136
7 III-Nitride LED Quantum Efficiency Improvement Technology
efficiency. On the other hand, Joule heat will be generated when current flows inside
the chip. Joule heat is generated in places where current is dense. The difference in
material and structure will cause different light energy absorbed by different regions
of the chip, which will also cause the difference in heat generation. The distribution
of current will affect the temperature field of the chip. In many cases, one of the main
mechanisms of chip failure is electromigration caused by local overheating, aging
of materials, and the like. To this end, it is necessary to conduct in-depth research on
various factors that can affect the current distribution of the LED.
Both the p-GaN and n-GaN regions of the LED chip should have a relatively thick
layer of electrically conductive material to ensure that the current injected into the
chip through the electrode can spread evenly before flowing into the active region.
This layer of electrically conductive material is called the “current spreading layer”.
It is the current spreading layer that determines the difference in current distribution
of the LED. For example, in a GaN-based lateral structure LED, the ITO transparent
electrode is a current spreading layer on the p-GaN region side. p-GaN is too thin
and the conductivity is too poor to function as a current spreading. n-GaN layer is
the current spreading layer on n side of the region. In a GaN-based vertical structure
LED, n-GaN is a current spreading layer on n type region, and a NiAg metal film
on the p side region is almost an ideal conductor. It is, therefore, not necessary to
consider the current expansion effect.
The conductivity of the current spreading layer is described by the square resistance in . The smaller the square resistance, the higher the conductivity and stronger
the current spreading capability are. E.g. sheet resistance of a commonly used 300 nm
thick ITO film on p-GaN is about 10 . The sheet resistance of a 2 μm thick n-GaN
is also around this value. The square resistance is inversely proportional to the thickness of the film and proportional to the resistivity of the material. In order to obtain a
more uniform current distribution, the square resistance of the current spreading layer
should be as low as possible. For a light-emitting device, one should also consider
its absorption of light in the design of the current spreading layer. This issue is often
related to the square resistance.
7.4.2 Current Distribution Theory
Here are some basic knowledges of the current distribution in LED devices. First,
the current-voltage characteristics of the diode need to be introduced since this
characteristic is related to the current distribution of the diode.
We know that the current-voltage characteristic curve of an ideal diode can be
expressed in the form of Eq. 7.11 [28]. Suppose that GaN-based LED satisfies the
current-voltage relationship, the IV curve of LED is shown in Fig. 7.19. Although the
actual situation can be slightly different, the trend of voltage changes with current is
the same. The dynamic resistance r d1 varies with current is also shown in the figure.
As the injection current increases, the dynamic resistance r d1 gradually decreases.
7 III-Nitride LED Quantum Efficiency Improvement Technology
efficiency. On the other hand, Joule heat will be generated when current flows inside
the chip. Joule heat is generated in places where current is dense. The difference in
material and structure will cause different light energy absorbed by different regions
of the chip, which will also cause the difference in heat generation. The distribution
of current will affect the temperature field of the chip. In many cases, one of the main
mechanisms of chip failure is electromigration caused by local overheating, aging
of materials, and the like. To this end, it is necessary to conduct in-depth research on
various factors that can affect the current distribution of the LED.
Both the p-GaN and n-GaN regions of the LED chip should have a relatively thick
layer of electrically conductive material to ensure that the current injected into the
chip through the electrode can spread evenly before flowing into the active region.
This layer of electrically conductive material is called the “current spreading layer”.
It is the current spreading layer that determines the difference in current distribution
of the LED. For example, in a GaN-based lateral structure LED, the ITO transparent
electrode is a current spreading layer on the p-GaN region side. p-GaN is too thin
and the conductivity is too poor to function as a current spreading. n-GaN layer is
the current spreading layer on n side of the region. In a GaN-based vertical structure
LED, n-GaN is a current spreading layer on n type region, and a NiAg metal film
on the p side region is almost an ideal conductor. It is, therefore, not necessary to
consider the current expansion effect.
The conductivity of the current spreading layer is described by the square resistance in . The smaller the square resistance, the higher the conductivity and stronger
the current spreading capability are. E.g. sheet resistance of a commonly used 300 nm
thick ITO film on p-GaN is about 10 . The sheet resistance of a 2 μm thick n-GaN
is also around this value. The square resistance is inversely proportional to the thickness of the film and proportional to the resistivity of the material. In order to obtain a
more uniform current distribution, the square resistance of the current spreading layer
should be as low as possible. For a light-emitting device, one should also consider
its absorption of light in the design of the current spreading layer. This issue is often
related to the square resistance.
7.4.2 Current Distribution Theory
Here are some basic knowledges of the current distribution in LED devices. First,
the current-voltage characteristics of the diode need to be introduced since this
characteristic is related to the current distribution of the diode.
We know that the current-voltage characteristic curve of an ideal diode can be
expressed in the form of Eq. 7.11 [28]. Suppose that GaN-based LED satisfies the
current-voltage relationship, the IV curve of LED is shown in Fig. 7.19. Although the
actual situation can be slightly different, the trend of voltage changes with current is
the same. The dynamic resistance r d1 varies with current is also shown in the figure.
As the injection current increases, the dynamic resistance r d1 gradually decreases.