Varactor Diodes

When any PN junction diode is reverse-biased, majority carriers are swept away from the diode’s junction and a relatively wide depletion area is formed.  When the diode is subjected to varying reverse bias voltage, the width or thickness of this depletion region will also vary.  When the reverse bias voltage increases in value, the depletion region becomes wider.  When the reverse voltage decreases, the depletion region becomes narrower.  The depletion region acts like an insulator since it provides an area through which no conduction can take place.  It also effectively separates the N and P sections of the diode in the same way that a dielectric separates the two plates of a capacitor.  In fact, the entire PN junction diode is basically a small electronic capacitor that changes its capacitance as its depletion region changes in size.

Ordinary PN junction diodes possess only a small amount of internal junction capacitance and in most cases this capacitance is too small to be effectively used.  However, special diodes are constructed so that they have an appreciable amount of internal capacitance and are used in much the same way that ordinary capacitors are used in electronic circuits.  These special diodes are commonly referred to as VARACTOR DIODES or simply VARACTORS, or VARICAP on CAPACITANCE DIODES.