What type of semiconductor device is designed for use as a voltage-controlled capacitor?
Why A reverse-biased PN junction has a depletion region that acts as the dielectric between two conducting regions, so the junction behaves like a capacitor. Increasing the reverse voltage widens that depletion layer, which increases the plate spacing and lowers the capacitance. A varactor (varicap) diode is built to exploit this effect, and it is commonly used to tune VFOs, PLLs, and to build frequency multipliers.
Watch out A Zener diode also operates reverse biased, which makes it tempting, but it is designed to hold a constant voltage at breakdown for regulation. A tunnel diode provides negative resistance for oscillators, and an SCR is a latching power switch.
Varactor = VARiable reACTOR: more reverse volts, wider gap, less capacitance.