The
PN junction
In the previous tutorial we saw how
to make an N-type semiconductor material by doping a silicon atom with small
amounts of Antimony and also how to make a P-type semiconductor material by
doping another silicon atom with Boron. This is all well and good, but these
newly doped N-type and P-type semiconductor materials do very little on their
own as they are electrically neutral.
However, if we join (or fuse) these
two Semiconductor Materials together
they behave in a very different way merging together and producing what is
generally known as a “PN Junction“.
When the N-type semiconductor and
P-type semiconductor materials are first joined together a very large density
gradient exists between both sides of the PN junction. The result is that some
of the free electrons from the donor impurity atoms begin to migrate across
this newly formed junction to fill up the holes in the P-type material
producing negative ions.
However, because the electrons have
moved across the PN junction from the N-type silicon to the P-type silicon,
they leave behind positively charged donor ions ( ND ) on
the negative side and now the holes from the acceptor impurity migrate across
the junction in the opposite direction into the region where there are large
numbers of free electrons.
As a result, the charge density of
the P-type along the junction is filled with negatively charged acceptor ions
( NA ), and the charge density of the N-type along the
junction becomes positive. This charge transfer of electrons and holes across
the PN junction is known as diffusion.
This process continues back and
forth until the number of electrons which have crossed the junction
have a large enough electrical charge to repel or prevent any more charge carriers from crossing over the junction. Eventually a state of equilibrium (electrically neutral situation) will occur producing a “potential barrier” zone around the area of the junction as the donor atoms repel the holes and the acceptor atoms repel the electrons.
have a large enough electrical charge to repel or prevent any more charge carriers from crossing over the junction. Eventually a state of equilibrium (electrically neutral situation) will occur producing a “potential barrier” zone around the area of the junction as the donor atoms repel the holes and the acceptor atoms repel the electrons.
Since no free charge carriers can
rest in a position where there is a potential barrier, the regions on either
sides of the junction no become completely depleted of any more free carriers
in comparison to the N and P type materials further away from the junction.
This area around the PN Junction is now called the Depletion Layer.
The
PN junction
The total charge on each side of a PN
Junction must be equal and opposite to maintain a neutral charge condition
around the junction. If the depletion layer region has a distance D, it
therefore must therefore penetrate into the silicon by a distance of Dp for the
positive side, and a distance of Dn for the negative side giving a relationship
between the two of Dp.NA = Dn.ND in
order to maintain charge neutrality also called equilibrium.
PN
junction Distance
As the N-type material has lost
electrons and the P-type has lost holes, the N-type material has become
positive with respect to the P-type. Then the presence of impurity ions on both
sides of the junction cause an electric field to be established across this
region with the N-side at a positive voltage relative to the P-side. The
problem now is that a free charge requires some extra energy to overcome the
barrier that now exists for it to be able to cross the depletion region
junction.
This electric field created by the
diffusion process has created a “built-in potential difference” across the
junction with an open-circuit (zero bias) potential of:
Where: Eo is the zero
bias junction voltage, VT the thermal voltage of 26mV at room
temperature, ND and NA are the impurity concentrations
and ni is the intrinsic concentration.
A suitable positive voltage (forward
bias) applied between the two ends of the PN junction can supply the free
electrons and holes with the extra energy. The external voltage required to
overcome this potential barrier that now exists is very much dependent upon the
type of semiconductor material used and its actual 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. This potential barrier will always exist
even if the device is not connected to any external power source, as seen in
diodes.
The significance of this built-in
potential across the junction, is that it opposes both the flow of holes and
electrons across the junction and is why it is called the potential barrier. In
practice, a PN junction is formed within a single crystal of material
rather than just simply joining or fusing together two separate pieces.
Electrical contacts are also fused
onto either side of the crystal to enable an electrical connection to be made
to an external circuit. Then the resulting device that has been made is called
a PN junction Diode or Signal Diode.
In the next tutorial about the PN
junction, we will look at one of the most interesting applications of the PN
junction is its use in circuits as a diode. By adding connections to each
end of the P-type and the N-type materials we can produce a two terminal device
called a PN Junction Diodewhich can be biased by an
external voltage to either block or allow the flow of current through it.
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