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The variable-capacitance diode is a product that makes use of the capacity characteristics of the depletion layer. The depletion layer occurs in the pn junction of the diode when the voltage is applied in the reverse direction, and the thickness varies in proportion to the reverse voltage.
Therefore, as the reverse voltage applied increases, the capacitance decreases. This is the same function as increasing the distance between the two electrodes of the capacitor. Conversely, if the reverse voltage becomes low, the capacitance increases.
It is used for tuning circuits etc. Since the frequency characteristics are changed by this capacitance change, a large capacity change ratio is required as compared with a normal diode.
The important characteristics of a variable-capacitance diode are not the forward voltage VF and switching characteristics as in the case of a general diode, but the capacitance value and its variation (voltage dependence).