Varicap

A varicap, also known as a varactor diode or tuning diode, is a specific type of diode that acts as a voltage-controlled capacitor. Unlike a regular diode, which is optimized for conducting current, a varicap is designed to exploit the relationship between the applied voltage and the width of its depletion region. The depletion region is a zone within the semiconductor material where charge carriers have been depleted, and its width changes with the applied reverse bias voltage.

When a reverse bias voltage is applied across a varicap, the depletion region widens. As the depletion region widens, it effectively increases the distance between the conductive regions, behaving similarly to an increase in the distance between the plates of a capacitor, thus decreasing its capacitance. Conversely, decreasing the reverse bias voltage narrows the depletion region, increasing the capacitance. This voltage-dependent capacitance is the key characteristic that makes varicaps useful.

Varicaps are primarily used in electronic circuits where capacitance needs to be adjusted electronically. Common applications include tuning circuits in radio and television receivers, voltage-controlled oscillators (VCOs), frequency synthesizers, and phase-locked loops (PLLs). By varying the voltage applied to the varicap, the resonant frequency of an LC circuit can be precisely controlled, allowing for tuning across different frequencies without the need for mechanical tuning components. The range of capacitance variation and the voltage control range are important parameters for selecting the appropriate varicap for a given application.