Effect of Temperature on LED Performance

Compared with traditional light sources, LED light sources are solid cold light sources with the characteristics of long life, high light efficiency, no radiation, low power consumption, good impact and vibration resistance, and high reliability. Today, when green lighting is advocated globally, LED is sought after by the world as a new type of green lighting source. However, with the rapid development of LED, how to further improve the life and reliability of LED is indeed an urgent matter. Starting from theory and experiment, this paper studies and analyzes the influence of temperature on LED performance, and proposes methods to improve the life and reliability of LED.

1 LED light-emitting principle
LED is a semiconductor diode. The core light-emitting part is a PN junction composed of P-type and N-type semiconductors. In addition to the general PN junction characteristics, LED also has light-emitting characteristics. Under forward voltage, electrons are injected from the N region into the P region. Holes are injected from the P region into the N region. The minority carriers entering the other region recombine with the majority carriers to emit photons. The light-emitting principle is shown in Figure 1.

2 Effect of temperature on LED performance
2.1 Effect of temperature on LED luminous intensity
When the temperature of the PN junction rises, the lattice vibration amplitude of the semiconductor increases. When the vibration energy of the atom is higher than a certain value, the electron will exchange energy with the lattice atoms (or ions) when it transitions from the excited state to the ground state, and a non-radiative transition will occur. The probability of this process increases exponentially with the increase in temperature. Therefore, when the temperature rises to a certain level, the non-radiative transition of the semiconductor increases, the internal quantum efficiency decreases, and the luminous intensity decreases. When
the temperature rises, the epoxy resin used for LED packaging will gradually denature and turn yellow, thereby affecting the light transmittance of the epoxy resin, resulting in a decrease in external quantum efficiency and a decrease in luminous intensity.
2.2 Effect of temperature on LED forward voltage
Forward voltage is an important parameter for determining LED performance. Usually, the forward voltage of InGaA1P LED at 20 mA forward current is between 1.8 and 2.2 V, and the forward voltage of InGaN LED is between 3.0 and 3.6 V. Under the small current approximation, the forward voltage of the LED can be expressed as:

where Vf is the forward voltage, If is the forward current, I0 is the reverse saturation current, q is the electron charge, k is the Boltzmann constant, Rs is the series resistance, and n is a parameter that characterizes the perfection of the PN junction, which is between 1 and 2. The reverse saturation current I0 on the right side of formula (1) is closely related to temperature. The I0 value increases with the increase of temperature, resulting in
a decrease in the forward voltage Vf.

3 Experimental research
Here, we take the red and blue light elliptical lamp as an example to study the effect of temperature on LED performance. The experimental design is: use the crystal AIGaInP red chip Es-SAHRPN10 and the silan InGaN blue chip SL-NBIT0300 to trial-produce red and blue elliptical lamps respectively. Ordinary silver glue is used for red light solid crystal glue, ordinary insulating glue is used for blue light solid crystal glue, iron bracket is used for bracket, and ordinary epoxy resin is used for packaging. Take 20 finished blue elliptical lamps, number 1 to 20, and then use JF light intensity tester to test the normal light intensity Iv and forward voltage Vf of elliptical lamps No. 1 to 20 at room temperature and 20mA. After testing the data, the 20 tubes are divided into two groups: lamps No. 1 to 10 are lit at room temperature and 20 mA for 168 hours; lamps No. 11 to 20 are placed in an 85℃ constant temperature box and also lit at 20 mA for 168 hours. After the experiment, the JF light intensity tester was used to test the normal light intensity Iv and forward voltage Vf of each group at room temperature and 20 mA. Then the red oval lamp was also subjected to the above grouping experiment.

4 Experimental data and analysis
Tables 1 to 4 are the data tested in the experiment.

From the above experimental data, we can see the influence of PN junction temperature and ambient temperature on LED luminous intensity and forward voltage.
Under 85℃, the average light decay of blue light is 12.68%, and the forward voltage drops by 0.05 V, while the average light decay of blue light at room temperature is only 3.32%, and the forward voltage drops by 0.01 V; for red light, the same trend is also seen, the average light decay under 85℃ is 9.97%, and the forward voltage drops by 0.031 V, while the average light decay at room temperature is 1.89%, and the forward voltage drops by 0.004 V. This shows that as the ambient temperature rises, the LED light decay is serious and the forward voltage drops greatly.
Tables 1 and 3 also reflect the effect of the internal temperature increase of the LED on the light decay when the red and blue LEDs are continuously lit at room temperature and 20mA. At the same time, it can be seen that the light decay of blue light is more serious than that of red light. This phenomenon can be explained from two aspects: First, the solid crystal glue used for blue light is insulating glue, while the solid crystal glue for red light is silver glue. The thermal conductivity of the insulating glue is very weak, resulting in the blue light heat dissipation is not as good as the red light. The PN junction temperature of the blue light is slightly higher than that of the red light. The higher the temperature, the more serious the light decay; second, under ultraviolet light exposure, the physical properties of the epoxy resin will change, resulting in a decrease in transmittance, and the blue chip will emit part of the short-wave light. Under long-term short-wave irradiation, the transmittance of the epoxy resin decreases and the luminous intensity decreases.

5 Conclusions
Through the research experiments in this article, it can be seen that temperature has a great influence on the performance of LEDs.
When the temperature rises, the light decay of the LED is serious and the life is reduced. Therefore, in LED packaging, in order to improve the life and reliability of LED, the temperature and heat dissipation of LED should be considered. A series of methods can be used to reduce thermal resistance to achieve the purpose of heat dissipation; such as using copper brackets with good thermal conductivity to replace iron brackets; for single-electrode chips, high thermal conductivity silver glue can be used; for blue and white light, anti-UV and anti-yellowing epoxy resin is used for packaging;
and when the temperature rises, the forward voltage of LED decreases. In actual use, if constant voltage drive is used, the forward voltage is reduced, resulting in an increase in driving current, which will reduce the life of LED on the one hand, and cause poor consistency performance. When used on large screens (such as traffic lights and display screens), the driving current of each LED lamp is different, resulting in different brightness, which will cause problems such as flower screens and obvious mosaic areas. Therefore, in actual use, constant current drive should be used to improve the life and reliability of LED.