①Causes of heating of LED devices
Like traditional light sources, LEDs also generate heat when they work. Under the action of an external electric field, the electrons in the N-type semiconductor gain energy to overcome the potential barrier at the PN junction and recombine a large number of holes in the P-type semiconductor. After recombination, the electron returns to a low-potential energy state and releases energy at the same time. The form of energy release may be in the form of radiation, or it may be in the form of non-radiation. If the energy is released in the form of non-radiation, it will cause the vibration of the semiconductor crystal lattice, that is, it will be converted into heat. In addition, electrons encounter resistance when they migrate in the semiconductor, which is another source of heat.
②The influence of temperature on LED light output
According to experiments, when the ambient temperature is low, the light output of the LED will increase; and high temperature will cause its light output to decrease. When the ambient temperature is too high or the working current is too large, the temperature of the LED chip will increase. At this time, the electron and hole concentration inside the PN junction, the forbidden band width and the electron migration rate and other microscopic parameters will change, which will affect the light output of the LED. Figure 1 shows the relationship between luminous flux output (referred to as luminous output) and PN junction temperature under the condition of constant LED operating current.
When the temperature of the PN junction of the LED increases, the band gap of the material will decrease. This indicates that the output of radiation recombination will shift to the long wave direction. At the same time, the increase in PN junction temperature will also cause the forward voltage drop of the LED to decrease. This means that once the LED in the loop has excessive temperature rise, the response of the PN junction to this will further increase the temperature of the LED. Once the temperature of the LED chip exceeds a certain value, the entire LED will be damaged. The value of this temperature is called the maximum junction temperature or critical temperature. The critical temperature of LEDs of different materials is different, even the same material, packaging structure and other factors will affect the critical temperature.
③ Commonly used LED heat dissipation technology
In the LED lighting system, the heat sink is very important. To solve the heat dissipation problem of LEDs, one can improve the luminous efficiency of LED chips, reduce non-radiative recombination, and fundamentally reduce heat generation; second, improve the LED structure or adopt a heat sink. In theory, any material that can conduct heat can be used as a heat sink for LEDs. Most metals are good conductors of heat, so a metal frame can be used as a heat sink (Figure 2). At the same time, the contact area between the LED and the heat sink should be increased as much as possible to ensure good thermal contact between the LED and the heat sink.
The most commonly used heat dissipation method for high-power LEDs is the flip chip structure. The structure of the traditional LED determines that most of the heat of the chip is dissipated through the metal pins, and cannot be dissipated through the substrate with poor thermal conductivity. After adopting the flip chip structure, the heat can be dissipated through the heat sink located at the bottom of the L and ED. Figure 3 shows the internal structure of the flip-chip LED. The LED chip is soldered on the insulating heat sink on the surface, and the electrical connection with the lead frame is completed by gold wire bonding. The heat sink is encapsulated with plastic material, and the outside of the chip is sealed with a thermally stable, insulating and optically transparent resin material, and then the entire LED device is attached to the substrate, and then an external heat sink is used to improve the heat dissipation effect. A small part of the heat energy of this structure is directly dissipated to the surrounding environment through the sealing material, and most of the heat is dissipated to the surrounding environment through the light-emitting layer (heat generation) → substrate → chip bonding layer → substrate → heat dissipation fins. The packaging material has a great influence on the heat dissipation effect. Therefore, looking for packaging materials with excellent thermal conductivity and optimizing the thermal system of the device from the material aspect is an important way to improve the heat dissipation efficiency.