3-16
Figure 3-12.—PN junction.
The size of the depletion region in a varactor diode is directly related to the bias. Forward biasing
makes the region smaller by repelling the current carriers toward the PN junction. If the applied voltage is
large enough (about .5 volt for silicon material), the negative particles will cross the junction and join
with the positive particles, as shown in figure 3-13. This forward biasing causes the depletion region to
decrease, producing a low resistance at the PN junction and a large current flow across it. This is the
condition for a forward-biased diode. On the other hand, if reverse-bias voltage is applied to the PN
junction, the size of its depletion region increases as the charged particles on both sides move away from
the junction. This condition, shown in figure 3-14, produces a high resistance between the terminals and
allows little current flow (only in the microampere range). This is the operating condition for the varactor
diode, which is nothing more than a special PN junction.
Figure 3-13.—Forward-biased PN junction.
Figure 3-14.—Reverse-biased PN junction.
