Dark currents in MNS capacitors are studied as a function of metal electrode material and insulator thickness. Dark currents sensitively reflect different electrode materials for thin (∼200 Å) nitride films. Thus, it is found that high-work-function metals increase conduction under metal positive bias by enhanced hole injection and low-work-function metals increase conduction under metal negative bias by enhanced electron injection. Similar polarity differences are observed betwen n-type and p-type degenerate Si substrates. These contact differences disappear as the nitride becomes thicker and the thickness of trapped space-charge layers near the contacts becomes small compared to the nitride thickness.