Q21. What type of PN diode is formed by using a fine metal wire and a section of N-type semiconductor
PN JUNCTION OPERATION
Now that you are familiar with P- and N-type materials, how these materials are joined together to
form a diode, and the function of the diode, let us continue our discussion with the operation of the PN
junction. But before we can understand how the PN junction works, we must first consider current flow in
the materials that make up the junction and what happens initially within the junction when these two
materials are joined together.
Current Flow in the N-Type Material
Conduction in the N-type semiconductor, or crystal, is similar to conduction in a copper wire. That
is, with voltage applied across the material, electrons will move through the crystal just as current would
flow in a copper wire. This is shown in figure 1-15. The positive potential of the battery will attract the
free electrons in the crystal. These electrons will leave the crystal and flow into the positive terminal of
the battery. As an electron leaves the crystal, an electron from the negative terminal of the battery will
enter the crystal, thus completing the current path. Therefore, the majority current carriers in the N-type
material (electrons) are repelled by the negative side of the battery and move through the crystal toward
the positive side of the battery.
Figure 1-15.Current flow In the N-type material.
Current Flow in the P-Type Material
Current flow through the P-type material is illustrated in figure 1-16. Conduction in the P material is
by positive holes, instead of negative electrons. A hole moves from the positive terminal of the P material
to the negative terminal. Electrons from the external circuit enter the negative terminal of the material and
fill holes in the vicinity of this terminal. At the positive terminal, electrons are removed from the covalent
bonds, thus creating new holes. This process continues as the steady stream of holes (hole current) moves
toward the negative terminal.