A Quantitative Theory of 1/f Type Noise Due to Interface States in Thermally Oxidized Silicon

It has long been recognized that states at the Si-Si0 2 interface which exchange charge with the silicon can give rise to 1// type noise. Recently, Sah and Hielscher 1 have shown by experiment that the 1/f noise of a metal -Si0 2 -silicon (MOS) capacitor is directly related to interface state density and capture conductance over the energy gap. Random capture and emission of carriers by interface states results in fluctuations of trapped charge. In an MOS capacitor, these charge fluctuations cause random changes in admittance constituting noise. These charge fluctuations can be calculated from the dispersion of interface state time constants. A major obstacle to a quantitative theory of 1/f type noise arising from interface states has been the lack 2019 2020 T H E BELL SYSTEM T E C H N I C A L J O U R N A L , NOVEMBER 19G7 of an experimentally established mechanism for interface state time constant dispersion. This obstacle has recently been removed. With the MIS conductance technique, 2 - 3 - 4 accurate small-signal measurements have been made of interface state density and capture conductance over the middle half of the energy gap in the Si-Si0 2 system. A large interface state time constant dispersion was observed in the depletion and accumulation regions. An explanation which quantitatively fits these measurements essentially without any arbitrary adjustable parameters is that the dispersion arises from a random distribution of surface potential over the plane of the interface.