Discussion on Four Technical Problems of Semiconductor Lighting

Semiconductor lighting sources have entered the field of lighting in batches, but there are many problems, mainly energy efficiency, reliability, light color quality and cost. The energy efficiency and light color quality are rich in content, such as visual comfort and intelligence. Dimming control, etc., will not be described here. This article will discuss the main technical problems that need to be solved, which can be attributed to “three highs and one low”, that is, high luminous efficiency, high color rendering, high reliability and low cost technical problems. The realization of low cost is also a technical problem. To solve these four technical problems, we need to take a series of measures in all aspects of the semiconductor lighting industry chain, such as the adoption of new technologies, new structures, new processes, new materials, etc. Only the technical routes and directions that should be adopted are mentioned here. Product innovation in LED companies has helped.

First, how to achieve high light efficiency

The efficacy of semiconductor lighting, or energy efficiency, is an important indicator of energy efficiency. At present, the industrialization level of LED devices can reach 120-140lm/W, and the total energy efficiency of lighting fixtures can be greater than 100lm/W. This is still not high, the energy-saving effect is not obvious, and there is still a large distance from the theoretical value of the semiconductor device light effect of 250 lm / W. To achieve high light efficiency, we must solve related technical problems from all aspects of the industrial chain, mainly to improve internal quantum efficiency, external quantum efficiency, package light output efficiency and lamp efficiency. This article will focus on epitaxy, chip, package, lamp, etc. A discussion of the technical issues to be resolved.

1. Improve internal quantum efficiency and external quantum efficiency

The following measures are taken to improve internal quantum efficiency and external quantum efficiency.

(1) Substrate surface roughening and non-polar substrate

Growth of GaN using nanoscale patterned substrates, "oriented" patterned substrates or non-polar, semi-polar substrates reduces the effects of dislocations and defect densities and polar fields, and improves internal quantum efficiency [1].

(2) Generalized homogeneous substrate

GaN is grown on Al2O3 sapphire substrate by HVPE (Hydride Liquid Phase Epitaxy) as a mixed homogeneous substrate GaN/Al2O3. On the basis of epitaxial growth of GaN, the dislocation density can be greatly reduced by 106-107 cm-2. And greatly improve the internal quantum efficiency. Nichia, Cree and Peking University are both in research and development [2].

(3) Improve the quantum well structure

Control the variation and variation of the In composition, optimize the quantum well structure to increase the electron and hole overlap probability, increase the radiation recombination probability, and adjust the transport of unbalanced carriers to improve the internal quantum efficiency.

(4) Chips with new structure

Adopting the new structure requires the chip to emit light on six sides, adopting new technology on the chip interface to perform various surface roughening methods, reducing the probability of photon reflection on the chip interface, and increasing the surface light transmittance to improve the external quantum efficiency of the chip.

2. Improve package light extraction efficiency and reduce junction temperature

(1) Phosphor efficiency and coating process

The photoexcitation efficiency of the phosphor is still not high, and the yellow powder can reach about 70%. The efficiency of the red powder and the green powder is low and needs to be further improved. In addition, the coating process of the phosphor is very important, and it has been reported that the phosphor powder having a thickness of 60 μm is uniformly coated on the surface of the chip, and the excitation efficiency is high.

(2) COB package

At present, the light source of semiconductor illumination adopts various forms of COB package, and it is an urgent task to improve the light extraction efficiency of the COB package. It is reported that the second generation (some called the third generation) COB matrix structure package has a luminous efficiency of 120 lm/ W or above. If a flip-chip and a six-sided illuminator are used for total reflection, the luminous efficiency can be more than 160 lm/W.

(3) Reduce the junction temperature

According to relevant reports, the luminescence amount at the junction temperature of 25 °C is set to 100%. When the junction temperature rises to 60 °C, the luminescence amount is only 90%, and when it rises to 140 °C, it is only 70%, so it is necessary to increase the package. Heat dissipation measures to maintain a low junction temperature and maintain high luminous efficiency.

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