Question Video: Understanding Electrons in a Doped Semiconductor Physics • 9th Grade

The diagram shows a lattice of silicon atoms that contains one atom of boron. The diagram shows the effect of the presence of the boron atom on the bound electrons in an adjacent silicon atom. What is the effective relative electronic charge of the boron atom after it has had an effect on the adjacent silicon atom?

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Video Transcript

The diagram shows a lattice of silicon atoms that contains one atom of boron. The diagram shows the effect of the presence of the boron atom on the bound electrons in an adjacent silicon atom. What is the effective relative electronic charge of the boron atom after it has had an effect on the adjacent silicon atom?

To begin, we should recognize that this diagram shows a semiconductor that’s been doped with boron. If the lattice consisted only of silicon atoms, it would be a pure semiconductor. But here, the semiconductor contains boron as an impurity. Because of this, we should recall some properties of such a doped semiconductor. To start, it’s important to remember that a neutral atom of boron contains three electrons in its outermost electron shell, so we say that it’s a trivalent atom.

When a boron atom is introduced to a silicon lattice, each of the four silicon atoms surrounding boron shares one of its outermost electrons through a covalent bond. This sharing contributes four other electrons to boron’s outermost shell, resulting in a total of seven out of eight possible electrons. We can see this illustrated in the given diagram, as there are seven outermost electrons immediately surrounding the boron nucleus. Since it does not contain the full eight electrons that would make the shell complete, we can see that there is one electron hole, or vacancy, in the shell.

It’s important to note that at this point boron and the surrounding silicon atoms remain electrically neutral. However, the presence of this electron hole means that the boron atom is very likely to accept a nearby electron to complete the shell, and this usually happens very quickly. The accepted electron could be a free electron from the lattice. But typically, it’s actually an electron from an adjacent silicon atom. This is the effect that the question statement is referring to.

In this case, the eighth electron isn’t merely shared with boron through a covalent bond. Rather, it becomes one of the boron atom’s own electrons. And thus, once the boron atom has had its effect on the lattice, the originally neutral atom becomes a negatively charged ion, often called a negative acceptor ion. Thus, with the addition of this one electron, which we know has an effective charge of negative one, the effective relative electronic charge of the boron atom is negative one.

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