The VARACTOR is another of the active two-terminal diodes that operates in the microwave range.
Since the basic theory of varactor operation was presented in NEETS, Module 7, Introduction to Solid-
State Devices and Power Supplies, Chapter 3, only a brief review of the basic principles is presented here.
The varactor is a semiconductor diode with the properties of a voltage-dependent capacitor.
Specifically, it is a variable-capacitance, pn-junction diode that makes good use of the voltage
dependency of the depletion-area capacitance of the diode.
In figure 2-42A, two materials are brought together to form a pn-junction diode. The different energy
levels in the two materials cause a diffusion of the holes and electrons through both materials which tends
to balance their energy levels. When this diffusion process stops, the diode is left with a small area on
either side of the junction, called the depletion area, which contains no free electrons or holes. The
movement of electrons through the materials creates an electric field across the depletion area that is
described as a barrier potential and has the electrical characteristics of a charged capacitor.
Figure 2-42A.Pn-junction diode as a variable capacitor.
External bias, applied in either the forward or reverse direction, as shown in figure 2-42B and C,
affects the magnitude, barrier potential, and width of the depletion area. Enough forward or reverse bias
will overcome the barrier potential and cause current to flow through the diode. The width of the
depletion region can be controlled by keeping the bias voltage at levels that do not allow current flow.
Since the depletion area acts as a capacitor, the diode will perform as a variable capacitor that changes
with the applied bias voltage. The capacitance of a typical varactor can vary from 2 to 50 picofarads for a
bias variation of just 2 volts.
Figure 2-42B.Pn-junction diode as a variable capacitor.