The interface state density of silicon metal‐oxide‐semiconductor (MOS) transistors operated at room, liquid‐nitrogen, and liquid‐helium temperatures is investigated using the conventional subthreshold slope technique. The magnitude of the interface state density found of the order of 1013–1014/eV cm2at liquid‐helium temperature, suggests that such states correspond to the localized states situated in the band tails below the mobility edge of the two‐dimensional subband. Moreover, the interface state densities found by the subthreshold slope have been confirmed using the dynamic transconductance technique.