Video: Physics Past Exam • 2017/2018 • Pack 1 • Question 19A

Physics Past Exam • 2017/2018 • Pack 1 • Question 19A

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Video Transcript

How is a p-type semiconductor created?

This name, p-type, stands for positive type, which tells us which kind of charge is in excess on the semiconductor. If we were to take a common semiconductor material such as silicon, which we know has four electrons in its valence electron shell, when silicon atoms are joined together in a lattice structure, the atoms inside the bulk material have a full valence shell of eight electrons. And not only that, each pair of silicon atoms shares a pair of valence electrons. In this way, there’s neither an excess of positive or negative charge in silicon by itself.

However, if we replace the central silicon atom with an atom having a different valence electron structure, we can create a net charge to this bulk material. Recalling that silicon has four valence electrons, if we wanted to create an overall positively charged semiconductor, we would choose an atom that has fewer valence electrons than silicon.

In particular, if we choose an atom that has three electrons in its valence shell, such as boron, and then dope the silicon with this impurity, we see that, in one location where there used to be an electron, there is now an electron hole. This hole, the absence of an electron where it would be, gives the semiconductor an overall positive charge.

We can say then that this type of semiconductor is created by adding trivalent atoms, that is, atoms with three electrons in their valence shell, to increase the concentration of positive holes in the semiconductor. That’s the way p-type semiconductors are created.

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