The flatband capacitance (C_FB) is defined as the total measured capacitance of a Metal-Oxide-Semiconductor (MOS) structure or a similar semiconductor device when the applied gate voltage is exactly equal to the flatband voltage V_FB.

This flatband voltage represents the specific potential needed to establish the flatband condition, where the energy bands within the semiconductor substrate are flat all the way to the oxide interface.

Achieving this condition means there is no net electric field in the semiconductor, the surface potential is zero, and consequently, there is no accumulation or depletion of mobile charge carriers at the interface.

The total capacitance of the MOS structure is fundamentally a series combination of the oxide capacitance (C_ox) and the capacitance arising from the semiconductor (C_s).

At the precise flat-band condition, the semiconductor’s contribution (C_s,FB) is not zero, but is represented by the Debye capacitance ($C_{Debye}$), which accounts for the residual distribution of majority carriers near the interface. Therefore, the flatband capacitance (C_FB) is the series combination of the oxide capacitance (C_ox) and the Debye capacitance (C_Debye).

In practical measurements, (C_FB) is an essential, theoretically calculated value used to locate the flatband voltage on a measured capacitance-voltage (C-V) curve, which in turn is critical for characterizing fixed charges and other non-idealities within the device.

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