Electron Mobility in Inversion and Accumulation Layers on Thermally Oxidized Silicon Surfaces

Accurate modeling of MOS devices requires quantitative knowledge of carrier mobilities in surface inversion and accumulation layers. Optimization of device structures and accurate circuit simulation, particularly as technologies push toward fundamental limits, necessitate an understanding of how impurity doping levels, oxide charge densities, process techniques, and applied electric fields affect carrier surface nobilities. It is the purpose of this paper to present an extensive set of experimental results on the behavior of electron surface mobility in thermally oxidized silicon structures. EmpiricaI equations are developed which allow the calculation of electron mobility under a wide variety of substrate, process, and electrical conditions. The experimental results are interpreted in terms of the dominant physical mechanisms responsible for mobility degradation at the Si/SiO₂ interface. From the observed effects of process parameters on mobility roll-off under high vertical fields, conclusions are drawn about optimum process conditions for maximizing mobility. The implications of this work for performance limits of several types of MOS devices are described.