This article is part of The engineer’s complete guide to capacitors. If you’re unsure of what type of capacitor is best for your circuit, read How to choose the right capacitor for any application.
What is a varactor diode?
Variable capacitors like air capacitors and trimmer capacitors are adjustable manually. In contrast, the capacitance of a varactor diode is controlled by a reverse bias voltage. Since the controlling voltage can be applied automatically, manual tuning is no longer required. This helped to launch digital radios that employed microprocessors or microcontrollers to handle the digital tuning using a digital-to-analog converter.
The varactor diode is also referred to as a voltage variable capacitor, or VVC. The diode’s name “varactor” is a contraction for “variable reactor.” Similarly, “varicap” is a contraction for “variable capacitor.”
Operation of a varactor diode
A semiconductor diode has a positive-acting p region doped with acceptor impurities. It also has a negative-acting n region doped with donor impurities. At the p-n junction interface, a depletion region is formed. The depletion region is devoid of majority (holes and electrons) charge carriers and can be viewed as being an insulator. The p and n regions are like the plates of a capacitor and the depletion region behaves like the dielectric. The capacitance (C) is determined by the dielectric constant (ε) times the plate area (A) divided by the distance (d) between the plates according to C=εA/d.
A reverse bias voltage causes the depletion region to widen. This is like increasing the distance between the plates and causes the capacitance to decrease. The reverse bias voltage controls the capacitance.
The control the reverse bias exerts on varactor diode capacitance is non-linear. It is a function of the doping profile used in the manufacturing process. The doping profile indicates the doping density in relation to the p-n junction. In the image that follows, the linearly graded, abrupt, and hyperabrupt profiles are shown.
The capacitance associated with a reverse-biased p-n junction is called “junction capacitance.” Junction capacitance slows down the switching speed of rectifier and switching diodes. To minimize the effect of reverse bias on junction capacitance, linearly graded doping profiles are used. To increase the effect of reverse bias on junction capacitance, abrupt junction doping profiles are used in the case of varactors. The hyperabrupt doping profile is used to increase the sensitivity of a varactor diode.
Applications of varactor diodes
Due to their small capacitance values, varactor diodes are used in radio frequency (RF) design applications. While they’re found in a wide variety of circuits, there are two main areas of application: voltage-controlled oscillators (VCOs) and RF filters.
Voltage-controlled oscillators are used in many RF applications, notably in phase locked loops (PLLs). PLLs are used in FM (frequency modulation) demodulators or in frequency synthesizers. RF filters use varactor diodes to tune the filters. One important design application is the tracking filter, used in the front end of receivers to track the incoming received signal. Tracking filters are also used in spectrum analyzers to display the spectral content of complex signals.
Alternatives to varactor diodes
Varactor diodes are important electronic devices. The choice in many cases is not an alternative technology, but the design-specific application specifications and the varactor diode that can meet its needs.