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Semiconductors part 2

Watch this to learn about the role of semiconductors in LEDs for Higher Physics (just a little bit more screen-time…)

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Imagine a world without screens.

A world in which you get your information from paper – like the vast majority of human history.

Screens surround us in our lives, perhaps even sometimes too much. They are on your TVs, phones, tablets, laptops, drones, in hospitals, in advertising, in factories – everywhere.

Many of these screens rely on basic LED technology – the simple physics behind applying semi-conductors to make colored light.

So keep your eyes on the screen as we unpack this…

This is thinkfour.

When we create doped semiconductors we add or remove additional electrons which lurk just outside their conduction and valence band in order to make the flow of electrons easier.

To make semiconductors really useful we need to combine these N-type and P-type semiconductors. If we sandwich them together we create a PN junction, this is the foundation of all modern electronics…because this creates a diode.
When we make a PN junction the additional electrons from the N-type semi-conductor fill the holes in the P-type. This makes the P-type slightly negatively charged and the N-type slightly positive. In this area there are no more free electrons or holes and we call it the depletion zone. This creates a small potential difference or voltage across the junction. This is typically about 0.7v and we call this the barrier potential.

Now, if we take this PN junction and connect it to a battery 2 things can happen:
The n-type is closest to the negative terminal of the cell and the p-type is closest to the positive terminal.

- in this instance the electrons flow out of the negative terminal towards the n-type. Any holes in the n-type are filled by the electrons from the cell and a current flows. If we orientate the junction this way it is called forward bias.

The junction is orientated the other way around with the p-type closest to the negative terminal. This time the holes in the p-type are filled by the electrons from the cell and no movement across the junction occurs, there is no current flowing. This is called reverse bias.
This PN junction therefore allows current to flow in only one direction around a circuit depending on its orientation. This is a diode.

The last thing you need to consider is the case of a special diode, an LED. In LEDs when the electrons and holes move across the junction sometimes they combine and this releases energy in the form of a photon. The energy of the photon depends on how big the bandgap is for that material a big bandgap releases a high energy photon so blue or violet, whereas a small bandgap produces lower energy photons like red. It is worth remembering that the bandgap determines the colour of the light emitted and one semi- conductor can only release one colour of light.

Without the technological input from LEDs our world would feel very different.
By manipulating the physics of semi-conductors, we make light and, in doing so, we make windows of communication.

That said, screens are not everything and despite the exciting physics, I would also strongly encourage you to relax without one for a while; pick up a book or go for a walk, and, just occasionally, leave your phone behind.

This was thinkfour, thanks for watching.

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