Causes of Heat Generation in LED

- Sep 29, 2019-

Like traditional light sources, semiconductor light-emitting diodes (LED) generate heat during operation, depending on the overall luminous efficiency. Under the action of external electric energy, electroluminescence occurs when the radiation of electrons and holes is combined. The light emitted near the P-N junction needs to pass through the semiconductor medium and packaging medium of the chip itself to reach the outside (air). In the end, only about 30-40% of the input energy is converted into light energy, while the remaining 60-70% of the energy is converted into heat energy in the form of lattice vibration caused by non-radiation recombination.

The higher the chip temperature, the stronger the non-radiative recombination and the weaker the luminescence efficiency. Because people subjectively think that high-power LED has no heat, in fact it does. There is so much heat that problems occur during use. In addition, many people who use high-power LED for the first time do not know how to solve the thermal problem effectively, which makes the reliability of products become a major problem. So, does LED generate heat? How much heat can it produce? How much heat does LED generate?

Under the forward voltage, the electrons get energy from the power supply. Driven by the electric field, the electrons overcome the electric field of PN junction and transit from N region to P region. These electrons recombine with the holes in P region. Because the free electrons drifting to the P region have higher energy than the valence electrons in the P region, the electrons return to the low energy state when recombining, and the excess energy is emitted in the form of photons. The wavelength of the photon emitted is related to the energy difference Eg. It can be seen that the luminescent region is mainly near the PN junction, and the luminescence is due to the energy released by the recombination of electrons and holes. In a semiconductor diode, electrons encounter resistance throughout the journey from entering to leaving the semiconductor region. Simply in principle, the physical structure of semiconductor diodes is simple in principle. The number of electrons emitted by the negative source of the physical structure of semiconductor diodes is equal to the number of electrons returned to the positive. For ordinary dipoles, the recombination of electron-hole pairs occurs because the photon spectrum released is not in the visible range due to the energy level difference Eg.

Electrons will consume power because of the existence of resistance in the internal path of the bipolar body. The power consumed conforms to the basic laws of electronics:


In formula: RN is N-zone bulk resistance

VTH is the opening voltage of PN junction

RP is the resistance of P region

The heat generated by the power consumed is:

Q = Pt

In formula: t is the time when the dipole is electrified.

In essence, LED is still a semiconductor diode. Therefore, when the LED is working in the forward direction, its working process conforms to the above description. Its power consumption is:


U LED is the forward voltage at both ends of the LED light source.

I LED is the current flowing through the LED.

These consumed power is converted into heat.


In the formula: t is the power-on time

In fact, the energy released by an electron in the P region when it recombines with a hole is not directly supplied by an external power supply, but because the energy level of the electron in the N region is higher than that of the valence electron in the P region when there is no external electric field. When it reaches the P region and recombines with the hole to become the valence electron in the P region, it releases so much energy. The size of Eg is determined by the material itself and has nothing to do with the external electric field. The effect of external power supply on electrons is only to push them to move in direction and overcome the effect of PN junction.

The heat production of LED is not related to light efficiency; there is no relationship between a few percent of the electric power to produce light and the rest of the electric power to generate heat. Through understanding the concept of thermal generation, thermal resistance and junction temperature of high-power LED, deducing theoretical formula and measuring thermal resistance, we can study the actual packaging design, evaluation and product application of high-power LED. It should be pointed out that heat management is the key problem at the present stage when the luminous efficiency of LED products is not high. To fundamentally improve the luminous efficiency and reduce the generation of heat energy is the only way to draw money from the bottom of the kettle. This requires the progress of technology in chip manufacturing, LED packaging and application product development.

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