As with traditional light sources, semiconductor light-emitting diodes (LEDs) also generate heat during operation, depending on the overall luminous efficiency. Under the action of external energy, the electrons and holes radiate complex electroluminescence, and the light radiated near the P-N junction passes through the semiconductor medium and the encapsulation medium of the chip itself to reach the outside (air). Integrated current injection efficiency, radiated quantum efficiency, chip external light extraction efficiency, etc., and ultimately only about 30-40% of the input power into light energy, the remaining 60-70% of the energy mainly non-radiation composite lattice vibration Form of heat transfer.
While the chip temperature increases, it will enhance the non-radiative composite, and further weaken the luminous efficiency. Because, people subjectively believe that high-power LED no heat, in fact, indeed. A lot of heat, so that problems in the use of the process. Plus a lot of people who use high-power LED for the first time, the heat problem does not know how to effectively solve, making product reliability become the main problem. So, what happens to the heat? How much heat can it produce? How much heat does LED produce?
LED in the forward voltage, the electrons from the power to obtain energy, driven by the electric field, to overcome the PN junction of the electric field, from the N area transition to the P area, these electrons and P area of the hole compound. Since the free electrons drifted into the P region have more energy than the valence electrons in the P region, the electrons return to the low energy state at the time of recombination, and the excess energy is released in the form of photons. The wavelength of the emitted photon is related to the energy difference Eg. It can be seen that the luminescence area is mainly near the PN junction, and the luminescence is the result of the release of energy from electrons and holes. A semiconductor diode, the electrons entering the semiconductor area to leave the semiconductor area of all the distance, will encounter resistance. Simply from the principle point of view, the physical structure of the semiconductor diode simply from the principle point of view, the semiconductor body of the physical structure of the negative source of electrons and back to the cathode of the number of electrons are equal. Ordinary diodes, in the case of electron - hole pairs, are due to the energy level difference Eg, the released photon spectrum is not within the visible range.
Electrons in the diode inside the road, will be due to the presence of resistance and power consumption. The power consumed by the basic law of electronics:
P = I2 R = I2 (RN ++ RP) + IVTH
Where: RN is the N-zone resistance
VTH is the PN junction opening voltage
RP is the P zone body resistance
The power consumed by the heat is:
Q = Pt
Where t is the time the diode is energized.
In essence, the LED is still a semiconductor diode. Therefore, the LED in the positive work, its work process in line with the above description. It is consumed by its electric power:
P LED = U LED × I LED
Where: U LED LED light source at both ends of the forward voltage
I LED is the current flowing through the LED
These consumed electric power is converted to heat release:
Q = P LED × t
Where t is the energization time
In fact, the energy released by the electrons in the P-zone and the holes is not directly supplied by the external power source, but because the electron is in the N-zone, its energy level is greater than that of the P-region when there is no external electric field Price level higher than Eg. When it reaches the P area, and the hole complex and become the P area of valence electrons, it will release so much energy. The size of Eg is determined by the material itself, independent of the external electric field. The effect of external power on electronics is only to push it to do directional movement and overcome the effect of PN junctions.
LED heat production and light effect has nothing to do; there is no one percent of the electric power to produce light, the remaining part of the electric power generated a thermal relationship. Through the high-power LED heat generation, thermal resistance, junction temperature concept understanding and theoretical formula derivation and thermal resistance measurement, we can study the high-power LED's actual packaging design, evaluation and product applications. It should be noted that thermal management is the LED products in the luminous efficiency is not high at this stage of the key issues, from the fundamental to improve the luminous efficiency to reduce the heat generated is drastic, which requires chip manufacturing, LED packaging and application product development The progress of all aspects of technology.