Related knowledge of LED junction temperature

1. What is the junction temperature of LED?

The basic structure of LED is a semiconductor PN junction. Experiments show that when current flows through an LED element, the temperature of the PN junction will rise. Strictly speaking, the temperature of the PN junction area is defined as the junction temperature of the LED. Usually, since the element chips are very small, we can also regard the temperature of the LED chip as the junction temperature.

Light-emitting diodes (LEDs) have been widely used in transportation, advertising, and instrument displays due to their high brightness, low power consumption, long life, high reliability, easy driving, energy saving, and environmental protection. They have now been used in special lighting [1][2] and will become the main light source in general lighting [3]. At present, the production and use of LEDs in the world are showing a rapid upward trend, but LEDs have a heating phenomenon. As the working time and working current of LEDs increase, their luminous intensity and luminous flux will decrease, their life will decrease, and the excitation efficiency of white light will also decrease [4]. This is mainly due to the increase in LED junction temperature. In 2002, Hong et al. [5] found that the peak wavelength shift of AlGaInP red LEDs has a linear relationship with the change in junction temperature. For white light LEDs, as the junction temperature increases, the intensity of yellow light and blue light emitted by the LED decreases at different rates. The total energy and blue light energy ratio (W/B) of white light LEDs is related to the junction temperature.

2. What are the causes of LED junction temperature?

When the LED is working, there may be five situations that cause the junction temperature to rise to varying degrees:

a. The electrode structure of the component is bad, the material of the window layer substrate or the junction area and the conductive silver glue all have a certain resistance value. These resistors are stacked together to form the series resistance of the LED component. When the current flows through the PN junction, it will also flow through these resistors, thereby generating Joule heat, causing the chip temperature or junction temperature to rise.

b. Since the PN junction cannot be extremely perfect, the injection efficiency of the component will not reach 100%. That is to say, when the LED is working, in addition to the P region injecting charges (holes) into the N region, the N region will also inject charges (electrons) into the P region. Under normal circumstances, the latter type of charge injection will not produce a photoelectric effect, but will be consumed in the form of heat. Even if the useful part of the injected charge will not all turn into light, a part of it will combine with impurities or defects in the junction area and eventually turn into heat.

c. Practice has proved that the limitation of light extraction efficiency is the main reason for the increase of LED junction temperature. At present, advanced material growth and component manufacturing processes have enabled most of the input electrical energy of LED to be converted into light radiation energy. However, since the refractive coefficient of LED chip materials is much larger than that of the surrounding medium, most of the photons (>90%) generated inside the chip cannot smoothly overflow the interface, but are totally reflected at the interface between the chip and the medium, return to the inside of the chip, and are finally absorbed by the chip material or substrate through multiple internal reflections, and become heat in the form of lattice vibration, causing the junction temperature to increase.

d. Obviously, the heat dissipation capacity of LED components is another key condition that determines the junction temperature. When the heat dissipation capacity is strong, the junction temperature decreases. Conversely, when the heat dissipation capacity is poor, the junction temperature will rise. Since epoxy glue is a low thermal conductivity material, it is difficult for the heat generated at the PN junction to be dissipated upward to the environment through transparent epoxy. Most of the heat is dissipated downward through the substrate, silver paste, tube shell, epoxy bonding layer, PCB and heat sink. Obviously, the thermal conductivity of the relevant materials will directly affect the heat dissipation efficiency of the component. For an ordinary LED, the total thermal resistance from the PN junction area to the ambient temperature is between 300 and 600℃/w. For a power LED component with a good structure, the total thermal resistance is about 15 to 30℃/w. The huge difference in thermal resistance indicates that ordinary LED components can only work normally under very small input power conditions, while the dissipated power of power components can be as large as watts or even higher.

3. What are the ways to reduce LED junction temperature?

a. Reduce the thermal resistance of the LED itself; b. Good secondary heat dissipation mechanism; c. Reduce the thermal resistance between the LED and the installation interface of the secondary heat dissipation mechanism; d. Control the rated input power; e. Reduce the ambient temperature. The input power of the LED is the only source of the thermal effect of the component. Part of the energy becomes radiant light energy, and the rest eventually becomes heat, thereby raising the temperature of the component. Obviously, the main method to reduce the temperature rise effect of the LED is to try to improve the electro-optical conversion efficiency (also known as external quantum efficiency) of the component so that as much input power as possible is converted into light energy. Another important way is to try to improve the heat dissipation capacity of the component so that the heat generated by the junction temperature can be dissipated to the surrounding environment through various channels.