A PN junction is a type of semiconductor, which means that its ability to conduct electricity is between that of a metal, which readily conducts, and an insulator, which doesn’t. PN junctions are particularly useful for controlling current flow because they only allow it to pass in one direction, maximizing the efficiency of our equipment and forming an essential component of modern electronics. But how do they actually work?

 

To make a PN junction one simply has to structure two variations of Silicon together. Silicon is a crystalline solid with a valency of four, meaning that each atom forms a bond with four of its neighbors. This is what holds the structure together.

Credit: PadaKuu.com [1]

Bonds between atoms occur when they share electrons, which are small electric charges that form the basis of current. The two variations of Silicon needed to make the junction are P doped and N doped silicon. In N doped silicon we introduce phosphorous atoms in place of Silicon atoms. Phosphorous has 5 valence electrons, meaning that it forms four bonds with the other Silicon atoms but still “donates” an extra electron.

 

In P doped Silicon we introduce Boron atoms with a valency of 3. This means that there is a “hole” in place of one of its electrons when bonding with the other silicon atoms. This empty space has the potential to “accept” another electron to fill its hole.

 

Credit: DevXplained[2]

 

A way of looking at it is to say an n type region is doped with “donor impurities”, atoms that will donate their electrons and the p type region is doped with “acceptor impurities” that want to accept another electron. But electrons are negatively charged so when Phosphorous donates its spare electron it becomes a more positive ion (charged particle) and when Boron accepts another electron it becomes a more negative ion. Like charges repel and opposite charges attract.

 

So what happens when we join the two together? The electrons and holes themselves are not actually fixed in place but are free to wander or “diffuse through the material”. That may seem counter intuitive in the case of a hole given that it is the absence of an electron but the following diagram illustrates it nicely.

 

Credit: Electronic Devices (Floyd)[3]

 

So some of the electrons from the n type region wander into the p type region, where they are accepted by acceptor impurities, changing them into negative ions. Some of the holes from the p type region wander to the n type region where electrons are donated to them, creating positive ions. We now have positive charges built up in the n type region and negative charges built up in the p type region.

 

Credit: Byju’s[4]

 

The effect of creating this charge imbalance in the “depleted region” is that diffusion of the electrons and holes is opposed. An electron in the n type region will not be able to cross the boundary because it is repelled by the build-up of charge on the p type side of the depleted region and similarly for holes.

 

However, if you are to connect the positive terminal of a battery to the p type side and a negative terminal to the n type side then you will “forward bias” the PN junction. This applies an electric field across the diode that essentially forces electrons and holes to overcome the force of repulsion and traverse the barrier. This has the effect of shrinking the depleted region as the free electrons can balance the positively charged ions and the holes can balance the negatively charged ions. Current can now flow. However the voltage applied across the diode needs to be large enough for them to overcome this “barrier potential” in the first place.

 

If the diode is then reversed biased, this is where the p type region is connected to the negative terminal and the n type region is connected to the positive terminal. The holes and electrons are attracted away from the depleted region, which grows and impedes the current even more.

 

Hence in order to let current flow in one direction, one simply needs to forward bias the diode and to make it impede current (become a resistor) one must reverse bias the junction.

 

References

[1]        “Bond structure of semiconductors, intrinsic and extrinsic semiconductors; | PadaKuu.com.” https://padakuu.com/bond-structure-of-semiconductors-intrinsic-and-extrinsic-semiconductors-794-article (accessed Apr. 20, 2023).

[2]        “P-N Junction | DevXplained.” https://devxplained.eu/en/blog/p-n-junction (accessed Apr. 20, 2023).

[3]        T. Floyd, Electronic Devices. Pearson, 2005.

[4]        “General Data Protection Regulation(GDPR) Guidelines BYJU’S.” https://byjus.com/physics/p-n-junction/ (accessed Apr. 20, 2023).

 

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