Conductance associated with interface states in MOS tunnel structures

Abstract An experimental study has been made of the MOS conductance associated with the interface states in the SiSiO 2 system, using as variables the oxide thickness, δ, and the measurement frequency. It is observed that, as δ is increased, a conductance that is independent of frequency (δ ≲ 15 A ) changes to one which is strongly frequency-dependent (δ ≳ 20 A ) . Direct tunnelling into the interface states is observed for δ = 20 A . These results are in agreement with theoretical predictions. It is found that the interface states equilibrate with the silicon bulk for δ ≳ 25 A , though the majority and minority carriers in the silicon surface region can only be regarded as in thermal equilibrium for δ ≳ 35 A . The conclusion is reached that, for the purpose of investigating interface states, the effect of direct tunnelling into the states does not provide a very promising tool.