Updated June 29,
Varactors are voltage-variable
capacitors. The word "varactor" is a form of portmanteau.
Think of the varactor as a diode
where you can vary its reverse-biased junction capacitance. Typically
you can achieve a range where the capacitance doubles from one voltage
extreme to the other, e.g. a varactor could supply between 1 pF
and 2 pF. Varactors can be discrete devices (ask your favorite diode
supplier) or supplied monolithically as part of a MMIC design (SiGe
technology usually offers varactors).
While we wait for content on
how a varactor works, here's
information on how to make a voltage-variable phase shifter