Surface Accumulation in MOS capacitor

Surface Accumulation (VGB> VFB)
An applied positive gate voltage larger than the flatband voltage (which will be defined shortly)
(VGB> VFB) induces positive charge on the “metal” gate and negative charge in the
semiconductor. The only negative charges available are electrons and they accumulate at the
surface. The electron concentration at the surface is above the bulk value, thus leading to a
condition that is called surface accumulation. The charge distribution and equivalent circuit is
shown in Figure 3. The flatband voltage(VFB) is the voltage at which there is no charge on the
plates of the capacitor and hence there is no electric field across the oxide. It’s numerical value
depends on the doping of the semiconductor and on any residual interface charge that may exist
at the interface between the semiconductor and the insulator. When the surface of the
semiconductor is accumulated, a plot of the charge per unit area (QN) at the semiconductor /
oxide interface versus the applied voltage (VGB) is linear and the slope is the oxide capacitance
per unit area., Cox.,

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