Carrier quantization at flat bands in MOS devices

While the modeling of carrier quantization in strong inversion and accumula tion has received extensive attention in the literature, it is commonly ass umed that near flat bands a classical model is acceptable, due to the small surface electric field. We show that this picture is not correct, The pres ence of the abrupt potential discontinuity at the Si/SiO2 interface causes a "dark space" of a few nanometers, where the carrier concentration is much smaller than in the bulk, This quantum effect causes a significant attenua tion of the capacitance in the near-flat-band region for channel doping con centrations above 10(17) cm(-3). The effect is also important at the polysi licon side, where a high doping concentration is used. The nonnegligible ef fects of the dark space on the C-V curve of MOS devices are shown.