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A p-n junction is formed by combining an N-type and P-type semiconductors together in very close contact. The term junction refers to the region where the two
types of semiconductor meet. It can be thought of as the border region between the P-type and N-type blocks as shown in the
following diagram:
+--------+--------+
| P-type | N-type |
+--------+--------+
|
p-n junction
The p-n junction possesses some interesting properties which have useful applications in modern electronics. In particular, it
is commonly used as a diode, an electrical device that allows an electricity flow in one
direction but not in the other (opposite) direction. This property is explained in terms of the forward-bias and
reverse-bias effects, where the term bias refers to an application of electric voltage to the p-n junction.
Forward-bias
Forward-bias occurs when the P-type block is connected to the positive terminal of a battery and the
N-type block is connected to the negative terminal, as shown below.
+--------+--------+
+-----| P-type | N-type |-----+
| +--------+--------+ |
| |
| | |
+-------------| |-------------+
positive | negative
terminal terminal
With this set-up, the 'holes' in the P-type
region and the electrons in the N-type region are pushed towards the junction.
The positive charge applied to the P-type block repels the holes, while the negative charge applied to the N-type block repels
the electrons. As electrons and holes are pushed towards the junction the distance between them decreases. This weakens the
junction barrier thereby reducing its resistance. As the barrier between the two blocks is sufficiently reduced, the electrons
will be able to overcome the junction barrier and enter the P-type region (moving leftwards from one hole to the next, with
reference to the above diagram).
This enables an electric current to flow. An electron starts flowing around from the negative terminal to the positive
terminal of the battery. It starts at the negative terminal, moving towards the N-type block. Having reached the N-type region it
enters the block and makes its way towards the p-n junction. The junction barrier can no longer keep the electron in the N-type
region due to the forward-bias effect (in other words, the barrier produces very little electrical resistance against the flow of electrons). The electron will therefore cross the
junction and move ahead into the P-type block. Once inside the P-type region, the electron will jump from one available hole to
the next, making its way to the positive terminal of the power supply. This process will be repeated over and over again allowing
the electric current to flow.
Reverse-bias
Connecting the P-type region to the negative terminal of the battery and the N-type region to the
positive terminal, produces the reverse-bias effect. The connections are illustrated in the following diagram:
+--------+--------+
+-----| P-type | N-type |-----+
| +--------+--------+ |
| |
| | |
+-------------| |-------------+
negative | positive
terminal terminal
Because the P-type region is now connected to the negative terminal of the power supply, the 'holes' in the P-type region are pulled away from the
junction. Similarly, because the N-type region is connected to the positive terminal, the electrons will also be pulled away from
the junction.
This effectively widens the p-n junction. Because the barrier is now wider, the electrical resistance against the flow of electrons will be higher. For this reason there will be no
(or minimal) flow of electric current.
Summary
The forward-bias and reverse-bias properties of the p-n junction imply that it can be used as a diode. A p-n junction diode allows an electric current to flow in one direction, but not in the opposite direction.
When the p-n junction is forward-biased, electric current flows freely due to reduced resistance of the p-n junction. When the
p-n junction is reverse-biased, however, the junction barrier (and therefore resistance) becomes greater and current flow is
minimal.
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