Energy Bands Description
In gaseous substances, the arrangement of molecules are spread apart and are not so close to each other. In liquids, the molecules are closer to each other. But, in solids, the molecules are closely arranged together, due to this the atoms of molecules tend to move into the orbitals of neighbouring atoms. Hence, the electron orbitals overlap when atoms come together.
In solids, several bands of energy levels are formed due to the intermixing of atoms in solids. We call these set of energy levels as energy bands.
Formation of Energy Bands
In an isolated atom, the electrons in each orbit possess definite energy. But, in the case of solids, the energy level of the outermost orbit electrons are affected by the neighbouring atoms.
When two isolated charges are brought close to each other, the electrons in the outermost orbit experiences an attractive force from the nearest or neighbouring atomic nucleus. Due to this reason, the energies of the electrons will not be at the same level, the energy levels of electrons are changed to a value which is higher or lower than that of the original energy level of the electron.
The electrons in the same orbit exhibit different energy levels. The grouping of this different energy levels is known as energy band.
However, the energy of the inner orbit electrons are not much affected by the presence of neighbouring atoms.
Classification of Energy Bands
The electrons in the outermost shell are known as valence electrons. These valence electrons contain a series of energy levels and form an energy band known as valence band. The valence band has the highest occupied energy.
The valence electrons are not tightly held to the nucleus due to which a few of these valence electrons leave the outermost orbit even at room temperature and become free electrons. The free electrons conduct current in conductors and are therefore known as conduction electrons. The conduction band is one that contains conduction electrons and has the lowest occupied energy levels.
Forbidden Energy Gap
The gap between the valence band and the conduction band is referred to as forbidden gap. As the name suggests, the forbidden gap doesn’t have any energy and no electrons stay in this band. If the forbidden energy gap is greater, then the valence band electrons are tightly bound or firmly attached to the nucleus. We require some amount of external energy that is equal to the forbidden energy gap.
The figure below shows the conduction band, valence band and the forbidden energy gap.
Gold, Aluminium, Silver, Copper, all these metals allow an electric current to flow through them.
There is no forbidden gap between the valence band and conduction band which results in the overlapping of both the bands. The number of free electrons available at room temperature is large.
Glass and wood are examples of the insulator. These substances do not allow electricity to pass through them. They have high resistivity and very low conductivity.
The energy gap in the insulator is very high up to 7eV. The material cannot conduct because the movement of the electrons from the valence band to the conduction band is not possible.
Germanium and Silicon are the most preferable material whose electrical properties lie in between semiconductors and insulators. The energy band diagram of semiconductor is shown where the conduction band is empty and the valence band is completely filled but the forbidden gap between the two bands is very small that is about 1eV. For Germanium, the forbidden gap is 0.72eV and for Silicon, it is 1.1eV. Thus, semiconductor requires small conductivity.
Energy Band Theory
According to Bohr’s theory, every shell of an atom contains a discrete amount of energy at different levels. Energy band theory explains the interaction of electrons between the outermost shell and the innermost shell. Based on the energy band theory, there are three different energy bands:
- Valence band
- Forbidden energy gap
- Conduction band
Frequently Asked Questions – FAQs
The valence band and the conduction band overlap in _____.
What is the energy that a valence electron should have to jump from valence band to conduction band called?
What is the energy gap between the valence and conduction band termed as?
The band in which the electrons move freely is known as _____.
What is a band model?
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