7.3 Principle of Semiconductor Detectors
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and such semiconductor is referred to as a p-type semiconductor. Therefore, unlike
doping silicon with a phosphorus atom, each boron atom decreases the concentration
of the electron by one. Since Fermi energy is related to the number of electrons present
in the system, and since Fermi energy of intrinsic silicon is equal to (1/2)E g , any
decrease in electrons due to boron doping will decrease Fermi energy from its intrinsic
Fermi level. A decrease in the Fermi level is shown by shifting its position from the
intrinsic level toward the valence band. In other words, Fermi energy will shift toward
the valence band as we increase the concentration of boron atoms in silicon. In order
to distinguish the shifting of Fermi energy due to boron from phosphorus, the former
is referred to as p E F . For practical purposes, like phosphorus, doping with boron is
done such that the difference between the Fermi energy ( p E F ) and the uppermost
level of valence band (E V ) is maintained at about 0.1 eV (Fig. 7.1e, f).
Thus, we see that silicon can be doped either to make n-type with its Fermi energy
almost near the conduction band (Fig. 7.1c), or it can be doped to make p-type with
its Fermi energy almost close to its valence band value (Fig. 7.1d). In other words,
while n-type material can be viewed as material having excess of electrons, p-type
material can be viewed as having a very low concentration of free electrons (or having
a large concentration of holes). Thus, if an n-type material is brought in contact with
a p-type material, there would be a natural flow of excess electrons from n-type to
p-type, until an equilibrium has been established.
7.4 Formation of p : p Junction
What is the effect of flow of electrons on joining n- and p-type materials? It is
necessary to realize that if there is an electron transfer from one material to another,
the material losing electrons becomes positively charged and the other becomes negatively charged. When we join n- and p-type materials, and if electron transfer occurs
in the fashion expressed earlier, then both materials become electrically charged. This
behavior is very difficult to digest, as materials cannot become electrically charged
by simply joining them! In reality, electron transfer does occur, but instead of an
electron leaving its parent material, it accumulates at its interface. As a result, the
neutrality of the material is maintained and yet electron transfer occurs.
7.4.1 Formation of Space Charge Region
Since, in solid material, atoms are rigidly fixed in a given lattice configuration, any
movement of electrons automatically creates a positive charge at the lattice site from
where the electron has moved out. Therefore, it is not unreasonable to assume that
while electrons accumulate at the interface of two semiconductors, an equivalent
amount of positive charge would be created within n-semiconductor. It can be confirmed mathematically that these disturbances due to the transfer of electrons take
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and such semiconductor is referred to as a p-type semiconductor. Therefore, unlike
doping silicon with a phosphorus atom, each boron atom decreases the concentration
of the electron by one. Since Fermi energy is related to the number of electrons present
in the system, and since Fermi energy of intrinsic silicon is equal to (1/2)E g , any
decrease in electrons due to boron doping will decrease Fermi energy from its intrinsic
Fermi level. A decrease in the Fermi level is shown by shifting its position from the
intrinsic level toward the valence band. In other words, Fermi energy will shift toward
the valence band as we increase the concentration of boron atoms in silicon. In order
to distinguish the shifting of Fermi energy due to boron from phosphorus, the former
is referred to as p E F . For practical purposes, like phosphorus, doping with boron is
done such that the difference between the Fermi energy ( p E F ) and the uppermost
level of valence band (E V ) is maintained at about 0.1 eV (Fig. 7.1e, f).
Thus, we see that silicon can be doped either to make n-type with its Fermi energy
almost near the conduction band (Fig. 7.1c), or it can be doped to make p-type with
its Fermi energy almost close to its valence band value (Fig. 7.1d). In other words,
while n-type material can be viewed as material having excess of electrons, p-type
material can be viewed as having a very low concentration of free electrons (or having
a large concentration of holes). Thus, if an n-type material is brought in contact with
a p-type material, there would be a natural flow of excess electrons from n-type to
p-type, until an equilibrium has been established.
7.4 Formation of p : p Junction
What is the effect of flow of electrons on joining n- and p-type materials? It is
necessary to realize that if there is an electron transfer from one material to another,
the material losing electrons becomes positively charged and the other becomes negatively charged. When we join n- and p-type materials, and if electron transfer occurs
in the fashion expressed earlier, then both materials become electrically charged. This
behavior is very difficult to digest, as materials cannot become electrically charged
by simply joining them! In reality, electron transfer does occur, but instead of an
electron leaving its parent material, it accumulates at its interface. As a result, the
neutrality of the material is maintained and yet electron transfer occurs.
7.4.1 Formation of Space Charge Region
Since, in solid material, atoms are rigidly fixed in a given lattice configuration, any
movement of electrons automatically creates a positive charge at the lattice site from
where the electron has moved out. Therefore, it is not unreasonable to assume that
while electrons accumulate at the interface of two semiconductors, an equivalent
amount of positive charge would be created within n-semiconductor. It can be confirmed mathematically that these disturbances due to the transfer of electrons take