Understanding the Varactor Diode

A Varactor diode, also known as a varicap or tuning diode, is a special type of diode designed to behave like a voltage-controlled capacitor. Unlike regular diodes primarily used for rectification, varactors are optimized to maximize their capacitance variation when the reverse bias voltage applied across them changes.

Varactor Diode Operation Under Reverse Bias

The key characteristic of a Varactor diode is its ability to act as a variable capacitance specifically under Reverse Bias conditions.

• Depletion Region: When a reverse bias voltage is applied to a PN junction diode, a depletion region forms around the junction. This region is essentially devoid of free charge carriers and acts like a dielectric (insulator) between the two conductive semiconductor regions (acting like plates).
• Capacitance Change: The width of this depletion region ($W$) is directly proportional to the applied reverse bias voltage ($V_{RB}$). Specifically, $W \propto \sqrt{V_{RB}}$.
• Voltage-Controlled Capacitance: The capacitance ($C$) of this structure is inversely proportional to the width of the depletion region ($W$). Therefore, the capacitance varies with the reverse voltage: $C \propto \frac{1}{W} \propto \frac{1}{\sqrt{V_{RB}}}$.
• Application: By varying the reverse bias voltage, the width of the depletion region changes, consequently changing the diode's capacitance. This property makes varactors extremely useful in applications like tuning circuits in radio and TV receivers, frequency multipliers, and phase-locked loops, where capacitance needs to be electronically adjusted.

Operation Under Other Bias Conditions

Let's look at why the other options are not suitable:

• Forward Bias: When a Varactor diode is forward biased, the depletion region narrows significantly, and the diode starts conducting current. The capacitance is primarily determined by the charge stored in the junction, which depends heavily on the forward current. This behaviour is not stable or predictable enough to be used as a variable capacitance, and the diode's low forward resistance dominates.
• Zero Bias: With zero bias, the depletion region has a fixed width determined by the built-in potential. This results in a constant capacitance, not a variable one.
• Both Forward and Reverse Bias: As explained, the variable capacitance behaviour is specifically associated with the reverse bias region due to the modulation of the depletion width. It does not reliably function as a variable capacitor in forward bias.

Therefore, the Varactor Diode acts as a variable capacitance precisely when it is under Reverse Bias.