Understanding the PN Junction

Whenever someone decides to learn electronics, the first question that comes to his mind may be – “Where shall I begin?“. I would say, one shall 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).

Now you might have got an idea about why researchers have arrived at making two types of extrinsic semiconductors – p-type  (which can accept electrons) and n-type (which can donate electrons). Lets see what all interesting phenomena happen when we form a junction using a p-type and n-type semiconductor.

Industrially there are several different ways to make a pn junction. For the ease of understanding, I will explain it in a simple sentence. We usually make it using a single wafer of Si or Ge. This means, we first convert a silicon (pure, intrinsic) wafer to a p-type semiconductor by doping it with a trivalent impurity (Boron or Indium) on one side. Then we dope this p-type semiconductor with a pentavalent impurity (Phosphorous or Arsenic or Antimony ) to form an n-type region on the same wafer. Thus we have made a p-type and n-type semiconductor on the same wafer, resulting in formation of a junction (between p-type and n-type semiconductors) on the same silicon wafer.

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.

1) Diffusion  2) Formation of space charge  3) Drift 

How diffusion occurs ?

In an n-type semiconductor, the majority carriers are negative charge carriers or electrons. In a p-type semiconductor, majority carriers are holes or positive charges. When a junction is formed in a silicon wafer by doping, a concentration gradient occurs between p-type and n-type materials. This results in electrons moving from n side to p side and holes moving from p side to n side through the junction (call it as “initial movement“). When an electron leaves the n-side region, it leaves behind an ionised donor (a positive charge ) at the n-side. Similarly when a hole is diffused to n-side, it leaves behind an ionised acceptor (a negative charge) at the p-side.  This movement of electrons from n-side to p-side (n–>p) and the movement of holes from p-side to n-side is called (p–>n) “diffusion” and it results in a current named as “diffusion current“.

How space charge formation occurs ?

We have seen that an electron moving from n to p (n–>p) leaves behind a positive charge at the n-side of the junction. Similarly a hole moving from p-side to n-side (p–>n) leaves behind a negative charge at the p-side of the junction. When more and more electrons leaves the n-region & more and more holes leaves the p-region, a region of positive and negative charges is formed at the junction. Positive charges get accumulated near the n-side junction and negative charges get accumulated near the p-side junction. This region is known as “depletion” region. It has been named so because the region is formed by the “initial movement” of electrons and holes, where they “depleted” their original positions leaving behind +ve and -ve charges at the junction.

How drift occurs?

We have seen that there is a layer of -ve charges accumulated at the p-side of junction and a layer of +ve charges accumulated at the n-side of the junction. This results in the formation of an electric field directed from positive charge to negative charge. This electric field causes electrons to move from p side to n side (p–>n) and the holes to move from n side to p side (n–>p). This motion of charge carriers due to electric field is known as “drift” The current resulting from the flow of electrons and holes due to this electric field (generated by depletion region) is known as “drift current”. If you observe carefully, you can easily see that drift current is opposite in direction to the diffusion current.

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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.

Here you may find a good video with animation, which explains the formation of pn junction. You do watch this video. It will help you to grasp concepts behind pn junction even better.