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7 Non-conventional Detection Techniques
of crystallization (i.e., lattice dimension of the crystal), band separation would be
observed. The difference between the upper energy level of the valence band and
lower energy level of the conduction band is known as the band gap and is express in
terms of “eV”. Hence closer the atoms, greater would be the band gap. This suggests
that every material exhibits different band gap as they crystallize into different but
specific lattice dimensions.
7.3.2 Fermi Energy in a Material
What is the magnitude of energy of electrons occupying the valence band? This
is a difficult question to answer, because it involves a huge amount of mathematics
to arrive at some meaningful conclusion. But it can be confirmed that if the material
is highly pure (i.e., intrinsic semiconductor), the maximum energy electrons can
occupy being in the valence band at room temperature is equal to half of the band
gap. In other words, if we wish to excite electrons from the valence band to the
conduction band, we need to supply only (1/2)E g . This energy is normally called
Fermi energy (E F ) of electrons of an intrinsic semiconductor.
Can we alter this energy? Yes, this can be altered, if the intrinsic semiconductor is
doped with atoms possessing valence either greater or smaller than that of the host
atoms. For example, the energy of valence electrons of silicon (which is four) can
be altered by adding either boron (which has three valences) or phosphorus (which
has five valences). Again, an explanation of the reasons for the alteration of energy
of electrons requires various concepts. Therefore, without going into much detail,
we can assume that the Fermi energy of an electron is related to the concentration
of dopant (like phosphorus in silicon) present in the semiconductor. The addition of
phosphorus, for example in silicon, makes it n-type, because each phosphorus atom
creates one additional free electron in silicon. It is possible to increase the energy
of valence electrons of silicon by such doping so much that its energy is almost
equal to the lowest energy level of the conduction band. Under this condition, pure
silicon which is an insulator can become a conductor like metal without altering
its semiconducting properties. However, for practical purposes, the concentration
of doping is maintained such that the difference between the Fermi energy of the
electron and lowest energy level of the conduction band (E c ) is about 0.1–0.2 eV
(Fig. 7.1c, d).
7.3.3 n- and p-Type Materials
If silicon is doped with boron, then each boron atom creates a scarcity of one electron
(i.e., increases concentration of positive charge known as hole) at the site where boron
is substituted. This type of semiconductor thus has a large concentration of holes
7 Non-conventional Detection Techniques
of crystallization (i.e., lattice dimension of the crystal), band separation would be
observed. The difference between the upper energy level of the valence band and
lower energy level of the conduction band is known as the band gap and is express in
terms of “eV”. Hence closer the atoms, greater would be the band gap. This suggests
that every material exhibits different band gap as they crystallize into different but
specific lattice dimensions.
7.3.2 Fermi Energy in a Material
What is the magnitude of energy of electrons occupying the valence band? This
is a difficult question to answer, because it involves a huge amount of mathematics
to arrive at some meaningful conclusion. But it can be confirmed that if the material
is highly pure (i.e., intrinsic semiconductor), the maximum energy electrons can
occupy being in the valence band at room temperature is equal to half of the band
gap. In other words, if we wish to excite electrons from the valence band to the
conduction band, we need to supply only (1/2)E g . This energy is normally called
Fermi energy (E F ) of electrons of an intrinsic semiconductor.
Can we alter this energy? Yes, this can be altered, if the intrinsic semiconductor is
doped with atoms possessing valence either greater or smaller than that of the host
atoms. For example, the energy of valence electrons of silicon (which is four) can
be altered by adding either boron (which has three valences) or phosphorus (which
has five valences). Again, an explanation of the reasons for the alteration of energy
of electrons requires various concepts. Therefore, without going into much detail,
we can assume that the Fermi energy of an electron is related to the concentration
of dopant (like phosphorus in silicon) present in the semiconductor. The addition of
phosphorus, for example in silicon, makes it n-type, because each phosphorus atom
creates one additional free electron in silicon. It is possible to increase the energy
of valence electrons of silicon by such doping so much that its energy is almost
equal to the lowest energy level of the conduction band. Under this condition, pure
silicon which is an insulator can become a conductor like metal without altering
its semiconducting properties. However, for practical purposes, the concentration
of doping is maintained such that the difference between the Fermi energy of the
electron and lowest energy level of the conduction band (E c ) is about 0.1–0.2 eV
(Fig. 7.1c, d).
7.3.3 n- and p-Type Materials
If silicon is doped with boron, then each boron atom creates a scarcity of one electron
(i.e., increases concentration of positive charge known as hole) at the site where boron
is substituted. This type of semiconductor thus has a large concentration of holes