We have compared the current-voltage characteristics of silicon nitrides prepared from the two gas combinations N-2/NH3/SiH4 (N-2-SiNx) and He/NH3/SiH4 (He-SiNx) at temperatures between 100 and 350 degrees C. A downstream plasma enhanced chemical vapor deposition reactor with a non-ECR microwave plasma source has been used. While N-2-SiNx with reasonable electrical properties requires deposition temperatures of about 350 degrees C, the characteristics of He-SiNx even improve at decreasing process temperatures. Almost identical current-voltage characteristics are found for both N-2-SiNx and He-SiNx prepared at 350 degrees C exhibiting an Ohmic behavior at low fields and a Poole-Frenkel (PF) conduction at high fields. At a deposition temperature of 100 degrees C the He-SiNx with a dielectric strength of 1.13x10(7) V/cm and an onset field strength of the PF conduction of 6.9x10(6) V/cm is in contrast to the N-2-SiNx with a dielectric strength of 4.3x10(6) V/cm and a PF onset field strength of as low as 2x10(5) V/cm resulting in many orders of magnitude higher current flow. In order to find a correlation between the dielectric and the structural properties of the silicon nitrides several analyses are performed. The He-SiNx proves to be superior to the N-2-SiNx concerning refractive index, mass density and buffered HF etch rate at every deposition temperature and the deterioration with decreasing deposition temperature is weaker. All SiNx films are found to be nitrogen-rich at a N:Si ratio of 5:3. As expected, the hydrogen content increases with decreasing deposition temperature, being lower for the He-SiNx than for the N-2-SiNx. In all layers most of the hydrogen is bonded to the nitrogen atoms. A simple model is proposed that explains the difference between the two kinds of low-temperature SiNx by the microscopic void structure of the material.