In the periodic table, we can see three major categories of metals. They are metals, non-metal, and semi-metals. So, what makes a substance a metal, a non-metal and a semi-metal? that helps us to understand how the electrons behave in those substances and determine their conductivity. As you are already aware that metals are good conductors. Now, there is a range of substances in the metals category and some are more conductive than others, example gold is a better conductor than iron. And in non-metals, we clearly have substances that are very good insulators. They do not conduct any electricity at all but then there are the semiconductors like silicon and germanium, they do conduct electricity as long as the conditions are right.
In this article, I am going to explain specifically what factor that determines their conductivity, what is the band theory. Hopefully, in this article, you get a better understanding of what is occurring at the molecular level that determines the substances ability to conduct whether it is a metal, a non-metal or a semi-metal. Before we start to discuss band theory, Let's refresh about valence band, conduction band, and the forbidden energy gap.
The electrons in the inner shells of the atom are strongly bound to the nucleus. Generally, A band which is occupied by the valence electrons or a band having highest energy is known as valence band. The valence band may be partially or completely filled and this band can never be empty.
In some materials, the valence electrons are loosely attached to the nucleus and even at room temperature some of the valence electrons can leave the valence band. These are called free electrons. They are responsible for conduction of current in the conductor and therefore it is called conduction electrons. The band occupied by these electrons is called a conduction band. this band may be the empty band or partially filled band.
The separation between the valence band and conduction band is called the forbidden energy gap. If an electron is to be transferred from valence band to conduction band, external energy is required which is equal to the forbidden energy gap.
Imagine now our valency and our conduction bands represented by two boxes. So, in the first case, we are going to represent a conductor. So, here is a conductor has a conduction band that overlaps the valence band, you can see it is represented by an increasing amount of energy as we go up. So, because they overlap any electrons that exist in the valence band is automatically in the conduction band. There is no gap for them to have to traverse in order to become conductive. So, the outermost electrons are free to conduct and hence it is a conductor.
What about an insulator? all insulator has a valence band and a conduction band but there is quite a large separation or forbidden gap in this case, for the insulator. Because this gap is quite large and it can range six to nine electron volts. So these electrons are unable to get into the conduction band and therefore unable to become conductive and it is an insulator.
So, what about semiconductors? a semiconductor also has a valence band and a conduction band. There is also a gap, however, the forbidden gap is much smaller and so for an electron to move from the valence band to conduction band, it requires an amount of energy to move from the valence band to conduction band. But the energy can be gained through thermal energy. So, for example, if I have a semiconductor such as germanium, it only needs about 1.1 eV to traverse it. How do we manage to do that? just we need to heat up the substance as long as it is above zero degrees Kelvin. Then there may be enough energy for electrons to move from the valence band to conduction band and of course, once they are in the conduction band, they are free to become conductive and they will conduct if I apply the electric field to them or apply a potential difference across them.
In this article, I am going to explain specifically what factor that determines their conductivity, what is the band theory. Hopefully, in this article, you get a better understanding of what is occurring at the molecular level that determines the substances ability to conduct whether it is a metal, a non-metal or a semi-metal. Before we start to discuss band theory, Let's refresh about valence band, conduction band, and the forbidden energy gap.
Valence Band, Conduction Band, and Forbidden Energy Gap
In some materials, the valence electrons are loosely attached to the nucleus and even at room temperature some of the valence electrons can leave the valence band. These are called free electrons. They are responsible for conduction of current in the conductor and therefore it is called conduction electrons. The band occupied by these electrons is called a conduction band. this band may be the empty band or partially filled band.
The separation between the valence band and conduction band is called the forbidden energy gap. If an electron is to be transferred from valence band to conduction band, external energy is required which is equal to the forbidden energy gap.
Band Theory of Metals, Semi-conductors, and Insulators
What about an insulator? all insulator has a valence band and a conduction band but there is quite a large separation or forbidden gap in this case, for the insulator. Because this gap is quite large and it can range six to nine electron volts. So these electrons are unable to get into the conduction band and therefore unable to become conductive and it is an insulator.
So, what about semiconductors? a semiconductor also has a valence band and a conduction band. There is also a gap, however, the forbidden gap is much smaller and so for an electron to move from the valence band to conduction band, it requires an amount of energy to move from the valence band to conduction band. But the energy can be gained through thermal energy. So, for example, if I have a semiconductor such as germanium, it only needs about 1.1 eV to traverse it. How do we manage to do that? just we need to heat up the substance as long as it is above zero degrees Kelvin. Then there may be enough energy for electrons to move from the valence band to conduction band and of course, once they are in the conduction band, they are free to become conductive and they will conduct if I apply the electric field to them or apply a potential difference across them.
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