In this work, we have conducted a systematic investigation of leakage current and electrical breakdown of plasma enhanced chemical vapor deposited (PECVD) silicon nitride, both for planar films and deposited films on the vertical sidewall for the application of the vertical thin film transistors. The thickness evolution of physical properties and electrical characteristics of silicon nitride films in the range of 50 to 300 nm are investigated. Electrical breakdown strength for 150-300nm thick films was approximately 7 MV/cm, whereas the value dropped to ~3MV/cm for 50nm thick films deposited under the same process conditions. It is shown that the early failure of the thin nitride is accompanied by the increase of the pinholes number. For the vertical thin film transistors, the experimental result shows the reliability and leakage current of the gate dielectric depends on the step coverage of the silicon nitride film on the vertical sidewall.