Video Transcript
In this video, we will learn how to
describe the electrical bonds in pure semiconductor materials. Here, the type of conducting we’re
thinking about is electrical conduction. A material that’s not a good
conductor of electricity is called an insulator. Examples of insulators include
glass, plastic, and wood. None of these materials easily
permits the flow of electric charge. Materials that are highly
conductive are called conductors. All the metals we find on the
periodic table are conductors. Because of how well they facilitate
the flow of charge, most electric circuits are made of metals. There are some materials though
that are neither good insulators nor good conductors. They fit somewhere in between and
are called semiconductors.
A pure semiconductor is one that is
made up of just one atomic element. A semiconductor made of silicon,
for example, is a pure semiconductor. Though many different materials can
be used to make a semiconductor, silicon is one of the most common. Silicon is the second most abundant
element in Earth’s crust. And it’s also the eighth most
abundant element in the universe overall. We’ll keep in mind that not every
pure semiconductor is made of silicon. But for the rest of our lesson,
we’ll use this element as an example of such a material.
Silicon’s usefulness as a
semiconductor comes down to its atomic structure. If what we see now is the nucleus
of a single silicon atom, then to complete the picture, we’ll want to show how the
electrons of this atom are distributed. A silicon atom’s electrons occupy
three different energy levels. In the first level, there are two
electrons. In the second level, there are one,
two, three, four, five, six, seven, eight electrons. And lastly, in the third layer,
there are one, two, three, four electrons. This outermost energy level is
called the valence level or the valence shell.
It’s the occupancy of electrons in
the valence layer that determines how a silicon atom interacts with other atoms
nearby. From an energy perspective, it’s
favorable for this outermost energy level to have eight electrons or zero
electrons. Under either of those conditions,
this atom would be fairly electrically stable. That is, it will be unlikely to
either gain electrons from or lose electrons to its environment. That gaining and losing is part of
what it means to be electrically conductive.
A neutral silicon atom like this
though has four electrons in its valence shell. It’s neither very close to having a
full outermost shell of eight electrons or an empty one with zero electrons. Since it’s largely the electrons in
this outermost shell that determine how this atom interacts with nearby atoms, when
we draw a single atom, it’s not unusual just to draw in its valence shell of
electrons and ignore the other interior ones. When a pure semiconductor is made
of silicon, it’s composed of many, many silicon atoms arranged in what’s called a
lattice.
A lattice is an orderly structure
of rows and columns. In a real silicon semiconductor,
there will be many more rows and columns than the ones we show here. When silicon atoms are arranged
like this, the atoms form what are called covalent bonds with their neighbors. To see how these bonds form, let’s
consider just two silicon atoms next to one another. Each of these atoms has four
electrons in its valence shell. It’s possible though for some of
these electrons to be shared in between the two atoms. Let’s say, for example, that this
electron here and this electron here are part of a covalent bond between these two
silicon atoms. That would mean that these
electrons in a sense belong to both atoms. We represent that by drawing in
these atoms at the intersection points between these two atoms’ valence
orbitals. Due to this covalent bond, we would
now say that each of these silicon atoms has five electrons in its valence
shell.
We’re getting closer, in other
words, to having a full or complete valence shell of eight electrons. This concept of electron sharing
through covalent bonds is very important in what’s called the bulk material of a
lattice. Let’s consider the silicon atom in
our lattice that’s completely surrounded by other atoms. This atom is capable of forming
four covalent bonds: one with this atom, one with this atom, one with this atom of
silicon, and one with this one. Through these bonds, four electrons
are now added, we could say, to the orbital of the central atom of silicon. That is, they’re shared with that
atom so that now this central silicon atom has one, two, three, four, five, six,
seven, eight electrons in its valence shell. Energetically then, this central
atom is unlikely to either lose or gain electrons.
If we apply this idea of electron
sharing across all the silicon atoms in this lattice, we can see that any interior
atoms — here we have just one — have eight electrons in their valence shell. And those on the borders have
either six or seven valence electrons. If we think of a realistically
sized silicon semiconductor, we know that there are more rows and columns than we’ve
drawn in here, many more. In a realistically sized sample,
most of the atoms would be interior atoms like this, with eight electrons in their
valence shell.
If those electrons tend to stay in
place, and energetically they are likely to, then this sample of silicon will not
easily transfer electrons from one part of the sample to another. At very low temperatures, even down
to absolute zero, silicon is a poor conductor. However, in a sample of silicon
above this minimum temperature, energy is available to displace some of the
electrons in the bulk of the sample. For example, if energy is added to
this electron in the central silicon atom, that electron could be ejected from the
valence orbital. An electron released like this is
called a free electron. Energetically, it’s free to roam
about the entire silicon semiconductor.
When it leaves, the electron leaves
behind what is called a hole. Even though a hole is just an
absence of an electron, we can think of it as having an effective positive
charge. Whenever a free electron is
created, a hole is created with it. In a pure semiconductor sample,
there are always exactly as many free electrons as there are holes. Even though this central silicon
atom has lost a negatively charged electron so that now it has an overall effective
positive charge, we typically don’t call such atoms ions. The reason for this is that this
positively charged hole is likely to be quickly filled by another electron. And that electron could come from
somewhere else within this same silicon atom.
For example, this electron here
might move over to the positively charged hole. If this happened, that electron
would leave behind its own positively charged hole. And then that hole is likely to be
quickly filled by another electron. In this way, holes are actually
capable of moving across the silicon lattice. They can be just as mobile as
electrons.
