In order to study the role of aluminum and gold as the gate electrode material, we have deposited silicon nitride films onto p-type silicon substrates via r.f. glow-discharge deposition and have fabricated metal-insulator-semiconductor (MIS) devices on these films using aluminum or gold as the top electrode material. Considerable penetration of aluminum gate material into our silicon nitride films is observed. The resulting decrease of the effective thickness of the MIS devices have led to tunneling in these films. However, no appreciable penetration of gold gate material is observed. Our study demonstrates clearly that gold is superior to aluminum as the gate electrode material in MIS devices for being suitable even when poor quality silicon nitride films are used as insulator in MIS devices.