The low frequency and high frequency C-V characteristics curves of a MOS capacitor are shown in fig 5.2.
Fig 5.2 : Low & High Frequency C-V curves
The low frequency or quasi-static measurement maintains thermal equilibrium at all times. This capacitance is the ratio of the change in charge to the change in gate voltage, measured while the capacitor is in equilibrium. A typical measurement is performed with an electrometer, which measures the charge added per unit time as one slowly varies the applied gate voltage.
The high frequency capacitance is obtained from a small-signal capacitance measurement
at high frequency. The bias voltage on the gate is varied slowly to obtain the capacitance
versus voltage. Under such conditions, one finds that the charge in the inversion layer
does not change from the equilibrium value corresponding to the applied DC voltage. The
high frequency capacitance therefore reflects only the charge variation in the depletion
layer and the (rather small) movement of the inversion layer charge.
