a varactor diode
/ VAIR-ak-tor /
A varactor diode is a diode used backwards on purpose — not to block current, but to act as a capacitor whose value you can dial with a voltage. Every reverse-biased junction is already a tiny capacitor; the varactor is built to make that capacitance large, clean, and strongly tunable. It turns a DC control voltage into an adjustable capacitance.
Recall that reverse bias widens the depletion region — the insulating gap between the two charged sides of the junction. A capacitor's value falls as its plates move apart, so MORE reverse voltage means a WIDER gap means LESS capacitance. Sweep the reverse voltage and the capacitance glides smoothly over a range (say 2 V gives 40 pF, 10 V gives 10 pF). No moving parts, just a voltage knob for capacitance.
Its home is tuning. Put a varactor in an LC resonant tank and a control voltage now sets the resonant frequency — that is how radios, TVs, and phase-locked-loop oscillators tune electronically instead of with a mechanical variable capacitor. The catch: the capacitance change is nonlinear with voltage, the usable range is modest, and the signal swing must stay small so the 'capacitor' value does not wobble with the signal itself.
A varactor showing 40 pF at 2 V and 10 pF at 10 V, placed in a tank with a 1 uH coil, tunes resonance from f = 1/(2 times pi times sqrt(L times C)) of about 25 MHz up to ~50 MHz as you raise the control voltage.
Reverse voltage sets a varactor's capacitance, which tunes an LC circuit.
A varactor is a capacitor, not a switch — keep it reverse-biased at all times. If the signal ever forward-biases the junction it stops being a clean capacitor (it conducts and distorts), and because C depends on voltage, large signals make it nonlinear.