The portal has been deactivated. Please contact your portal admin.

Lesson Video: Pure Semiconductors Physics • 9th Grade

In this video, we will learn how to describe the electrical bonds in pure semiconductor materials.

12:46

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.

Nagwa uses cookies to ensure you get the best experience on our website. Learn more about our Privacy Policy.