In this case, P-type’s charge
density along the junction is filled with negatively charged acceptor
ions (NA), and N-type’s charge density along the junction becomes
positive. This charge relocation of electrons and holes across the PN
junction is called diffusion. P and N layers’ width depends on how
heavily each side is doped with acceptor density NA, and donor density
ND, individually.
This progression
remains to and fro until quantities of electrons which have overlapped
the junction have sufficient electrical charge to prevent additional
charge carriers from over-passing the junction. Ultimately a state of
equilibrium (electrically neutral situation) will occur creating a
“potential barrier” zone around the area of the junction as the donor
atoms prevent the holes and the acceptor atoms prevent the electrons.
As no free charge carriers can rest in a position where there is a potential barrier, the regions on either sides of the junction now become completely depleted without any free carriers in comparison to the N and P type materials further away from the junction. This area around the PN Junction which is known as Depletion Layer.
The PN Junction
Total
charge on both side of a PN Junction must be equal and reverse to keep a
neutral charge condition around the junction. If the depletion layer
region has a distance D, it must be entered 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 known as equilibrium.
PN Junction Distance
N-type
material became positive with respect to the P-type because of losing
electrons by N-type material and losing holes by the P-type. Then the
existence 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. A free charge needs additional
energy to overcome the barrier that now exists for it to be able to
cross the depletion region junction.
This electric field developed by the diffusion process has developed a “built-in potential difference” across the junction with an open-circuit (zero bias) potential of:
Here,
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.
An appropriate positive voltage (forward
bias) applied between the two ends of the PN junction can supply the
free electrons and holes with additional energy. The external voltage
needed to overcome this potential barrier that now exists is very much
reliant on the type of semiconductor material used and its definite
temperature.
Usually, 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 remain
even if the device is not plugged in with any external power source, as
observed in diodes.
As this built-in potential across the junction
withstands both the flow of holes and electrons across the junction, it
is known as potential barrier. In practice, a PN junction is formed
within a single crystal of material rather than just simply connecting
or combining together two distinct portions.
The result of this
process is that the PN junction has current-voltage (IV or I-V)
rectifier characteristics. Electrical contacts are fused on either side
of the semiconductor to establish an electrical connection to an
external circuit. The resulting electronics are often referred to as PN
junction diodes or simply signal diodes. We observed here that a
PN junction can be made by joining or diffusing together different
doped semiconductor materials to create an electronic device called a
diode. can be used as the basic semiconductor structure of the
rectifier, all kinds of transistors, LEDs, solar power. cells, and many
other semiconductor devices.