7.3 Principle of Semiconductor Detectors
121
above its initial energy (known as conduction levels). If the magnitude of “N ” is
the Avogadro number, i.e., 6.023 × 10
23 , then that many number of levels will be
created in valence and conduction energy levels. Incidentally, we know that the
chemical binding energy to hold a molecule stable is around 1–5 eV. Hence, as a
crude approximation one can say that the electrons responsible to form the molecule
must be occupying their energy within this energy value (i.e., 1–5 eV).
In other words, the depth of the “N ” energy levels which we have generated should
be related to this chemical binding energy. This means that 10
23 energy levels lie
within 1–5 eV depth. Hence, the difference between each energy level present within
either conduction or valence energy would be ≈10
−23 . This value is so small that
it becomes very difficult to differentiate one energy level from another (Fig. 7.1c).
Hence instead of calling them a level, we call them a band of energy levels. The
conduction and valence energy levels are recognized as conduction and valence
bands, respectively.
Where do these electrons after creating these two energy bands exit? All electrons
involved in this type of interaction would try to occupy either of these two bands.
Filling of electrons will start with the lowest energy of the valence band. Once this
band is filled, then it will try to occupy the conduction band. We know that one
energy level can occupy a maximum of two electrons (like one couple can have a
maximum of one wife making the number two), so if there were “N ” number of
electrons interacting with a similar number of electrons from other sets of atoms,
then these 2N number of electrons can easily occupy the valence band (which has
“N ” number of energy levels).
Why do they occupy the valence band first? When water is spilled over a table,
does it fly in the air or go to a lower height? Naturally, it follows the path which
needs the least resistance. Likewise, these electrons will tend to occupy the lower
energy band first and if some electrons are still left, then they will occupy the higher
energy band. Since a valence band has “N ” number of levels with a capacity to
accommodate 2N number of electrons, all electrons of the interacting atoms can find
their home easily within the valence band and thus the conduction band will remain
empty. Thus, the net conclusion of these discussions is that when two sets of atoms
come very close to each other, the electrons of these atoms interact forming valence
and conduction bands.
7.3.1 Formation of Band Gap
How much far apart can these two bands be found? It is very easy to visualize.
When a beautiful girl comes near your place, the level of excitation, i.e., creation of
a new energy level will certainly depend how close the lady is to you. The farther
away the girl is, the least amount of energy difference will be created. So is the case
with the atom. Closer the atom gets, greater is the interaction and hence larger is
the separation of the two new energy bands. Therefore, depending upon the nature
121
above its initial energy (known as conduction levels). If the magnitude of “N ” is
the Avogadro number, i.e., 6.023 × 10
23 , then that many number of levels will be
created in valence and conduction energy levels. Incidentally, we know that the
chemical binding energy to hold a molecule stable is around 1–5 eV. Hence, as a
crude approximation one can say that the electrons responsible to form the molecule
must be occupying their energy within this energy value (i.e., 1–5 eV).
In other words, the depth of the “N ” energy levels which we have generated should
be related to this chemical binding energy. This means that 10
23 energy levels lie
within 1–5 eV depth. Hence, the difference between each energy level present within
either conduction or valence energy would be ≈10
−23 . This value is so small that
it becomes very difficult to differentiate one energy level from another (Fig. 7.1c).
Hence instead of calling them a level, we call them a band of energy levels. The
conduction and valence energy levels are recognized as conduction and valence
bands, respectively.
Where do these electrons after creating these two energy bands exit? All electrons
involved in this type of interaction would try to occupy either of these two bands.
Filling of electrons will start with the lowest energy of the valence band. Once this
band is filled, then it will try to occupy the conduction band. We know that one
energy level can occupy a maximum of two electrons (like one couple can have a
maximum of one wife making the number two), so if there were “N ” number of
electrons interacting with a similar number of electrons from other sets of atoms,
then these 2N number of electrons can easily occupy the valence band (which has
“N ” number of energy levels).
Why do they occupy the valence band first? When water is spilled over a table,
does it fly in the air or go to a lower height? Naturally, it follows the path which
needs the least resistance. Likewise, these electrons will tend to occupy the lower
energy band first and if some electrons are still left, then they will occupy the higher
energy band. Since a valence band has “N ” number of levels with a capacity to
accommodate 2N number of electrons, all electrons of the interacting atoms can find
their home easily within the valence band and thus the conduction band will remain
empty. Thus, the net conclusion of these discussions is that when two sets of atoms
come very close to each other, the electrons of these atoms interact forming valence
and conduction bands.
7.3.1 Formation of Band Gap
How much far apart can these two bands be found? It is very easy to visualize.
When a beautiful girl comes near your place, the level of excitation, i.e., creation of
a new energy level will certainly depend how close the lady is to you. The farther
away the girl is, the least amount of energy difference will be created. So is the case
with the atom. Closer the atom gets, greater is the interaction and hence larger is
the separation of the two new energy bands. Therefore, depending upon the nature