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Different types of semiconductors

In our previous article, we looked at the characteristics of a semiconductor.  We’re now going to move on to looking at different types of semiconductors and how they work.

N-type and P-type semiconductors

The image below shows an n-type conductor:

n-type conductor

Phosphorus has one more electron than silicon does, so when it replaces some silicon atoms within the crystal, this “extra” electron is free to move when a voltage is applied.  The electron, which is negative, is attracted towards the positive terminal, creating a current:

, Different types of semiconductors

The image below shows a p-type conductor:

p-type semiconductor

Boron has one fewer electron than silicon, so when it replaces some silicon atoms within the crystal, it leaves a space, or vacancy, where there is no electron.  This vacancy is called a hole, and it can be classified as a virtual particle.

How does current flow in a p-type semiconductor?

In this conductor, the holes appear to be moving.  The neighbouring electron is then pulled towards the hole to fill it.  When the electron has moved into the hole, it leaves behind a new hole, as you can see in the image below:

current flow in a p-type conductor

This process is repeated, creating holes in the Boron and Silicon.

As this process is repeated, electrons move towards the positive pole while holes move towards the negative pole.  Only the electrons are actually moving, but the holes have positively charged particles:

, Different types of semiconductors

As a result, both p-type and n-type semiconductors can have current flow, but they aren’t as conductive as metal.  What this means, is that we would have no use for semiconductors if the only purpose was current flow or conductivity.

The advantages of a semiconductor include its ability to allow or stop current flow based on certain conditions.  In fact, the basic principle behind a semiconductor is in its rectification behaviour using a p-n junction.

Forward Bias

Voltage is applied in the forward direction of the p-n junction:

Forward Bias

When the voltage is applied to the p-n junction, the holes and electrons can be moved towards the interface.

When holes and electrons meet at the interface, the electrons jump into holes and both are eliminated.  After those electrons are eliminated, more flow into the n-layer, and electrons flow out from the p-layer, creating new holes. This is repeated, enabling the current to continue to flow.

Reverse Bias

Voltage is applied in the reverse direction of the p-n junction:

reverse bias

Here is the region that doesn’t have any holes or electrons, called the depletion layer.  Voltage is applied to the p-n junction so that n becomes plus.  Since the holes and electrons move away from one another, they don’t meet at the interface, and the current can’t flow.

A region forms close to the interface, called the depletion layer, which doesn’t have any holes or electrons, and this produces voltage-withstanding. This lets us know that there is rectification behaviour in the p-n junction.

Keep an eye out for our next articles, which will give you even more interesting facts on semiconductors and their characteristics.

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