The electronic properties of silicon nitride/hydrogenated amorphous silicon (SiN/a-Si:H) interfaces are studied with complementary techniques: quasistatic capacitance measurements achieved on c-Si/SiN/a-Si:H/Al MIS structures, and dark conductivity, steady-state photoconductivity and modulated photocurrent ((MPC) experiments performed on glass/a-Si:H and glass/SiN/a-Si:H samples fitted with two coplanar Al electrodes, using the same SiN and aSi:H layers as in the MIS structures. Results of bias annealing experiments on the: MIS structures are explained in the framework of the defect-pool model taking account of a fixed positive charge in the insulator, which should yield a slight electron accumulation in the a-Si:H close to the SiN/a-Si:H interface under zero bias equilibrium conditions. This electron accumulation is clearly put into evidence from the experiments carried out on the coplanar samples, where we observe that the conductivities in the dark and under illumination are much higher in presence of the bottom SIN layer. The SiN layer also induces a significant decrease of the density of states above the Fermi level determined from MPC, which also confirms the changes in the defect density stated by the capacitance measurements and in agreement with the defect-pool model predictions.