bias temperature stress, and a current density-voltage characteristic which is approximately symmetric with respect to polarity. It is argued that in these silicon-rich silicon nitride films the dominant defects are silicon dangling bonds which can trap either electrons or holes. A different class of behavior was observed, however, for nitrogen-rich silicon nitride films deposited by plasma enhanced chemical vapor deposition. These films have a much smaller and asymmetric hysteresis loop in the capacitance-voltage curve, an asymmetric flat-band voltage shift under bias temperature stress, and an asymmetric current density-voltage characteristic. In addition, the breakdown electric field was found to be higher for these films for positive than negative gate biases. Our proposed explanation for these observations is that silicon dangling bonds, which can trap either electrons or holes, are greatly reduced in nitrogen-rich films and the dominant residual traps are hole traps.