What type of semiconductor device is designed for use as a voltage-controlled capacitor?

A varactor diode (often called a varicap) is a specialized semiconductor designed to act as a voltage-controlled capacitor. When reverse-biased, the width of its depletion region changes based on the applied voltage. As the reverse voltage increases, the depletion region widens, which increases the physical distance between the conductive layers and effectively decreases the capacitance. This unique property makes varactors essential components in tuning circuits, voltage-controlled oscillators (VCOs), and phase-locked loops. Other diodes listed serve entirely different purposes. Zener diodes are used for constant voltage regulation, tunnel diodes act as negative resistance elements in high-frequency oscillators, and silicon-controlled rectifiers (SCRs) are used for switching large amounts of power.