9.2.1.1. p-n Junction

Table of Contents

Junction Formation

A p-n junction is the boundary formed when p-type semiconductor and n-type semiconductor are joined in one crystal. The p-type side has many holes as its majority carriers, and the n-type side has many electrons as its majority carriers. The junction is important because the carriers do not remain evenly distributed after contact.

As soon as the two sides are joined, electrons from the n-side tend to diffuse into the p-side, where electron concentration is lower. At the same time, holes from the p-side diffuse into the n-side. When an electron meets a hole, they recombine. This removes mobile charge carriers near the boundary.

The result is a narrow region around the junction where very few mobile carriers remain. This region is called the depletion region, or depletion layer. It contains fixed charged ions left behind by the doping atoms. On the n-side, donor atoms become positively charged after giving up electrons. On the p-side, acceptor atoms become negatively charged after accepting electrons.

Rendering scientific sketch…

Formation of a p-n junction and depletion region

Built-In Electric Field

Because the depletion region contains fixed charges, it creates an internal electric field. This field points from the positive donor ions on the n-side toward the negative acceptor ions on the p-side. The field opposes further diffusion of electrons and holes.

This creates a balance between two effects. Diffusion pushes carriers across the junction because of concentration differences. The electric field pushes them back in the opposite direction. At equilibrium, these two effects cancel, and there is no net current across the junction.

The junction therefore develops a built-in potential difference, often called the barrier potential. This potential makes it harder for majority carriers to cross the junction.

In an unbiased p-n junction at equilibrium, diffusion of carriers creates a depletion region, an internal electric field, and a built-in potential barrier. Net current is zero at equilibrium.

Depletion Region

The depletion region is central to how the junction works. It is called depleted because it has lost most of its mobile electrons and holes. What remains are immobile ionized dopant atoms.

A wider depletion region generally means a stronger barrier to carrier motion. The width depends on the doping of each side. Heavily doped regions usually produce a narrower depletion layer, while lightly doped regions produce a wider one.

The depletion region acts somewhat like an insulating barrier inside the semiconductor. It is not a perfect insulator, but it strongly reduces the flow of majority carriers unless an external voltage changes the barrier.

Carrier Motion at the Junction

There are two important types of carrier motion in a p-n junction, diffusion and drift. Diffusion happens because carriers move from a region of high concentration to a region of low concentration. Drift happens because charged particles respond to the electric field in the depletion region.

At equilibrium, diffusion current and drift current are equal in magnitude and opposite in direction. This is why there is no net current even though carriers are still moving microscopically.

Minority carriers also matter. In the p-side, electrons are minority carriers. In the n-side, holes are minority carriers. These minority carriers can be swept across the junction by the built-in electric field.

Energy Picture

The junction can also be understood using energy. Before contact, the p-type and n-type materials have different carrier distributions. After contact and equilibrium, the energy bands bend near the junction. This bending reflects the built-in electric field and potential barrier.

For beginners, the key idea is simple. The junction creates an energy barrier that majority carriers must overcome to cross easily. External voltage can lower or raise this barrier, which is the basis of diode action.

A p-n junction works because the built-in potential barrier controls the motion of charge carriers across the boundary.

Summary of Main Features

The p-n junction has a few essential features that should be kept clearly in mind.

Feature	Meaning
p-side	Region with holes as majority carriers
n-side	Region with electrons as majority carriers
Diffusion	Carriers move due to concentration differences
Recombination	Electrons and holes cancel each other
Depletion region	Region near junction with few mobile carriers
Built-in electric field	Internal field caused by fixed ions
Barrier potential	Potential difference opposing majority-carrier flow
Equilibrium	Drift and diffusion balance, net current is zero

Key ideas of a p-n junction:
$$\text{diffusion} \rightarrow \text{depletion region} \rightarrow \text{electric field} \rightarrow \text{potential barrier}$$
These arise naturally when p-type and n-type semiconductors are joined.

Simple Conceptual Picture

A useful mental picture is to imagine two crowded regions of carriers brought into contact. Electrons from the n-side and holes from the p-side initially spread into the other side. This leaves behind fixed charges near the junction. Those fixed charges create an electric field that resists further spreading. Very quickly, a self-balanced structure forms.

That self-balanced structure is the p-n junction. It is the physical foundation of semiconductor diodes and many other electronic devices.

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