In an isolated atom, electrons are present in energy levels but in solid atoms are not isolated there is the interaction among each other due to this energy level split into different energy levels. The quantity of these different energy levels depends on the number of interacting atoms. The formation of the energy band is due to the splitting of sharp and closely compact energy levels. This is a decrease in nature. The order of energy levels in a band is 1030. The range of energy possessed by electrons in a solid is known as the energy band. Or it can also be defined as in gaseous substances their molecules are not closely arranged and in liquid also their molecules are not that much closer to each other. Whereas in the case of solid they have the strongest intermolecular force due to this their atom or molecule tends to move into orbitals of neighbouring atoms. Hence when they come together their electronic orbitals overlap. They have several bands of different energy levels, which are formed by the intermixing of atoms in solids and this set of energy levels is known as energy bands.
We have understood that materials can be classified into conductors, insulators. and semiconductors. We can make the study of their properties easier by plotting out their electron energies. It has been observed that energy states in these materials lead to the formation of energy bands, unlike the phenomenon that is observed in free atoms. In a free atom, each atom has a distinct energy level.
A major determinant of the properties of these materials will be the position of the electrons. If the electrons are inside the conductivity band, we can proceed to study the properties of the materials. In insulators, the electrons inside the valence band are separated from the conduction band by the forbidden energy gap. In contrast to this, the valence band overlaps the conduction band in metals. In semiconductors, this forbidden energy gap is so small that any thermal or other excitations can bridge the gap.
It has been noticed that the presence of a doping material in semiconductors can increase their conduction capacity by leaps and bounds. An important factor that is taken into consideration as part of the band theory is the Fermi level, which demonstrates the highest accessible electron energy levels at lower temperatures. The position of the Fermi level with respect to the conductivity band is a crucial parameter.
Different Types of Energy Bands
Valence Band: Valence electrons are those electrons that are present in the outermost shell. They have different energy levels and form an energy band known as the valence band. They occupy a maximum amount of energy.
Conduction Band: As outermost electrons are not tightly held to the nucleus due to which sometimes they leave the outermost orbit at room temperature and become free electrons. These free electrons tend to conduct current in conductors and this is the reason they are known as conduction electrons. Therefore the conduction band is that band that contains conduction electrons and has the lowest occupied energy. A wide bandgap tells us about different conditions required to energize valence electrons to conduction bands. In the case of metals both valence band and conduction band overlap each other, electrons can promptly bounce between the two groups which show the material is profoundly conductive.
Forbidden Energy Gap: The gap present between the conduction band and valence band is known as the forbidden energy gap. If the forbidden energy gap is more, then the valence band electrons are tightly bound or attached to the nucleus.
Conduction Band in Semiconductor and Metals
In metals, the conduction electrons are compared to the valence electrons of given constituent molecules. Whereas in semiconductors at low temperatures, the conduction band has no electrons. Origination of conduction electrons is due to thermal excitation from a lower energy band or impurity atoms in the crystal.
What is a Conduction Band?
Band of electron orbitals that electrons can jump from a lower energy level to a high energy level when they get excited and this band is known as the conduction band. When electrons are in an excited state, they have enough energy to move freely in the material. Due to this flow of electrons, there is a flow of electric current. There is an energy gap between the highest occupied energy state of the valence band and the least energy state of the conduction band and this gap is known as bandgap and is used for the electrical conductivity of the material. This vast gap symbolizes that a large amount of energy is required to excite electrons to the conduction band. After this again valence band and conduction band coverup as they do in the case of metals, as electrons can jump to and fro to both groups which indicate the material is highly conductive.
We have already defined resistivity in the chapter on Current Electricity. The reciprocal of resistivity is conductivity and it comes into play here. Conductivity is a factor used to measure the flow of electrons in any given material. Copper has a very high conductivity of 5.95 x 107 W-1m-1, thereby allowing electricity to flow more freely than aluminium. Aluminium has a slightly lower conductivity of 3.77 x 107 W-1m-1 when compared to copper.
Difference Between the Valence Band and Conduction Band
Conduction Band Valence Band :
The energy bands are of higher energy levels. This band is formed by a series of energy levels containing valence electrons
In this band, electrons are partially filled. Here electrons are filled. Here is the empty band of minimum energy. Here is an empty band of maximum energy. Their electrons can gain energy from external electric fields. Here electrons cannot gain energy from external electric fields. The flow of current is due to the presence of such electrons. Valence bonds occupy the highest energy level at 00 K and this is called the Fermi level.