10
2 Basic Principles of LED
When two semiconductors combine to form a p–n junction, electrons will flow
from the n-region with high Fermi level to the p-region with low Fermi level, while
holes will flow from the p-region to the n-region. Therefore, the Fermi level in the
n-region will move downward and the Fermi level in the p-region will move up until
the Fermi level in the n-region and the p-region are equal. Under this condition, there
is a unified Fermi level in the p–n junction and the p–n junction is in equilibrium.
When a positive bias voltage is applied to a p–n junction, the applied positive
bias voltage basically falls in the barrier region. This is because that the carrier
concentration in the barrier region is very small and the resistance is very large. On
the other hand, the carrier concentration in the p and n regions outside the barrier
region is very high and the resistance is very small. The forward bias produces an
electric field opposite to the built-in electric field in the barrier region, which weakens
the electric field intensity in the barrier region, resulting in the injection of minority
carriers from both sides (n and p region) of the junction. The Fermi level in the n
region moves up but the Fermi level in the p region moves down, until the difference
of Fermi levels in the n and p regions equals to the difference between the builtin electric field and the applied electric field. The non-equilibrium carriers higher
than the concentration at the equilibrium state will recombine near the junction and
produce photons.
When a reverse bias voltage is applied to a p–n junction, the electric field generated
by the reverse bias in the barrier region is in the same direction as the built-in electric
field. The electric field in the barrier region is enhanced. A very small number of
carriers in the junction region could be driven away by the electric field. The Fermi
level in the n region moves down and the Fermi level in the p region moves up until
the difference between the built-in electric field and the applied electric field in the n
region is equal to the sum of the built-in electric field and the applied electric field in
the p region. The current of the p–n junction is also very small and tends to remain
unchanged. The LED does not emit light under reverse bias. It can be seen that, like
the ordinary diodes, LED has the characteristics of ‘ON’ under forward bias and
‘OFF’ under reverse bias.
2.2 Radiation and Non-radiation Recombination
Because of the interaction within the semiconductor, there are always a certain
number of electrons and holes in the equilibrium state of any semiconductor. From
the microscopic point of view, equilibrium state refers to the balance between the
microscopic processes caused by certain interactions within the system. It is precisely
these micro-processes that make the system transition from non-equilibrium state to
equilibrium state, resulting in the recombination of non-equilibrium carriers. There
are two mechanisms for the recombination of unbalanced carriers in LED. In the
process of recombination, the excess energy of electrons can be released in the form
of radiation (photon emission). This recombination is called radiation recombination, which is an inverse process of light absorption. The excess energy of electrons
2 Basic Principles of LED
When two semiconductors combine to form a p–n junction, electrons will flow
from the n-region with high Fermi level to the p-region with low Fermi level, while
holes will flow from the p-region to the n-region. Therefore, the Fermi level in the
n-region will move downward and the Fermi level in the p-region will move up until
the Fermi level in the n-region and the p-region are equal. Under this condition, there
is a unified Fermi level in the p–n junction and the p–n junction is in equilibrium.
When a positive bias voltage is applied to a p–n junction, the applied positive
bias voltage basically falls in the barrier region. This is because that the carrier
concentration in the barrier region is very small and the resistance is very large. On
the other hand, the carrier concentration in the p and n regions outside the barrier
region is very high and the resistance is very small. The forward bias produces an
electric field opposite to the built-in electric field in the barrier region, which weakens
the electric field intensity in the barrier region, resulting in the injection of minority
carriers from both sides (n and p region) of the junction. The Fermi level in the n
region moves up but the Fermi level in the p region moves down, until the difference
of Fermi levels in the n and p regions equals to the difference between the builtin electric field and the applied electric field. The non-equilibrium carriers higher
than the concentration at the equilibrium state will recombine near the junction and
produce photons.
When a reverse bias voltage is applied to a p–n junction, the electric field generated
by the reverse bias in the barrier region is in the same direction as the built-in electric
field. The electric field in the barrier region is enhanced. A very small number of
carriers in the junction region could be driven away by the electric field. The Fermi
level in the n region moves down and the Fermi level in the p region moves up until
the difference between the built-in electric field and the applied electric field in the n
region is equal to the sum of the built-in electric field and the applied electric field in
the p region. The current of the p–n junction is also very small and tends to remain
unchanged. The LED does not emit light under reverse bias. It can be seen that, like
the ordinary diodes, LED has the characteristics of ‘ON’ under forward bias and
‘OFF’ under reverse bias.
2.2 Radiation and Non-radiation Recombination
Because of the interaction within the semiconductor, there are always a certain
number of electrons and holes in the equilibrium state of any semiconductor. From
the microscopic point of view, equilibrium state refers to the balance between the
microscopic processes caused by certain interactions within the system. It is precisely
these micro-processes that make the system transition from non-equilibrium state to
equilibrium state, resulting in the recombination of non-equilibrium carriers. There
are two mechanisms for the recombination of unbalanced carriers in LED. In the
process of recombination, the excess energy of electrons can be released in the form
of radiation (photon emission). This recombination is called radiation recombination, which is an inverse process of light absorption. The excess energy of electrons