Influence of air humidity on the low-frequency capacitance of silicon MOS structures with a nanoscale oxide layer

•A new mechanism of the low-frequency conductivity in MOS weakly inverted structures with nanoscale oxides is proposed.•H2O induced surface states make the main contribution to the total structure capacitance.•Surface state capacitance depends linearly on air humidity.•Reasons for the observed linearity of the structure capacitance are outlined. The influence of air humidity on the low-frequency capacitance of Mо - SiO2 - NN+Si(111) structures with a donor concentration of 2∙1021 m−3 in the N silicon layer and a thin porous oxide, about 5 nm thick, has been investigated. It has been established that in weakly inverted structures the capacitance of surface states at the silicon - oxide interface to a great extent determines the structures low-frequency capacitance. Largely, these states are induced by water molecules adsorbed by a thin porous oxide layer from the air. The recharging of the surface states is carried out by minority charge carriers and is determined by the generation-recombination processes in the silicon space charge region. This allows the surface states capacitance, as well as the total structure capacitance, to linearly depend on the humidity of the surrounding gas medium. The obtained expression for calculation of the low-frequency capacitance of the studied structures adequately describes the experimental dependences and serves as a proof of the new low-frequency conductivity model proposed in the paper for such structures.