We compare the turn-on voltage, P-I, and EL responses between the MISLEDs made by Si-rich SiNx and SiOx films. Active layer thickness enlarged from 120 to 360 nm is achieved by lengthening deposition time from 10 to 30 min, which inevitably increases the forward turn-on voltage from 3 to 41 V. We observe that the forward turn-on voltage of SiNx based MISLED is only 10.43 V and that of SiOx based one is 69 V with the same film thickness of 100 nm. The tunneling-based carrier transport mechanism is dominated due to the exponential like V-I behavior, while the tunneling probability is strongly dependent on the height of the barriers between metal/dielectric and dielectric/nc-Si matrices. The P-I slope of SiNx and SiOx based MISLEDs are 1.6 and 115.2 mW/A, respectively. The SiNx MISLED reveals threshold current and voltage of only 4 A and 12 V due to lower barrier height of both ITO/SiNx and SiNx/nc-Si, whereas the threshold current and voltage of SiOx based MISLED are 400 A and 78 V, respectively. In comparison, the higher tunneling current through the SiNx MISLED fails to promote the larger external quantum efficiency of the MISLED, indicating that such lower barriers are not beneficial to the confinement of tunneling carriers and the enhancement of light-emission efficiency.