Video Transcript
The diagram shows a lattice of
silicon atoms in a semiconductor. The left side of the lattice has
been doped with donor atoms. This is called the n-side. The right side of the lattice has
been doped with acceptor atoms. This is called the p-side. The regions on either side of the
dividing line are of equal size, and the ion concentration is the same on both
sides. The semiconductor is at thermal
equilibrium.
Alright, before we get to our
question, let’s understand this information we’re given here. We’re told that this diagram shows
us a lattice of silicon atoms. And we can see that most of the
atoms here are indeed silicon, represented by the symbol Si. So we have the silicon lattice, and
we’re told that the left side, over here, has been doped with donor atoms. This means that some of the silicon
atoms in the lattice have been replaced by what are called impurities. These impurities are called donor
atoms because they have one more valence electron in their natural state than
silicon does. And therefore, when they’re added
into the lattice of silicon, they effectively donate that excess electron to the
lattice.
We’re told that this left side of
the lattice is called the n-side, so we can label it that way up top. Then, if we look at the right side
of our silicon lattice, we’re told that this too has been doped, but this time with
acceptor atoms. These are atoms which by themselves
have one fewer valence electron than silicon. And therefore, when they’re added
into the lattice structure, they create a vacancy or an electron hole. The reason they’re called acceptor
atoms is because they tend to accept free electrons into these holes. This right side is the p-side of
the semiconductor. Knowing that the p-side and the
n-side are of equal size and that they have the same ion concentration and that our
semiconductor overall is at thermal equilibrium, let’s now move on to our
question.
The first part of our question
asks, toward which side of the lattice will free electrons tend to move by
diffusion? And then part two says, toward
which side of the lattice will holes tend to move by diffusion? Alright, so considering this first
part about the movement of free electrons in our semiconductor, we learned earlier
that it’s the left side of the semiconductor, the n-side, that features what are
called donor atoms. We see these atoms here and
here. They’re atoms of phosphorus.
The reason these phosphors atoms
are called donors is that while silicon naturally has four electrons in its valence
shell, an atom of phosphorus has one more, five, which means that when these
phosphorus atoms are inserted into our lattice, each one contributes an extra, we
could call it, electron, one more than the silicon atoms they replaced. These extra electrons are separated
from the phosphorus atoms and begin to move about freely within the n-side of our
semiconductor.
So then, when it comes to which way
free electrons will move, we now know what side they’ll begin on. They’ll start on the n-side,
contributed by these donor phosphorus atoms. Negatively charged electrons are
drawn to positive charges, and it turns out that we can find such positive charges
over on the p-side. Here, instead of doping our silicon
lattice with phosphorus, we’ve done it with boron. Boron is different from silicon in
that it has only three valence electrons, one fewer than silicon. And so, when these boron atoms
replace silicon in the lattice, there’s one missing electron, we could say, in their
valence shell.
These vacancies are known as holes,
and they have an effective positive charge. And that is just what free
electrons are drawn to. So over on the n-side, we see this
free electron right here and this one here, one each contributed by the phosphorus
atoms. And we know that these negatively
charged objects will be drawn to the effectively positively charged holes over here
on the p-side. So that’s our answer for this first
part of our question.
And now let’s think about the
second part which asks, toward which side of the lattice will holes tend to move by
diffusion? Well, just as the negative free
electrons are drawn to the positive holes, so the positive holes will be drawn to
the negative free electrons. So the holes then starting out on
the p-side, the right side, will be drawn towards the left, and that, as we’ve seen,
is the n-side. By diffusion then, holes will be
drawn towards the n-side or the left side of our lattice.
