The P-N junction is the basic building
block of almost all integrated circuits and other types of semiconductor
devices that are manufactured nowadays, such as transistors, diodes, LEDs, etc.
The PN junction forms the basis of much of today's semiconductor technology.
Thanks to www.allaboutcircuits.com for the diagram!
The unique properties of the junction enable it to perform a wide variety of
functions in many different configurations. The P-N junction has the very
useful property that current is allowed to flow only in one direction across
the junction, but it is blocked from flowing in the other direction.
current consists of a flow of electrons, this means that electrons can flow in
only one direction.
If a block of a P-type semiconductor is
connected in series with a separate block of an N-type semiconductor through
wiring, the resulting circuit would have no unique properties. They would both
conduct normally in both directions, as the number of electrons is balanced by
the number of protons in both blocks, and neither block has any net charge.
However, when a single semiconductor
crystal is manufactured such that it is doped with P-type material on one end,
and with N-type material on the other end with a junction in between, there is
a drastic change in the semiconductor’s properties.
The N-type material contains
an excess number of free electrons, which are negative majority charge
carriers, and these are free to move around in the crystal - a few of them
can even wander across the junction into the P-type material, and combine with
some of the holes in the region.
The free holes in the P-type material are
positive majority charge carriers. Because the electrons have moved across the
junction from the N-type material to the P-type material, they leave behind
positively charged donor ions on the negative side and the holes from the
P-type material have essentially migrated across the junction in the opposite
direction into the N-region where there are large numbers of free electrons. This
charge transfer of electrons and holes across the junction is known as
Eventually, an equilibrium state is
reached, as atoms in the P-type material near the junction are able to receive
an extra electron, and atoms in the N-type material are able to rid themselves
of an extra electron. Because of this, the
thin region of the P-type material near the junction has a net negative charge
because of the electrons attracted. Since electrons have been removed from the
N-type region near the junction, it takes on a localized positive charge. The
N-side has a positive voltage relative to the P-side. The total charge on each
side of the junction must be equal and opposite to maintain a neutral charge
condition around the junction.
After the charge on both sides reaches a
certain level, it will prevent any additional electrons and holes from crossing
the junction, because of the repelling nature in electric fields of the same
polarity. This thin layer on both sides of the junction where charge has been
built up is known as the depletion region, and has been depleted of positive
and negative majority charge carriers. The small voltage potential across this
junction creates a kind of insulator which separates the conductive P and N
Any free charge that happens to wander into
the depletion area will only find positive charges (the donor atoms) on the N-type
side and a negative charges (the acceptor atoms) on the P-type side.
exert a force on the free charges, driving them back to their 'own side' of the
junction away from the depletion region. The charges built up on both sides of
the junction tend to clear the depletion region of any free charges. A free
charge now requires some extra energy to cross the junction and overcome the
forces exerted by the donor/acceptor atoms.
If a positive voltage (forward bias) is applied
between the two ends of the PN junction such that the P side is positive and
the N side is negative, it would be able to supply the free electrons and holes
with the extra energy needed to cross the junction. Electrons in the N side are
attracted towards the positive voltage and are assisted to jump across the
depletion layer. Likewise, holes in the P side move towards the negative
voltage and jump the depletion layer. Once the voltage reaches this barrier
potential, the P-N junction will freely conduct current, limited only by the
external voltage required to overcome this potential barrier depends on the
type of semiconductor material, the amount of doping, and temperature.
Typically at room temperature the voltage across the depletion layer for silicon
is about 0.6 - 0.7 volts and for germanium is about 0.3 - 0.35 volts. Silicon
is usually the better choice, because it has lower leakage currents when
reverse biased. PN junction diodes can also be manufactured from other
semiconductor materials, but these are usually for specialized applications.
voltage is applied to the ends of the PN junction in the opposite direction, such
that the N side is positive and the P side is negative, there would be
essentially no current through the device. This is because the holes in the P
type region are attracted towards the negative potential that is applied to it.
Likewise, the electrons are attracted towards the positive potential which is
applied to the N type region. This actually pulls the holes and electrons away
from the junction itself and the depletion region will increase in width.
effect, this increases the barrier potential according to the applied (reverse
bias) voltage. The width of the depletion region is only able to increase up to
a certain point, and if the reverse bias voltage is large enough, it will
overcome this depletion layer potential, and freely conduct current in the same
way as with forward bias. This upper limit is known as the breakdown voltage,
and a large current here could actually damage the device because of the excess
In looking at the characteristic voltage-current
plot of the PN junction, it can be seen that in the forward direction (forward
biased), current is blocked until the forward barrier potential is reached, and
in the negative direction (reverse biased), current is blocked until the
breakdown potential is reached.
While the PN junction
provides excellent rectifying characteristics, it is not a perfect diode having
infinite resistance in the reverse direction and zero resistance in the forward
direction. In reality a small amount of reverse current does flow, although it
is usually very small, in the region of a few pico amps or microamps.
reverse leakage current results from what are called minority carriers, which
are the small number of electrons found in a P type region and holes in an N
type region. This is caused by impurities in the manufacturing process. The new methods of manufacturing nowadays
have reduced the number of minority carriers, as well as the levels of reverse
- James Reinholm