p-n Junction

Understanding the formation and functioning of a p-n Junction is paramount to analyzing the working of semiconductor devices. Let’s look at how a p-n Junction is formed and how it behaves under a bias (or an externally applied voltage).

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Formation of a p-n Junction

Let’s imagine a thin p-type silicon (p-Si) semiconductor wafer. Now, we add a pentavalent impurity to it. Due to this, a small part of the p-Si wafer is converted into an n-Si. Now, the wafer contains a p-region and an n-region with a metallurgical junction between the two. Two important processes take place during the formation of a p-n Junction:

1. Diffusion
2. Drift

In an n-type semiconductor, the concentration of electrons is more than that of holes. On the other hand, in a p-type semiconductor, the concentration of holes is more than that of electrons.

When a p-n junction is being formed, holes diffuse from the p-side to the n-side (p→n) while electrons diffuse from the n-side to the p-side (n→p). This happens due to the concentration gradient across p and n sides. This gives rise to a diffusion current across the junction. Let’s look at both the scenarios:

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Electron Diffuses from n→p

This diffusion leaves an ionized donor (or a positive charge) on the n-side. This donor is bonded to the surrounding atoms and is immobile. As more and more electrons start diffusing to the p-side, a layer of positive charge (or positive space-charge region) in the n-side of the junction is formed.

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Hole Diffuses from p→n

This diffusion leaves an ionized acceptor (or a negative charge) on the p-side. As more and more holes start diffusing to the n-side, a layer of negative charge (or negative space-charge region) on the p-side of the junction is formed.

Since the diffusion of electrons and holes across the junction depletes the region of its free charges, these space charge regions together are called the depletion region.

This process is depicted in the image above. The thickness of the depletion region is merely around one-tenth of a micrometre. Also, an electric field develops, directed from the p-side to the n-side of the junction. This is because of the positive space-charge region on the n-side and the negative space-charge region on the p-side of the junction.

This electric field is responsible for the movement of electrons from the p-side to the n-side and holes from the n-side to the p-side. This motion of charged carriers due to the electric field is called drift. Hence, a drift current starts which is opposite in direction to the diffusion current. This is also seen in the image above.

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The Last Stages of Formation of a p-n Junction

When the diffusion starts, the diffusion current is large and the drift current is very small. As diffusion continues, the space-charge regions on either side of the junction start extending. This strengthens the electric field and eventually the drift current. The process continues till diffusion current = drift current. This is how a p-n junction is formed.

Barrier Potential

In the state of equilibrium, there is no current in a p-n junction. A difference of potential develops across the junction of the two regions due to the loss of electrons by the n-region and the subsequent gain by the p-region. The polarity of the potential opposes the further flow of carriers to maintain the state of equilibrium. This is the Barrier Potential.

Solved Examples for You

Question: Why is a photodiode is used in reverse bias?

1. Photocurrent flows only in reverse bias
2. Thermally generated current is large in reverse biased as compared to the photocurrent
3. Photocurrent is large in forward bias as compared to the forward bias current
4. Photocurrent is significant in reverse bias as compared to the reverse bias current

Solution: Option (D)

When a photo-diode is reverse biased, the width of depletion layer increases as compared to forward biased and a small reverse current (dark current) flows through the diode. Now, when the light is incident on the junction, electron-hole pairs are generated in depletion layer in a big amount (due to broad depletion layer) and these charge carriers can easily cross the barrier, hence contribute to current across the diode. We can say that in reverse bias, diode changes the incident light to current, more significantly due to broad depletion layer i.e. photocurrent is significant in reverse bias as compared to the forward bias current.

1. Solar Cell
2. Photo-diode
3. LED( Light Emitting Diode)
4. None of these

Solution: Option (A) Solar Cell

Question: From the adjacent circuit, the output voltage is

1. 10 V
2. 100 V
3. 90 V
4. 0 V

Solution: Here, Zener diode is connected parallel to the input voltage source. Zener diode is known as a voltage regulating diode hence, the output voltage will be equal to the voltage across Zener diode i.e. 10V