In case of a single isolated atom, the electrons in any orbit possess definite energy.
However, an atom in a solid is greatly influence by the closely-packed neighboring
atoms. The result is that the electron in any orbit of such an atom can have a range of
energies rather than a single energy. This is know as energy band. The range of
energies possessed by an electron in a solid is know as energy band.
The concept of energy band can be easily understand by referring to Fig. 4.3.
Fig. 4.3 shows the energy levels of a single isolated atom of silicon. Each orbit
of an atom has a single energy. Therefore, an electron can have only single
energy corresponding to the orbit in which it exists. However, when the atom is in
a solid, the electron in any orbit can have a range of energies. For instance,
electrons in the first orbit have slightly different energies because no two
electrons in this orbit see exactly the same charge environment. Since there are
millions of first orbit electrons, the slightly different energy levels form a band,
called 1st energy band [See Fig. 4.3 (iii)]. The electron in the first orbit can have
any energy range in this band. Similarly, second orbit electrons form second
energy band and so on.
Important Energy Bands In Solids
As discussed before, individual K, L, M etc. energy levels of an isolated atom are
converted into corresponding bands when the atom is in a solid. Though there are a
number of energy bands in solids, the following are of particular importance [See Fig.
(i) Valence band:
The range of energies (i.e. band) possessed by valence
electrons is know as valence band.
The electrons in the outermost orbit of an atom are know as valence electrons. In a
normal atom, valence band has the electrons of highest energy. This band may be
completely or partially filled. For instance, in case of inert gases, the valence band
whereas for other materials, it is only partially fill. The partially band can
accommodate more electrons.
(ii) Contraction band:
In certain materials (e.g metals), the valence electrons are loosely attach to
the nucleus. Even at ordinary temperature, some of the valence electrons may
get detached to become free electrons. In fact, it is these free electrons which
are responsible for the conduction of current in a conductor. For this reason, they
are call conduction electrons.
The range of energies (e.i band) possessed by conduction band electrons is known as
All electrons in the conduction band are free electrons. Il a substance has empty
conduction band, it means current conduction is not possible in that substance
Generally, insulators have empty conduction band. On the other hand, it is partially filled
(iii) Forbidden energy gap:
The separation between conduction band and valence
band on the energy level diagram is known as forbidden energy gap.
No electron of a solid can stay in a forbidden energy gap as there is no allowed energy
state in this region The width of the forbidden energy gap is a measure of the bondage
of valence electrons to the atom. The greater the energy gap, more tightly the valence
electrons are bound to the nucleus. In order to push an electron from valence band to
the conduction band (ie: to make the valence electron free), external energy equal to
the forbidden energy gap must be supplied.
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