Varactor Diode

The depletion region of a reverse-biased junction acts as a capacitor whose width grows with reverse voltage, so the capacitance falls as voltage rises:

$C_j = \frac{C_{j0}}{\left(1 + V_R / V_{bi}\right)^m}$

where $m$ ≈ 0.5 for abrupt junctions (hyperabrupt varactors have larger $m$ for a wider tuning range). Varactors tune voltage-controlled oscillators (VCOs) in PLLs, radio receivers and FM modulators.