A varactor diode or varicap diode is a semiconductor diode that changes its junction capacitance significantly depending on the applied reverse bias voltage. Many semiconductor PN junction diodes are designed to exhibit specific electrical properties; the varactor diode or varicap diode is one such semiconductor specifically designed to exhibit unique junction capacitance characteristics. These types of diodes are useful in RF circuits for tuning signals; that is why a class of varactor diodes is also called Tuner diodes.
It should be noted that the desired operation of the varactor diode is achieved only in reverse bias condition, but why (we will answer this later in the article)? In reverse bias condition, the internal capacitance of the diode changes significantly with respect to applied reverse voltage. That is why it is not the voltage-current but the voltage-capacitance that is the most important characteristic of a varactor diode.
What is a varactor diode or varicap diode?
A varactor diode is a semiconductor diode whose internal capacitance changes with respect to applied reverse voltage. It is a voltage-controlled device that acts as a variable semiconductor capacitor. It is also called a varicap diode, tuner diode, tuning diode, voltcap, or voltage variable capacitance.
The varactor diodes are shown in electronic circuits by many symbols. The most popular symbols to indicate a varactor diode in a circuit are shown in Figure 2.