there is a depletion, or lack of free electrons and holes in this area, it is known as the
1-68. The loss of an electron from the N-type material created a positive ion in the N-
type material while the loss of a hole from the P-type material created a negative ion in the
P-type material. These ions are fixed in place in the crystal lattice structure and cannot
move. Therefore, they make up a layer of fixed charges on the two sides of the junction
(see Figure 1-17). On the N side of the junction there is a layer of positively charged ions.
On the P side of the junction there is a layer of negatively charged ions. An electrostatic
field, represented by a small battery in the figure, is established across the junction
between the oppositely charged ions. The diffusion of electrons and holes across the
junction will continue until the magnitude of the electrostatic field is increased to the point
where the electrons and holes no longer have enough energy to overcome it, and are
repelled by the negative and positive ions respectively. At this point, equilibrium is
established and for all practical purposes, the movement of carriers across the junction
ceases. For this reason, the electrostatic field created by the positive and negative ions in
the depletion region is called a barrier.
Figure 1-17. PN Junction Barrier Formation
1-69. The action just described occurs almost instantly when the junction is formed.
Only the carriers in the immediate vicinity of the junction are affected. The carriers
throughout the remainder of the N- and P-type materials are relatively undisturbed and
remain in a balanced condition.
1-70. An external voltage applied to a PN junction is called BIAS. For example, if a
battery is used to supply bias to a PN junction and is connected so that its voltage opposes
the junction field, it will reduce the junction barrier and therefore aid current flow through
the junction. This type of bias is known as FORWARD BIAS. Forward bias causes the
junction to offer only minimum resistance to the flow of current.
1-71. Figure 1-18 shows forward bias. Notice the positive terminal of the bias battery is
connected to the P-type material and the negative terminal of the battery is connected to the
N-type material. The positive potential repels holes toward the junction where they
neutralize some of the negative ions. At the same time, the negative potential repels
electrons toward the junction where they neutralize some of the positive ions. Since ions on
23 June 2005