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Understanding the PN Junction

Whenever some one decides to learn electronics, the first question he asks may be – “Where shall I begin?“. I will say, one should begin at a junction ;) the “pn junction“. We know semiconductor devices like transistors and diodes are the basic building units of any equipment that involves electronics, say tablet computers to the sophisticated MRI machines! How these basic units like transistors and diodes are formed ? or how are they made ? The answer lies in understanding “PN Junction”. A PN junction is the basic building block of many semiconductor devices like diodes and transistors.

Note:- I have written an interesting article which tells the story behind invention & discovery of PN Junction diode. If you like to read the story, follow here:- Story behind Invention & Discovery of PN Junction

How a PN Junction is formed?

Before explaining the formation of a PN junction, I would like to remind you about the real basic stuff :- Classification into Metals,Semiconductors and Insulators. The elements and other things around us (like copper,silver, gold, rubber, glass, water, oil etc) are classified into Conductors, Semiconductors and Insulators based on their electrical conductivity. Conductors have high electrical conductivity, where as insulators has the least electrical conductivity. Semiconductors are materials that have electrical conductivity in between conductors and insulators. The most common semiconductors are Germanium and Silicon. In its naturally occurring form, they are called intrinsic semiconductors. But an intrinsic semiconductor (a semiconductor in its natural form) is not suitable for making any electronic device.One primary reason for this is very low electrical conductivity of an intrinsic semiconductor at room temperature. Researchers had found a way to manipulate the pure semiconductor properties and thereby improve its electrical conductivity several times. This is achieved by a process named doping (by adding a small amount of impurity to Silicon and Germanium). The newly formed semiconductor (known as doped semiconductor) is called an Extrinsic semiconductor. An extrinsic semiconductor can be formed in 2 ways and hence there are 2 types of extrinsic semiconductor named 1) p-type semiconductor and 2) n-type semiconductor. A p-type semiconductor is formed by doping Germanium (Ge) or Silicon (Si) with a trivalent (number of valence electrons=3) element like Indium, Boron or Aluminium. An n-type semiconductor is formed by doping Ge or Si with a pentavalent (number of valence electrons=5) element like Arsenic or Antimony. You may now recall that Ge and Si are tetravalent ( number of valence electrons=4) elements. This means an n-type semiconductor will have an excess of electrons or negative charge carriers(surplus of electrons that can be donated to other elements) where as a p-type semiconductor will have a surplus of holes or positive charge carriers (you must understand that in reality a hole or a positive charge is representation of “absence of an electron” ). So a p-type semiconductor can accept electrons from a donor (an n-type semiconductor).

What all phenomena occurs during formation of a PN junction?

Three important phenomena occurs during formation of pn junction; as explained below.

Note:- While reading take a look at the picture given below frequently. It will help you to understand concepts quickly and better.

 
pn junction animated diagram of equilibriumd
 

The formation of PN Junction

As we have understood the concepts of diffusion, depletion region and drift, lets find out how the formation of PN junction gets completed. Can you guess which one out of the 3 processes (diffusion, drift and depletion region) occur first? Its obviously “diffusion”. It is because of the diffusion of charge carriers across the junction, there forms a “depletion” region at the junction. And the depletion region results in formation of an “electric field” and this electric field results in “drift”. So initially “diffusion current” will be the highest and drift current will be very small. Gradually as the “depletion region” formation continues, drift current builds up and diffusion current falls down. There comes a particular point of time, when diffusion current is exactly equal and opposite to drift current and the junction comes to a state of equilibrium. At this state, there is no “net current” and hence the formation of pn junction is complete.

How the equilibrium at PN junction is maintained?

We have come upto the point of formation of a complete PN junction and we learned how it reached equilibrium. How do you think the equilibrium is maintained? Well, lets do a quick rewind again. We have seen that electrons have moved from n-side to p-side (n–>p) during diffusion. So the n-region has lost its electrons, where as p-side has gained electrons. If we compare this, we can see that, n-region is positively charged (due to loss of electrons) compared to p-region (which is negatively charged due to gain of electrons). This results in a “potential difference” across the n-region and p-region at the junction. At the state of “equilibrium“, this potential difference reaches a particular state that it prevents any further flow of electrons from n-side to p-side. Talking in other way, we need to overcome this potential difference by using an external energy source (say a battery) to move any one more electron from n-side to p-side. If we there is no influence of external energy, the formed pn junction (kept alone) wont be able to overcome this potential difference by itself and hence it remains at the state of “equilibrium” with zero net current. This potential difference is called “barrier potential“. It is called so, because it raises a “barrier” to the further movement of electrons from n-side to p-side.