What type of semiconductor diode varies its internal capacitance as the voltage changes?

A varactor diode, also known as a variable capacitance diode, is specifically designed to vary its internal capacitance in response to changes in the applied voltage. This ability is primarily due to the way the diode is constructed and its biasing conditions. When reverse voltage is applied, the depletion region of the diode changes, which in turn alters the capacitance. This characteristic makes varactor diodes particularly useful in applications such as frequency modulation and tuning circuits, where the capacitance needs to be adjusted dynamically.

In contrast, other types of diodes, like the rectifier diode, do not have variable capacitance and are mainly used for allowing current to flow in one direction while blocking it in the reverse direction. An LED diode (light-emitting diode) functions by emitting light when current flows through it, and a thermistor diode is not commonly referenced; thermistors are temperature-sensitive resistors rather than diodes. Thus, they do not exhibit the voltage-dependent capacitance characteristic that defines the operation of a varactor diode.