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7 Non-conventional Detection Techniques
place within the vicinity of about 10,000 Å from the interface of the semiconductor.
It is assumed that a positive charge created within the n-semiconductor also lies in a
plane at about 10,000 Å from the interface. Distance between this plane and the interface is known as the width of space charge (w). The magnitude of “w” depends on
the difference between the Fermi levels of two semiconductors; larger the difference,
greater is the width of space charge. This model suggests the formation of two types
of charges present in two planes separated by the width of space charge. Moreover,
these planes behave like two parallel plate capacitors. Let’s assume for simplicity
that the potential difference created due to such charge separation in a semiconductor is about 1.0 V and the charges are separated by a distance of 1000 Å. Then, an
electrical field of 0.1 MV cm
−1 is created in the space charge width. Electrical field
of this magnitude will be formed in both n- and p-type Si semiconductors.
Under this situation, if we excite an electron from the valence band to the conductance band in the n-semiconductor, a hole will be created in the valence band. If
these carriers (i.e., electrons and holes) are created within the space charge region,
electrons will be immediately forced to collect at the other end of n-type material
(i.e., behind the interface) and holes would be forced to move to the interface of
n- and p-semiconductors. Similarly, if an electron of p-semiconductor is excited
within the space charge region, due to electrical field, the electron will move toward
the interface of n- and p-semiconductors and hole will move toward the bulk. If
excitation of electrons is done in both semiconductors simultaneously, electrons will
find its place at the backside of the n-semiconductor, and holes will be collected at
the backside of the p-semiconductor. If the backside of the n- and p-type materials
is connected to an ammeter, these accumulated carriers will move to get neutralized,
resulting in a flow of current.
7.4.2 Distribution of Carrier Concentration
Another aspect of this junction formation needs to be touched upon. Before n- and
p-type materials are joined, the concentration of electrons (in n-type) and holes (in
p-type) are uniformly distributed throughout the material. In other words, there is
no accumulation of these carriers at any specific place in the material.
However, when these materials are joined, some of the electrons get accumulated
at the interface (for reasons explained earlier) in n-type and holes in the p-type. If
we wish to represent the variation in concentration of these carriers in the material,
it would be reasonable to express their concentration in an exponential fashion. In
other words, after the junction formation, the concentration of electrons appears
to decrease exponentially as we go from the interface to the bulk of the material.
Similarly, the concentration of holes also decreases exponentially as we go from the
interface to the bulk of p-type material. Conceptually, a hole is a representation of
missing electrons, therefore, it may not be incorrect to represent the concentration of
holes as missing of electron. Pictorially, formation of p−n junction and variation in
7 Non-conventional Detection Techniques
place within the vicinity of about 10,000 Å from the interface of the semiconductor.
It is assumed that a positive charge created within the n-semiconductor also lies in a
plane at about 10,000 Å from the interface. Distance between this plane and the interface is known as the width of space charge (w). The magnitude of “w” depends on
the difference between the Fermi levels of two semiconductors; larger the difference,
greater is the width of space charge. This model suggests the formation of two types
of charges present in two planes separated by the width of space charge. Moreover,
these planes behave like two parallel plate capacitors. Let’s assume for simplicity
that the potential difference created due to such charge separation in a semiconductor is about 1.0 V and the charges are separated by a distance of 1000 Å. Then, an
electrical field of 0.1 MV cm
−1 is created in the space charge width. Electrical field
of this magnitude will be formed in both n- and p-type Si semiconductors.
Under this situation, if we excite an electron from the valence band to the conductance band in the n-semiconductor, a hole will be created in the valence band. If
these carriers (i.e., electrons and holes) are created within the space charge region,
electrons will be immediately forced to collect at the other end of n-type material
(i.e., behind the interface) and holes would be forced to move to the interface of
n- and p-semiconductors. Similarly, if an electron of p-semiconductor is excited
within the space charge region, due to electrical field, the electron will move toward
the interface of n- and p-semiconductors and hole will move toward the bulk. If
excitation of electrons is done in both semiconductors simultaneously, electrons will
find its place at the backside of the n-semiconductor, and holes will be collected at
the backside of the p-semiconductor. If the backside of the n- and p-type materials
is connected to an ammeter, these accumulated carriers will move to get neutralized,
resulting in a flow of current.
7.4.2 Distribution of Carrier Concentration
Another aspect of this junction formation needs to be touched upon. Before n- and
p-type materials are joined, the concentration of electrons (in n-type) and holes (in
p-type) are uniformly distributed throughout the material. In other words, there is
no accumulation of these carriers at any specific place in the material.
However, when these materials are joined, some of the electrons get accumulated
at the interface (for reasons explained earlier) in n-type and holes in the p-type. If
we wish to represent the variation in concentration of these carriers in the material,
it would be reasonable to express their concentration in an exponential fashion. In
other words, after the junction formation, the concentration of electrons appears
to decrease exponentially as we go from the interface to the bulk of the material.
Similarly, the concentration of holes also decreases exponentially as we go from the
interface to the bulk of p-type material. Conceptually, a hole is a representation of
missing electrons, therefore, it may not be incorrect to represent the concentration of
holes as missing of electron. Pictorially, formation of p−n junction and variation in