How PN Junction is formed?

Newly doped N-type and P-type semiconductor materials are electrically neutral. After adding these two semiconductor materials together they uniquely integrate with each and form a junction which is called “PN Junction “. In this case, a big inclination is formed between both sides of the junction. Some free electrons from the donor impurity atoms start to move to the newly formed junction for filling up the holes in the P-type material creating negative ions. Therefore, they leave behind positively charged donor ions (ND) on the negative side and now the holes from the acceptor impurity travel across the junction in the reverse direction into the region where there are lots of free electrons.

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.

Prasun Barua

Prasun Barua is an Engineer (Electrical & Electronic) and Member of the European Energy Centre (EEC). His first published book Green Planet is all about green technologies and science. His other published books are Solar PV System Design and Technology, Electricity from Renewable Energy, Tech Know Solar PV System, C Coding Practice, AI and Robotics Overview, Robotics and Artificial Intelligence, Know How Solar PV System, Know The Product, Solar PV Technology Overview, Home Appliances Overview, Tech Know Solar PV System, C Programming Practice, etc. These books are available at Google Books, Google Play, Amazon and other platforms.


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