What type of semiconductor diode varies its internal capacitance as the voltage applied to its terminals varies?

A varactor (also known as a varicap) is a semiconductor diode specifically designed to act as a voltage-controlled capacitor. When a reverse-bias voltage is applied across its terminals, the width of the depletion region at the PN junction changes. Because this depletion region acts like the dielectric in a capacitor, varying its width directly alters the diode's internal capacitance. This unique property makes varactor diodes incredibly useful in modern radio circuits. They are widely used in electronic tuning circuits, voltage-controlled oscillators (VCOs), and automatic frequency control (AFC) systems, replacing older mechanical variable capacitors. Other diode types serve entirely different purposes: Zener diodes provide a constant voltage reference, Silicon-controlled rectifiers (SCRs) are used for power switching, and Schottky diodes are used for high-speed switching or high-frequency mixing.