But there’s another way for a hole
to be filled besides accepting an electron that’s bound within the lattice. At any given instant, there are
likely many free electrons roaming throughout the silicon sample. One of these might be attracted to
the positively charged hole and then fill that hole. When a free electron fills a hole
in the lattice, the process is called recombination. And of course wherever that free
electron came from, it left behind a hole.
So, whenever a free electron is
created or a bound electron changes locations, a hole is left behind. Because holes have an effective
positive charge, they attract negatively charged electrons. A hole can be filled by a free
electron or by a bound electron. In a pure semiconductor sample, a
good way to control the total number of holes and, therefore, the total number of
free electrons is to change the temperature of that sample. The higher the temperature is, the
more energy can be transferred to electrons in the lattice. This results in more free electrons
and more holes. Raising the temperature of the
lattice makes it behave more like a conductor. On the other hand, decreasing the
temperature of the sample will mean that fewer free electrons and holes are
created. Free electrons and holes are what
allow electric charge to move through the lattice. When there aren’t many of them, the
lattice behaves more like an insulator.
Knowing all this about pure
semiconductors, let’s look now at an example.
An atom of silicon is part of an
object made of silicon atoms, as shown in the diagram. Only the electrons in the outermost
shells of the atoms are individually represented. How many of the electrons in the
outermost shell of an atom in the object form covalent bonds with adjacent
atoms?
Here, we see a lattice of silicon
atoms, where the red dots in the center represent silicon nuclei and the blue dots
around these red centers represent electrons. The electrons represented in our
diagram show only those in the outermost shells of these atoms. Using this representation, a single
silicon atom would look like this. There’s the red atomic nucleus, and
then there are four valence electrons. When many such atoms are combined
to form a lattice, the atoms of silicon form covalent bonds with one another. This involves the sharing of a pair
of electrons between a pair of atoms. Through these bonds, it’s possible
to effectively fill up the valence shell of a silicon atom.
Since each of the silicon atoms
starts out with four valence electrons, having an outermost shell with eight
electrons means that four have been added. And these four have been added
through shared covalent bonds. We see a demonstration of this with
this atom, which is located in the bulk of our silicon lattice. This atom forms a covalent bond
with this atom above it, this silicon atom to its right, this silicon atom below it,
and this silicon atom to its left. Each one of these four atoms
surrounding the central one shares an electron with that central atom. This is how the central silicon
atom effectively gains four valence electrons for a complete valence electron shell
of eight. In answer to our question then, we
can say that four electrons in the outermost shell of an atom in the object form
covalent bonds with adjacent atoms.
Let’s look now at another
example.
The diagram shows a lattice of
silicon atoms at a temperature of 300 kelvin. Which of the features labeled
in the diagram is a free electron? What is the effective relative
charge of the feature labeled B?
Looking at our diagram, we see
that this lattice of silicon atoms is represented using red and blue dots. The larger red dots represent
the nuclei of silicon atoms. The smaller blue dots in
between the larger red ones represent individual electrons in the outermost or
valence shells of these atoms. We’re told that this lattice of
atoms is at a temperature of 300 kelvin. This means that there is enough
thermal energy available to remove some of the bound electrons from these
silicon atoms and create free electrons out of them.
By way of example, a single
silicon atom has four valence electrons. These electrons are bound
within the valence shell. But if enough energy were
transferred to one of them, say to this electron here, then this electron could
leave the valence shell and become a free electron. When this happens, such an
electron leaves behind what is called a hole or a vacancy. Free electrons and holes go
together. Every time a free electron is
created, it leaves a vacancy behind. This is what we see in our
diagram at the points B and A.
Point A shows an electron being
liberated from an atom, that is, becoming a free electron, while point B shows
the vacancy or the hole left behind. We can answer then the first
part of our question. Because electrons here are
represented by blue dots and point A shows a blue dot leaving its bound state
within the orbital of an atom, we know that it’s feature A in the diagram that
shows us a free electron. Feature B shows what is left
behind after the electron is ejected. This is called a hole or a
vacancy. While this location previously
had a negative electric charge, the charge of the electron that occupied it, it
now has an effective relative positive charge.
If we think of the charge of
the electron by itself as negative one and the charge of the hole by itself as
zero since there’s nothing there, then we can see that to go from the charge of
the electron to the charge of what it leaves behind, we need to add one. This then is the relative
charge of the hole, the charge of the hole relative to an electron. The effective relative charge
of the feature labeled B is plus one.
Let’s now finish this lesson by
reviewing a few key points. In this lesson, we learned that
when it comes to electrical conductivity, there are effectively three classes of
materials: insulators that do not conduct electricity well; conductors that do; and
semiconductors, which lie somewhere in between these extremes. The most common, though certainly
not the only, material that semiconductors are made of is silicon.
Silicon’s usefulness as a
semiconductor material has to do with the number of valence electrons it possesses,
four. When silicon atoms are arranged in
rows and columns called a lattice, the silicon atoms interior to that lattice form
covalent bonds with their neighbors. And this allows them to share
electrons so that they effectively have a full valence shell of eight electrons. If enough energy is transferred
though to one of these valence electrons, that electron can be ejected from the atom
and become what’s called a free electron. When this happens, what is left
behind is called a hole or a vacancy. Compared to the released electron,
that hole has an effective positive charge.
At any given time, the number of
free electrons in a pure semiconductor and the number of holes is the same. A hole can be filled either by
another free electron coming from somewhere within the lattice or by a bound
electron transferring to occupy that hole. This is a summary of pure
semiconductors.
