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.
